Limits of Functions University Students’ Concept Development
2006:08 DOCTORAL T H E SI S
Limits of Functions University Students’ Concept Development
Kristina Juter
Luleå University of Technology Department of Mathematics
2006:08|: -1544|: - -- 06 ⁄08 -- Limits of Functions- University Students’ Concept Development
by
Kristina Juter
Department of Mathematics Luleå University of Technology SE-971 87 Luleå
February 2006 2 Abstract This PhD thesis consists of six articles and a concise synopsis that generally frame these articles. The theories and methods used are outlined and discussed in the synopsis to show their relation. The articles are published or accepted for publication (see the list of publications at page 9) with the following abstracts:
I: Limits of functions – how do students handle them? This paper aims at formulating and analysing the development of 15 students in their creations of mental representations of limits of functions during a basic mathematics course at a Swedish university. Their concept images are sought via questionnaires and interviews. How do students respond to limits of functions? The data indicate that some students have incoherent representations, but they do not recognise it themselves.
II: Limits of functions: Traces of students’ concept images Students at a Swedish university were subjects in a study about learning limits of functions. The students’ perceptions were investigated in terms of traces of concept images through interviews and problem solving. The results imply that most students’ foundations were not sufficiently strong for them to understand the concept of limit well enough to be able to form coherent concept images. The traces of the students’ concept images reveal confusion about different features of the limit concept.
III: Students’ attitudes to mathematics and performance in limits of functions The main aim of this article is to discuss the attitudes to mathematics of students taking a basic mathematics course at a Swedish university, and to explore possible links between how well such students manage to solve tasks about limits of functions and their attitudes. Two groups, each of about a hundred students, were investigated using questionnaires, field notes and interviews. From the results presented a connection can be inferred between students’ attitudes to mathematics and their ability to solve limit tasks. Students with positive attitudes perform better in solving limit problems. The educational implications of these findings are also discussed.
IV: Limits of functions as they developed through time and as students learn them today In this article, the ways first year university students’ develop the concept of limits are compared to the historical development of the concept. The
3 aim is to find out if the students perceive the notion as mathematicians of the past did as understandings of the concept evolved. The results imply that there are some similarities, for example, the struggle with rigour and attainability. Knowledge of such critical areas can be used to improve the students’ opportunities of learning limits of functions. Some teaching aspects are also discussed.
V: Limits of functions - students solving tasks Students solve problems to learn mathematics. How do they cope with the problems and explain their solutions? This study is about students at a Swedish university, solving tasks about limits of functions. The students are enrolled in their first university course in mathematics. They study algebra and calculus for 20 weeks fulltime. There are many possible problems for the students to handle including the notion of function, the formal definition and limits as unattainable values for functions. Many students do not possess enough knowledge about limits to solve and explain solutions to non-standard problems. Some students arrive at correct answers through erroneous reasoning.
VI: Limits and infinity: A study of university students’ performance Students’ work with limits of functions includes the concept of infinity. These concepts are complex but necessary for mathematics studies. A study of student solutions to limit tasks was conducted to reveal how students handled limits of functions and infinity at a Swedish university. As a background to the students’ and teachers’ situations we have also analysed textbooks and curricula. One result is that many students were unable to handle limits correctly. Some students gave correct answers with incorrect explanations to tasks. Textbooks used at upper secondary schools do not provide much theory or many tasks about limits and infinity so most students new to university do not have a developed image of these concepts.
4 Acknowledgements My supervisors, Professor Barbro Grevholm, Professor Lars-Erik Persson and Professor Gilah Leder, have all been there for me throughout the period of my research education and I wish to thank each of them for that. Barbro, I am very happy to have been able to discuss all sorts of issues with you and for your encouragement when I needed it. Thank you for tirelessly reading and commenting on my texts at all times. Lars-Erik, thank you for your support and your very positive attitude to me and my work. You really strengthened my confidence with your enthusiasm. Gilah, thank you for your constructive advice and all your efforts to help me, in particular during my visit at La Trobe University. It was really an adventure, which you made possible. Especially thanks for the kangaroo expedition at the golf course.
I am very grateful for the hospitality of the Mathematics department at Lund University for letting me take time and space to carry out my two studies. Especially thanks to Per-Anders Ivert, Bengt Gamstedt, Tomas Claesson, Kjell Elfström, Catarina Petersson and Agneta Ohlzon.
I want to thank ‘Luleå Tekniska Universitet’ for the generous team work together with ‘Högskolan Kristianstad’ that has given me the opportunity to do my doctoral studies in Kristianstad near my home.
I also want to thank ‘Vetenskapsrådet’ and ‘Riksbankens Jubileumsfond’ for funding my research studies and the board of ‘Forskarskolan i matematik med ämnesdidaktisk inriktning’ for making the project possible. I would also like to thank ‘Högskolan Kristianstad’ for supporting me.
There have been two important seminars during my research education where my thesis has been read and commented by an opponent. The first was a licentiate seminar in January 2004 and the second was a 90% seminar in January 2006. I would like to thank Professor Johan Lithner and Professor Mogens Niss respectively for their valuable remarks and suggestions at those seminars.
Last, but perhaps most, I want to thank every single one of the students who participated in the studies. Their endless willingness to answer my questions was inspiring.
Kristina
5 6 Contents
Abstract 3 Acknowledgements 5 Contents 7 List of Publications 9 Introduction 11 Background and Questions 11 Theoretical Overview 12 Limits of Functions 12 Learning 14 Terminology 20 The Empirical Study 21 Rationale of the Empirical Study 22 The First Study 22 The Second Study 24 Methodological Remarks 27 Analysis of Data 29 Results 30 Ethical Issues and Scientific Quality 33 Discussion 34 New Results for the Field 34 Implications for Teaching and Learning 35 Further Research 37 Conclusions 38 References 40 Appendix 47 Article I Article II Article III Article IV Article V Article VI
7 8 List of Publications I Juter, K. (2005a). Limits of functions – how do students handle them? Pythagoras 61, 11-20.
II Juter, K. (2005c). Limits of functions: Traces of students’ concept images, Nordic Studies in Mathematics Education 10(3-4), 65-82.
Parts of the content of this article was presented at a conference and published as: II* Juter, K. (2004b). University students’ perceptions of limits of functions. In B. Tadich, S. Tobias, C. Brew, B. Beatty & P. Sullivan (Eds.), MAV (The Mathematics Association of Victoria) annual conference 2004, Towards excellence in mathematics (pp. 228-236). Melbourne, Australia: The Mathematics Association of Victoria.
III Juter, K. (2005b). Students’ attitudes to mathematics and performance in limits of functions, Mathematics Education Research Journal 17(2), 91- 110.
IV Juter, K. (2006a). Limits of functions as they developed through time and as students learn them today, to appear in Mathematical Thinking and Learning.
V Juter, K. (2006b). Limits of functions - students solving tasks, to appear in Australian Senior Mathematics Journal 20(1).
Parts of the content of this article was presented at a conference and published as: V* Juter, K. (2004a). Limits of functions – How students solve tasks. In C. Bergsten & B. Grevholm (Eds.), Mathematics and language, Proceedings of MADIF 4, the 4th Swedish Mathematics Education Research Seminar (pp. 146-156). Linköping, Sweden: Svensk Förening för Matematikdidaktisk Forskning.
VI Juter, K., & Grevholm, B. (2006). Limits and infinity: A study of university students' performance. To appear in C. Bergsten, B. Grevholm, H. Måsøval, & F. Rønning (Eds.), Relating practice and research in mathematics education. Fourth Nordic Conference on Mathematics Education, Trondheim, 2nd-6th of September 2005. Trondheim: Sør- Trøndelag University College.
9 10 Introduction Background and Questions Learning mathematics is an endeavour requiring a number of abilities, which vary for different mathematical topics. How individuals learn mathematics can also vary. Some go through hard work followed by a period of enlightenment (Hadamard, 1949), whereas others simply endure hard work. Mathematics has several characteristic properties, e.g. the use of models describing the real world, the compact and unambiguous formulations for clear expositions, and the deductive reasoning in proving and problem solving. Each property offers its own set of challenges for mathematics learners, often including a transition stage, for example, from a model to the real world, from everyday language to a mathematical expression, or from one step to another in a mathematical proof. Poincaré (1983), also identifying a transition, claimed that mathematics strives from the particular to the general through, for example, increasingly sophisticated deductive reasoning. The essence of mathematics is not easily formulated since there are so many qualities defining mathematics. There are consequently many different perceptions of mathematics (see, for example, Benacerraf & Putnam, 1983). One aspect focused on in this thesis is transition, e.g. abstraction and generalisation. Mathematics at the university level is formally presented in textbooks and at lectures, at a pace that is often higher than students are used to from upper secondary school. An initial course in mathematics at Swedish universities usually comprises algebra and calculus including the notion of limits of functions, which has been proven to be difficult for students to learn (Cornu, 1991; Tall, 1991; Tall & Vinner, 1981). Many different aspects regarding the notion of limits and the nature of learning that cause these difficulties are addressed in this PhD thesis. My own experience, as a student and later on as a university mathematics teacher, implied to me that learning limits of functions entails time and effort, perhaps to a greater extent than other parts of basic calculus. I wanted to understand more about how students perceive and learn limits of functions. My overall research question became: How do students deal with the concept of limits of functions at the basic university level in Sweden? In an attempt to answer this vast question, I conducted two studies at a large Swedish university. No such study on limits had previously been done in Sweden and it was therefore compared mostly to foreign research results. It is important that teachers who work with university level mathematics education are aware of the learning situation of students and are prepared to meet them at their levels in their learning processes, meaning that the pre-knowledge of
11 students is crucial (Ausubel, 1968). The extensive research question has been partitioned into the following set of more manageable sub-questions:
x How do students’ perceptions of limits develop during a semester of university level mathematics? (Juter, 2003; 2004b; 2005a; 2005c; 2006a) x How do students’ developments of limits of functions compare to the historical development of limits of functions? (Juter, 2006a) x How do students solve limit tasks? (Juter, 2003; 2004a; 2004b; 2005c; 2006a; 2006b; Juter & Grevholm, 2004; 2006) x What is the relationship of the students’ attitudes to mathematics and their performance in limits of functions? (Juter, 2005b)
The first sub-question is closely related to the overall question since the concept development of students is measured through their actions and responses in problem solving and discussions. The second sub-question has had an historical aspect added that is used as a device to compare the developments of the students to historical developments. This is not the same as saying that individual concept development is equal to historical concept development, which is discussed further on. The third sub-question is about students’ problem solving and is thus a direct part of the overall question. The last sub-question takes on another aspect of the work of students as their attitudes are compared to their abilities in solving limit tasks.
Theoretical Overview The nature of the concept of limits is discussed in this section. A description of the learning and concept formation theories used in the study follow. The last part deals with terminology where the applications of some words in the articles are clarified.
Limits of Functions The limit concept is an important part of the foundations of mathematical analysis and not understanding it clearly might lead to problems when dealing with concepts such as convergence, continuity and derivatives. The limit concept is connected to many other concepts, e.g. infinity, functions and infinitesimals. If an individual grasps the concept of limits, the connected concepts become easier to work with, but it is difficult for students to make sense of the concept. In fact, even many great mathematics researchers have found it hard to accurately handle limits through time.
12 The history of limits of functions shows that it was not obvious how a definition of limits should be stated or even if limits were useful. Published in 1797, Lagrange tried to form a definition of derivatives without limits using only algebraic analysis by representing functions with power series (Katz, 1998). He realised that his reasoning was not always true, but did not consider the false cases important. This method became popular among some mathematicians in England at that time. However, Cauchy in France did not share their enthusiasm. He started from the beginning and emphasised the rigor of the theory. Cauchy’s definition, stated in 1821, made it possible for him to deal with continuity analytically rather than geometrically, as had been the tradition earlier. Leibniz and Newton both worked with limits about a century before Lagrange and Cauchy made their contributions to mathematics. They developed theories to solve problems with area and extreme values in different notations and forms but with the same core and they are considered to be the founders of infinitesimal calculus (Lund, 2002). The lack of rigor in their theories made it difficult to use them as bases for further development. Newton was, for example, criticised by Berkeley in 1734 for the flaws in his calculations with fluxions, where he was unable to state a proper definition of a limit (Lund, 2002). It was not until Weierstrass presented his definition that there was a stringent and clear definition of limits (Boyer, 1949). Weierstrass’s definition is often used today as follows: A is called the limit of f (x) as x o a , if for every H ! 0 there exists a G ! 0 such that f (x) A H for every x in the domain with 0 x a G . This definition is the one referred to in the work presented in this thesis. Many mathematicians have worked with limits at different times and with different approaches for hundreds of years with obstacles that have taken a long time to overcome. This gives us a picture of the nature of the concept of limits of functions. According to Tall (1992) it is a concept “which signifies a move to a higher plane of mathematical thinking” (page 11). It is a complicated concept because it can be considered as both an infinite process and a rigid entity, depending on the situation. Infinity and this duality demands flexibility in the mental representations we use when thinking of limits. An inexperienced individual who does not possess enough representations or flexibility in the representations cannot understand the crucial similarities and differences for handling certain situations. In that case, knowing which operations and manipulations are allowed and which are not allowed is difficult. Controversies surrounding the development of limits through history may somehow reappear in the learning of limits at an individual level in
13 modern days and are therefore worthwhile investigating. There are reasons to be cautious in comparing ontogenesis (the development of an individual) with phylogenesis (the development of mankind). Changes in societies and cultural differences influence the results of comparisons, which must be acknowledged (Furinghetti & Radford, 2002; Radford, 2000). Rigor and the notion of limits developed differently, and different times had different traditions and ways of expression. The fact that many years passed before Weierstrass rigorously defined limits of functions with symbols does not imply that limits were neglectfully treated before. The tradition of using words instead of symbols perhaps complicated expositions that nevertheless could be completely accurate and rigorous even before Newton and Leibniz were alive. My point is not that ontogenesis recapitulates phylogenesis. Comparing the different developments of limits of functions could however provide interesting results (Juter, 2006a). One example is the question of rigor, which Cauchy emphasised and according to critics Newton did not. It seems that some students do not believe in the importance of knowing the formal definition of limits to understand a basic course in calculus (Juter, 2003; 2005a; 2006a). They have an intuitive conception that is generally sufficient, which if not working in certain situations is not enough to make the students rethink their representations. Several researchers have reported on studies regarding students learning limits, e.g. Bezuidenhout (2001), Cornu (1991), Milani & Baldino (2002), Szydlik (2000), Tall (1993) and Williams (1991). Some of their results are presented and discussed, in connection with the results of my study, in the articles of this thesis.
Learning I have a social constructivistic view of learning in the sense that students learn as individuals constructing their own concept knowledge that is influenced by their environment. The constructed knowledge has to be reconstructed by either the students or the researcher to become observable (Morf, 1998) accomplished through solutions to tasks, discussions or other ways of expression. My aims of this study led me to choose theories about concept formation, from where the students’ learning of limits could be described. Learning is measured by the students’ development as seen through their responses to various tasks. Several theories suitable for my purposes, instead of one theory, have been combined to function as tools in the data analysis and presentations. The APOS theory, developed by Dubinsky in co-operation with other researchers (Asiala, Brown, Devries, Dubinsky, Mathews & Thomas, 1996; Asiala, Dubinsky, Mathews, Morics & Oktac, 1997; Cottrill, Dubinsky,
14 Nichols, Schwingendorf & Vidakovic, 1996), is a hierarchical model based on Piaget’s thoughts of assimilation and accommodation with four stages from action to scheme (APOS in Table 1). Sfard (1991) also introduced a hierarchical model, somewhat comparable to the APOS model, describing the development from perceiving a concept as a process to perceiving it as an object, if the concept has this dual property. Both models represent mathematical concept formation from a stage where the concept is considered a process to a stage where the concept can be seen as an object, where the learner can go from one representation to another depending on the circumstances he or she is in. Later, Tall (2004) proposed a theory of a model involving three worlds of mathematics (Three worlds in Table 1), that is similar to the APOS model but less rigid. The models differ in the number of stages and their descriptions, though each model describes a transition from just perceiving a mathematical concept through actions to regarding the concept as an object, which in turn can be connected to other concepts or formally abstracted in new situations.
Table 1. The stages in the models on concept formation. APOS Three worlds Action Embodied world
Process Proceptual world Object
Scheme Formal world
The action stage of the APOS model, similar to the embodied world in the Three worlds model, starts at a stage of perceiving phenomena or objects through exploration with actions performed without a clear sense of the outcome or the essence of the action. The proceptual world is a synthesis of the second and third stages in the APOS model. Procedural actions on mental conceptions from the first world become encapsulated through the use of symbols. The term procept denotes the symbol together with the process and the concept it represents and can be used dually as process or object depending on the current context (Gray & Tall, 1994). The formal world is reached when the individual can use formal expressions to define mathematical properties and is able to see concepts in completely new ways. The last stage of the APOS model concerns the linking of a concept to other concepts in a mental net or scheme.
15 The hierarchical structures are different in the two models. In Tall’s three worlds, some concepts are primarily founded in the embodied world, whereas developments of other concepts mainly occur in the proceptual world. Connections between representations from the different worlds enhance the flexibility of the concept’s whole representation, which is dynamic as new parts are created and existing parts are recreated from the individual’s experiences. The APOS model is not as flexible, since it emphasises the importance of proceeding through the levels in the order formally mentioned. However, it can still work as a tool to describe the students’ concept formation in limits of functions. Not all mathematical concepts can be regarded as an object and a process, e.g. a circle or an equivalence class that are both pure objects, though in limits this duality is very obvious. Limits can be handled through an explorative approach with tables of function values and graphs from the beginning and later as symbolically expressed entities. Learning limits of functions demands leaping between operational and static perceptions (Cottrill et al., 1996). There is a challenge in understanding the termination § 1 · sin¨ ¸ x of an infinite procedure as a finite object, such as lim © ¹ 1. The xof 1 x theories compared here have a somewhat hierarchical structure, but what is important is to reach all significant stages and be able to change between the different stages. Only then can an individual fully understand the concept if understanding of a mathematical concept is defined as Hiebert and Carpenter did (1992), i.e. to be something an individual has achieved when he or she can handle the concept as part of a mental network. The more connections between the mental representations, the better the individual understands the concept (Dreyfus, 1991; Hiebert & Carpenter, 1992). Each connection or creation of a mental representation can be conceived as an act of understanding (Sierpinska, 1990), and the strength or depth of this understanding of a concept can be measured in the quantity and quality of the acts of understanding for that particular concept. The models of learning used in the articles focus more on the APOS theory in the earlier stages of the study (Juter, 2005a), followed by Tall’s three worlds in the later stages (Juter, 2006a); the theories are described further in the articles. Tall’s three worlds (Tall, 2004) were inspired by, among others, the APOS model and Sfard’s model (Sfard, 1991). The use of these different learning models in the articles has not caused any inconsistencies because the foundations and core ideas are very similar. Tall’s model perceives a concept as a process and an object at the same stage, the proceptual world, with the transition between the perceptions in
16 itself being part of the stage, as opposed to the APOS model where the different perceptions are at different stages. I found Tall’s model to be less hierarchic and consequently more flexible, and I therefore chose to change my theoretical frame in 2004 when I first came in contact with Tall’s three worlds of mathematics. Theories about concept images and concept definitions (Tall & Vinner, 1981; Vinner, 1991) complemented the aforementioned theories. A concept image for a concept is an individual’s total cognitive representation for that concept. The concept image comprises all representations from experiences linked to the concept, of which there may be several sets of representations constructed in different contexts that possibly merge as the individual becomes more mathematically mature. I argue that one aspect of being mathematically mature is the ability to perform this type of abstraction. Multiple representations of the same concept can co-exist if the individual is unaware of the fact that they represent the same concept. Possible inconsistencies may remain unnoticed if the inconsistent parts are not evoked simultaneously. Parts of concept images, concept definitions, are the individual’s definitions of the concept, learned by rote or self constructed. Tall and Vinner (1981) distinguished personal concept definitions from formal concept definitions, with the latter generally being the established definition among mathematicians. Concept images are created as individuals go through the developments represented by the models of concept formation in Table 1. The model in Figure 1 shows how part of a concept image can be structured as I consider it. The three types of symbols used each represent a concept at the stage of one of Tall’s three worlds, as described in the figure. The concepts can be, for instance, geometric series, derivatives of polynomials, definitions of derivatives and limits of functions, theorems, proofs, and examples of topics of related concepts. More links and more representations of concepts exist around the formal world representations of concepts. There are also parts that are not very well connected to other parts. This situation can occur when individuals use rote learning as they try to cope with mathematics.
17 Embodied world
Proceptual world
Formal world
Figure 1. Model of part of a concept image at one time. Each node is a representation of a concept at one of the different stages of Tall’s three worlds.
Mental representations can be depicted in terms of topic areas as a complement to the levels of abstraction shown in Figure 1. A topic area refers to the areas of mathematics with components of a certain topic, e.g. ‘functions’ or ‘limits’. The components are such nodes as those in Figure 1. The sizes of topic areas vary according to what context they appear in, for instance large areas such as ‘functions’, or smaller areas such as ‘polynomial functions’. The classification in topic areas means sub-topic areas at several levels. A component in one topic area can in itself be a topic area. Weierstrass’s limit definition belongs to the topic area of ‘limit’, as do ‘limits of rational functions’ and the symbols used to express limits. The symbols also belong to the topic areas ‘derivatives’ and ‘continuity’. Topic areas overlap this way as illustrated in Figure 2. If a concept is represented in more than one topic area in a concept image and the topic areas the representations belong to are disjoined, then inconsistencies may occur in the way aforementioned. Inconsistencies can appear within a topic area as well, but they are easier to detect due to the
18 relatedness of the topic. The development of concept images never ends and the mental representations generate a dynamical system linked together at various levels. An example of a topic area, marked by a wider contour line in Figure 2 represents the topic area ‘limits’. It comprises a marked oval component representing the limit definition, which is also part of the topic areas ‘derivatives’, TA2, and ‘continuity’, TA3. The black rectangular component represents the definition of derivatives. Other classifications in topic areas can be done in more or less detail.
TA2
TA3
Embodied world
Proceptual world
Formal world
Figure 2. Topic areas and components with links in a model of a concept image. The marked part is the topic area ‘limits’. TA2 and TA3 represent the topic areas ‘derivatives’ and ‘continuity’ respectively.
The students’ concept images are expressed in terms of abstraction (Dubinsky, 1991) and generalisation (Dreyfus, 1991; Tall, 1991). Abstraction is cognitively more demanding, since it usually involves rebuilding mental representations. Generalisation means understanding the cores of more than one phenomenon to be equal in existing mental
19 constructions. Studying the actual mental networks or representations of a concept is not possible, but the students’ performances are determined by their capacities (Star, 2000) represented in their minds and we can therefore study the effects of their mental representations. I have worked with some additional theories in connection with the students’ problem solving (Hiebert & Lefevre, 1986; Lithner, 2000; 2003). I consider the students’ reasoning when they solve problems and discuss theory as expressions, or traces (Juter, 2005c), of their concept images and I found that these theories work well together.
Terminology The meanings of some frequently used words in the articles and this synopsis are explained in this section. Other terms used deemed to require an explanation are defined as they appear in the articles. The word perceive is described as follows in Collins Cobuild English Dictionary (1995): “If you perceive something, you see, notice, or realize it, especially when it is not obvious”, with the following example: “They strangely perceive television as entertainment”. The description is quite broad and I have used the word in this broad sense. The example given in the dictionary can be transformed to suit the topic of this thesis, for example: Some students perceive limits as the most important topic or a common perception is that limits are unattainable for functions. The words concept and notion are used synonymously in some of the articles. I consider a concept as Novak defines it: “a perceived regularity in events or objects, or records of events or objects, designated by a label” (Novak, 1998, page 36). The word understand is already mentioned in the previous section on learning where Hiebert and Carpenter (1992) defined understanding as something an individual has achieved when he or she handles a concept as a part of a mental network. This definition corresponds well with the theories of concept formation, since the definition is stated in the same context of mental representations of concepts. Collins Cobuild English Dictionary (1995) describes the word understand in a language context: “If you understand a language, you know what someone is saying when they are speaking that language”. Mathematics can be thought of as a language where symbols, which are sometimes words, are placed in various constellations to communicate different mathematical properties. Hence, to understand mathematics is to be aware of the meaning of each of the symbols used, and their specific constellation. Compared to Hiebert’s and Carpenter’s definition, the awareness of symbols can be seen as nodes in the concept image, of which the mental network is a part, and the
20 understanding of different constellations create the links between the nodes. The links themselves then build up knowledge of the topic and new nodes are created. Figure 3 illustrates an example of five nodes that in a certain context combine to create a sixth node. The five nodes with one link each represent the word limes, functions, the use of arrows in mathematics, the notion of infinity, and variables. Representations of the five nodes compose a constellation in the middle node, demanding an awareness of each component and their meaning in this specific constellation for it to make sense. A new node has then been formed and the individual has a better understanding if the links are strong and numerous through related parts of the concept image (Dreyfus, 1991; Hiebert & Carpenter, 1992; Sierpinska, 1990).
limes
f (x) t y=h(t) x lim f a xof
o f m
Figure 3. Example of nodes and links in a concept image where the middle node has been created from the five outer nodes.
The Empirical Study Two studies were conducted to reveal how students learn limits of functions. The methods used in the studies are outlined after a section about the overall intentions of the investigations. Methodological remarks acknowledging the methods’ limitations and a section about ethics and quality follow the descriptions of the studies. The last parts of the section describe the analyses performed and some results of the studies.
21 Rationale of the Empirical Study The students in the empirical study were enrolled in their first university level mathematics course. They had encountered the notion of limits of functions at upper secondary school, Swedish “gymnasieskolan”, at least in connection with derivatives and integrals (Skolverket, 2005). The treatment of limits can be intuitive or peripheral at this level and the students can have very different conceptions of limits upon arriving at university. Although the concept at university is presented from the beginning, the presentation is more formal than students are generally used to. I think this stage of mathematics education is vital for their creations of mental mathematics representations. Students are forced to work with their mental representations to make sense of the strict formulations they encounter in the course and to link new knowledge to already existing knowledge. I decided to conduct the study at a basic level because the foundations are built and established here and they are not easily changed (Smith, diSessa & Rochelle, 1993). The study has two parts, referred to as the first study and the second study. Different methods were used to triangulate the phenomenon of students learning the notion of limits of functions. The responses of the students to questionnaires and interviews and their actions are regarded as traces from their concept images and are used to assess their developments throughout the semester.
The First Study The first study was carried out in a 10-week course covering calculus and algebra during the spring of 2002. A total of 143 students were enrolled in the study. They filled out three questionnaires, Q1 to Q3, at the times indicated along the timeline. Field notes were taken through the 10 weeks when limits were taught. Limits were introduced in the course after the first questionnaire.
1 2 5 7 10 Q1 limits Q2 Q3 Timeline (weeks)
The first questionnaire, Q1, was designed to give information about the students’ previous education, attitudes to mathematics and their knowledge of limits with a few open limit questions before limits were taught in the course. The work of Mohammad Yusof and Tall (1994) influenced the
22 design of the attitudinal part. On the subject of limits, two questions were asked. The first one was:
In what circumstances have you met limits in your previous studies? Give as many examples as you can.
The second task was:
Explain thoroughly what the limit at a point x=a of a function f(x) means.
My intention with these questions was to get a picture of how well the students knew this concept and what other concepts they connected to it, to find out how developed their concept images were. The second questionnaire, Q2, was distributed to the students after they had seen the limit definition, proven some theorems and solved some limit problems. The first part concerned how the students interpreted their learning situation, with a focus on their troubles. They were asked what had been hardest to understand, why and what would have made understanding easier, as well as what had been the most important part in the analysis course so far and what the students thought was the most stimulating part. The design of the second part of the questionnaire was partly influenced by Tall (1980) and Szydlik (2000). The students were asked to find the limits (if they existed) in four problems and to explain what they had done as if they were explaining to a person who was just learning about limits. They were also asked if the functions in the problems could attain their limit values, if they exist, and to explain their claims. The aim of the four problems was for the students to show how well they could explain their calculations, and how separated functions and limits of functions were in their concept images. The last task was for the students to literally explain what a formal definition of a limit of a function means and to explain every symbol used. They were also asked to draw a picture where all the symbols were to be marked, with the aim to find out if they had different ways of thinking of limits as in Szydlik’s (2000) results. The students were finally given the third questionnaire, Q3, containing a few attitudinal questions followed by three limit problems to solve. The first problem was to define a situation of a function without a limit to see what the students changed in the definition they were used to from the textbook. That definition was stated in the questionnaire. The two following problems were accompanied by solutions that were either wrong or incomplete, or both wrong and incomplete. Here, I hoped to see if they
23 could handle their knowledge about limits in a non-traditional situation and how they handled a given solution, by either accepting it or challenging it with arguments of their own. Field notes were taken during the lectures (all students together) and in the classroom of one of the problem solving groups (20-30 students in each group). A grid over the classroom where the students had their problem solving sessions was developed, with each student being represented by a square in which I noted every comment, question, action or anything else important to the mathematical activity. This described fairly well the contributions of the students and the group’s teacher to the lessons. The field notes from the lectures were not as informative as those from the classroom, as there were too many students to use the grid system. The field notes from the lectures comprised information about what was taught and how it was taught. Students did not take much part in the activities during the lectures, but any remarks or questions were noted. The information from the field notes was used in the creation of tasks for the questionnaires, apart from being descriptions of the students’ environments (Juter, 2006a). The tasks were constructed to follow the course and span from easy to more demanding. Hence, all tasks were not created before the study commenced. Some tasks were developed as the course went along.
The Second Study Comprising 112 students, the second study was carried out during the autumn of 2002, i.e. the entire 20-week semester divided into two 10-week courses. The first course was the same as the first study and the second was a continuation course in algebra and calculus. The university offered an additional mathematics course that semester for students with biology, chemistry or some other non-mathematical main topic of study, so these students were not enrolled in the course of the present study, which therefore was a smaller group compared to the previous semester. Three questionnaires, E1 to E3, were handed out and two sets of interviews, I1 and I2, were conducted at the times indicated by the timeline.
1 2 7 10 16 20 E1 limits E2 I1 E3 I2 Timeline (weeks)
The second study was slightly different from the first study. The attitude part was reduced and some of the tasks were altered or removed to make the questionnaires sharper for their purposes. The task asking the students
24 to explain what a limit of a function is in the first questionnaire, Q1, was replaced by two easy tasks where some limits were to be considered in the questionnaire E1. This is because almost nobody in the first study was able to provide an answer to that task, possibly due to too vague experiences of limits at upper secondary school. The rest of the first questionnaire was left as in the first study. Parts of Q2 and Q3 were merged to form the second questionnaire, E2. The attitude questions were removed along with one of the four limit problems from Q2 and the reversed definition task from Q3. The removed tasks were not useful mainly because a majority of the students did not make any attempt to solve them in the first study. The task where a drawing and explanations of the symbols of the definition was expected from the students was also removed in the second study, because I wanted to use the time I had with the students as effectively as possible and I deemed this task to be too time consuming compared to the information it yielded to the study. The third questionnaire, called E3, was just one task where two fictitious students discussed a limit problem. One had a correct approach, the other a wrong approach.
E3: Two students discuss a problem. They do not agree on the solution. The problem discussed is about the following limit.
cos(x 2 ) lim xo0 10000
The student S1 claims that the limit exists and is zero with the following explanation:
I use the definition for limits and write |f(x) – A| where A is the limit. I try A = 0 since I think that is the limit. I get:
cos(x 2 ) 1 1 0 d < 10000 10000 9999
This is true for all x except x = 0, but that can never be the case since we then would have zero in the denominator in x2 , cos(x 2 ) so it is true for all x in the domain of f(x) = . This 10000
25 1 means that if we chose H = for all possible G with 9999 0 < |x – 0| < G, then the definition is met and the limit is zero.
The student S2 does not agree and claims that if one, for 1 example, chose H = then one can not find a G > 0 with 100000 |f(x) – A| < H where 0 < |x – 0| < G for all x in the domain, that is to say all x z 0. Therefore, the student S2 claims that cos(x 2 ) lim has no limit according to the definition of limits. xo0 10000
The students in the study were asked to determine who was right and why, and to state what was wrong in the incorrect approach. Field notes were taken just as in the first study during the lectures and problem solving sessions about limits of functions. Eighteen students were selected for the interviews, based on their responses to the first two questionnaires. Their answers were compared and sorted into groups with similar responses to the questionnaires. The students indicated in the second questionnaire if they were willing to participate in interviews. The selection was done based on these factors. If one type of answer was more frequent than another, then more students were chosen from the larger group to obtain a representative subset of the students. This way of choosing a sample depends on the students’ goodwill and it could be that some types of answers were not to be represented by an interviewed student. However, this was not the case in this study. The questionnaire results of the 34 students who had agreed to take part in the interviews were rather evenly distributed among the different types of responses from the entire group of students. Two interviews were planned with each of the eighteen students. One was conducted a few weeks into the second course and the other was directly after exams. Three students did not go through the second interview; hence, fifteen pairs of interviews were done. The duration of the first interview was between 30 and 60 minutes, depending on the interviewed student. I questioned the responses to the questionnaires and I assigned a few more limit tasks for the students to solve. The works of Williams (1991) and Tall and Vinner (1981) influenced me in the creation of two tasks. The first task comprised six statements about limits to respond to (Juter, 2003; 2005c). The second task was a limit problem with focus on attainability (Juter, 2005c). Other tasks were about the limit definition, e.g. if the students could state the formal
26 definition or recognise it next to an incorrect one (Juter, 2003). The tasks were of different character to work as instruments to measure the students’ perceptions of limits in different situations. The second interview was about 15-20 minutes long. The main topic of the discussions was the limit definition compared to the example with two fictitious students arguing about solutions in questionnaire E3. Both interviews were directed by a set of questions, with follow up questions adjusted to the students’ answers. The interviews were audio recorded and later transcribed in preparation for the analysis.
Methodological Remarks Many things can be said about the selections and choices in the method design. One could ask if the students who were willing to participate in the interviews were more positive to mathematics and perhaps even better at it than most students. If so, then the problems that are revealed in the study probably apply to a larger number of students. The students enrolled in this study presumably learned more about limits of functions than if they did not participate in the study. This is a problem that cannot be avoided if the method involves questionnaires or interviews. Any encounter influences the results (Cohen, Manion & Morrison, 2000). Acquiring information from the interviews and questionnaires without interacting with the students would not be possible. The students’ enrolment in the study may have helped them in their learning of limits, since the study did not retract from their studies, giving them additional opportunities to work with limits of functions through questionnaires and interviews. There are always difficulties to consider when an investigation involving people is done. When investigating human minds, it is not possible to obtain a complete description or to be sure that what a person says or does is what he or she would normally do (Jacobsen, 1993). This is important to consider, particularly in interview situations because interviews are parts of the developmental portraits of each student and are therefore individual in content and extent. Some students are happy to talk and do not feel embarrassed or anxious about their performance, whereas others become very nervous and do not want to talk about things they are unsure about. They may then find it convenient to disguise their true and perhaps complex thoughts in a short unrevealing answer. The tasks for the interviews were designed to gradually increase in difficulty, to relax the students and make them understand that I was interested in their thoughts rather than the correct answers to the tasks. Students who understand the researcher’s aims know something about him or her that may interfere with their interview responses; hence, it is important that the researcher keeps a
27 low profile. There is otherwise a risk that the interviewed student answers what he or she believes the researcher wants (Svenning, 1999). This drawback is compensated for by the possibilities to rephrase questions or ask the students to explain further, which, for example, questionnaires do not provide. Questionnaires were used repeatedly in the study to get as many responses as possible in a quantitative approach, as a complement to the qualitative part of the study. The quantitative data described the whole group from which the interviewed students were selected. Many of the questions and tasks used in the questionnaires were tested on students from other classes before they were used in the study to make them as efficient as possible for my purposes. I wanted the questionnaires to give as much information as possible, but not to be too time-consuming to the students or scare them off. Most questionnaire tasks were structured (Cohen, Manion & Morrison, 2000) for me to be able to analyse the large number of responses. Some tasks were more open and followed up at the interviews to provide additional information. Parts of the study, in both questionnaires and interviews, were replicated from foreign studies (Szydlik, 2000; Tall, 1980; Tall & Vinner, 1981; Williams, 1991), thereby enabling comparisons with existing results. Taking field notes was the third method of data collection. The researcher is passive and does not disturb the activity. Data from field notes is purely descriptive, as opposed to data from questionnaires and interviews created through interference by the researcher. I did not choose a socio cultural perspective even though the students’ actions were investigated through field notes. The notes were used to describe the environment from which the students constructed their individual knowledge. Different kinds of data gathering provided more aspects of the students’ learning situation, a triangulation that I needed to answer my questions. I have chosen to consider a rather wide question to start from, as mentioned in the introduction: How do students deal with the concept of limits of functions at the basic university level in Sweden? It has led to investigations, amongst others, regarding the individual progress of the 15 interviewed students during a semester (Juter, 2004b; 2005a; 2005c; 2006a), their problem solving (Juter, 2004a; 2006b; Juter & Grevholm, 2004; 2006) where they are considered as a group, high achieving compared to low achieving students (Juter, 2005a; 2005c; 2006a), historical development compared to individual development (Juter, 2006a), students’ attitudes compared to their achievements (Juter, 2005b) and theory linked to tasks (Juter, 2004a; 2006b; Juter & Grevholm, 2004; 2006). One might argue that each of these sub-investigations could provide a sufficient focus
28 for the outcome to become well contained within one theory, but I have chosen several theories to form the foundation from which I strive to make a point through my data. My aims through this study were to describe and analyse as accurately as possible the students’ learning situations within the frame of the course. As there are so many factors influencing the students, I decided to use multiple perspectives at the cost of a neat set of results.
Analysis of Data A computer program called NUD*IST (N6, 2003) was used in the process of classifying the solutions and responses of the students into different categories. Responses to tasks from questionnaires and interviews were transcribed and imported to the program. The categories of the limit tasks from the questionnaires were created from the students’ responses, and NUD*IST worked as a means to handle these categories. Some categories were created in advance to answer the questions and some became apparent from the data. The categories were adapted as the data was worked through until I had a representative set of categories to present the data. The interview categories were harder to create. The program had a search function that could be used to find keywords such as border. However, the students did not always speak in the terms I searched for, so besides the computer-based search, I read all transcripts and searched for passages fitting the different categories. Once a passage was found, the program was used to add the passage to the appropriate category. The categories were created to reveal as much as possible about the students’ developments of limits of functions through the duration of the semester with respect to a number of features of the concept, such as static or procedural perceptions, functions’ abilities to attain limit values, perceptions of the limit definition and strategies for problem solving in various contexts. That broad range of areas demands quite different systems of categories to work with; hence, they were designed to distinguish the chosen properties. Students’ responses were grouped with respect to the properties, and certain patterns became apparent (Strauss & Corbin, 1998) that were used to structure the categories. There is a danger of too much subjectivity in categorising student responses; therefore, other researchers also did parts of the categorisation to make sure that my judgements were reasonable. The students’ attitudes were also investigated and different sets of categories were used to measure them. The categories for the part inspired by Mohammad Yusof and Tall (1994) were decided in advance as the same categories Mohammad Yusof and Tall used to make comparisons possible (Juter, 2003; 2005b). The categories were arranged in a collection of
29 possible answers from one opinion to the opposite on a Likert scale (Cohen, Manion & Morrison, 2000). This gives the location of the students’ attitudes with respect to the question posed (Strauss & Corbin, 1998). The responses became easy to analyse and they were used to categorise the students as confident or unconfident in their work with mathematics (Juter, 2005b). Some openly formulated attitudinal questions were categorised according to the answers given after the answers were gathered. Examples of tasks and more informative descriptions of the study and the tasks can be found in the articles.
Results Some results are discussed here with more information about specific circumstances being provided in the articles referred to in the text. Most students could solve the routine tasks, but had trouble solving the non-routine tasks, requiring them to understand specific features of the notion of limits (Juter, 2003; 2004a; 2004b; 2005c; 2006b; Juter & Grevholm, 2004; 2006), for example, to decide whether a function can attain a limit value, or to determine what the different components in the definition represent. Algebra was more problematic than anticipated. Students failed in several cases of straightforward problem solving due to insufficiently developed algebraic skills. Manipulations with symbols were done in a manner implying that the students were unable to understand what the symbols represent or what the effects of the operations should be. One example of such an algebraic calculation is:
x 3 2 3 3 x 2 3 3 2x x x . x3 1 x3 1 x3 x3 x3
The student’s explanation showed that the two equal fractions were omitted and then the student overlooked the minus sign. Quite a few students carried out this type of calculation (Juter, 2006b) and many failed to differentiate surface properties (Lithner, 2003) from vital properties, leading the students to select unsuitable solving methods (Juter, 2006b). x 2 1 Examples are given in the responses to the task lim . Several students xo1 x 1 recognised the rational function and tried to divide numerator and denominator with what would have been the dominant term if x had tended to infinity. They did not consider what x tended to. They went for the solution they were used to from working with limits of rational functions,
30 where x tends to infinity, and found it difficult to see other possible solution methods. Building on foundations with these kinds of serious misrepresentations is hazardous. They affect the creation of new parts of the concept image linked to the erroneous part. As is, the notion of limits offers many kinds of challenges, and further complications can be too much for the students to handle. There was also some confusion of functions with limits of functions, which made the students give wrong answers to tasks or correct answers with wrong explanations often caused by misconceptions of infinity or the limit definition (Juter & Grevholm, 2006). Intuitive representations of the concepts, i.e. concepts not developed enough, in the embodied world (Tall, 2004; 2005) caused some of the misconceptions. The intuitive representations interfered with the students’ formal representations of limits. The meaning of the quantifiers in the formal limit definition seemed to be a mystery for many students throughout the entire semester (Juter, 2003; 2005a; 2006a). Few were able to state the definition or explain what it actually means, which is implying that they solved tasks without using the fundamental theory behind them. Can students fully understand the notion of limits if they do not understand its formal definition? Perhaps they did understand the concept from the problem solving in the course or through discussions with peers, but few students could actually connect tasks with the definition (Juter, 2005a; 2006a). They had serious problems in bridging the gap from intuitive or procedural perceptions to formal and static perceptions. Students’ developments need to have reached the proceptual world of Tall’s three worlds of mathematics (Tall, 2004) for them to be able to perceive limits both as objects and processes, and then the formal world to be able to understand, in the aforementioned sense (Collins Cobuild English Dictionary, 1995; Hiebert & Carpenter, 1992), what the components of the definition correspond to in the task. The transition to the third world of mathematics demands an ability of abstraction (Dubinsky, 1991). The components of the concept image need to be consistent, coherent and sufficiently elaborated, i.e. transparent to the student, for abstraction to be able to take place. This transparency means that students can switch between perceiving limits operationally and perceiving limits as entities (Asiala et al, 1996; Cottrill et al, 1996; Sfard, 1991; Tall, 2004). Mostly high achievers showed traces of abstraction in their work with limits, but there were low achieving students who also revealed evidence of some abstraction in their learning processes (Juter, 2005a; 2006a). There were other differences between high achievers and low achievers. High achieving students’ developments of the limit concept
31 resembled the historical development of limits (Juter, 2006a). Low achieving students did not display such a distinguished pattern. The students’ concept images were not necessarily coherent or true, though the students were not always aware of this. Some students actually showed great confidence in their own capability to handle limits, even after they had failed to solve problems or explain some theoretical property (Juter, 2003; 2005a). Successful routine problem solving can cause such reactions since the abilities of students become confirmed by their correct answers. One of the previously mentioned results is that many correct answers were wrongly justified. This undermines the creation of useful concept images and not many students in the study had a concept image of limits that allowed them to solve non-routine tasks or explain essential features of the definition. Limits of functions were considered the most important topic of the analysis part of the course, according to most students, and also the most difficult topic (Juter, 2005b). The first part probably inspires or encourages students to keep working despite their difficulties. There were attitudinal differences in the students’ abilities to solve tasks. Students with positive attitudes to mathematics performed better than other students in solving limit tasks, and students who were skilled limit problem solvers had a positive attitude to mathematics. Several types of gaps have become apparent from the results. There are gaps between students’ perceptions of limits as processes and objects, of limits as infinite and finite, and limits as intuitive and formal. There are gaps between limits at upper secondary school and limits at university. All these inconsistencies combined generate a range of possible problems for students to learn limits of functions. Most students cannot bridge the gaps on their own and it is therefore crucial that as few and as narrow gaps as possible exist in the students’ learning environment, i.e. textbooks, lectures, tasks and all other contexts in which they encounter limits. Input from the surrounding environment needs to be connected to the students’ concept images. Development from the students’ levels should be done by continuously increasing intricacy to avoid gaps. Students then have a better chance to understand mathematics, and particularly limits, with all its fundamental characteristics. The field notes revealed almost no interaction between the students and lecturer during the lectures. There were more discussion at the problem solving sessions, but few students were active in these discussions. Students who solved tasks at the blackboard were usually those who were active in the discussions. Some of the students who silently and inactively participated in the problem solving sessions were among those interviewed,
32 and explained their behaviour differently. One student did not want to go to the blackboard because he wanted to see other solution methods, preferably the teacher’s. Some students were shy and did not want to speak in front of the other students, with the risk of embarrassment. The learning environment was consequently not easily adjusted to the students’ unexposed levels of understanding.
Ethical Issues and Scientific Quality Ethics (Forskningsetisk policy, 2003) has been an important factor through the work presented. Nobody was forced or persuaded to participate in any part of the study and all students were informed that participation was not obligatory and that they could depart from the study at any time. Their identities have been protected with codes in the material. Only I have access to the key to the codes. All student names in all available documentation are fictitious. I tried to make the participants relaxed and confident with the environment where the interviews were conducted. It was important that no student felt diminished or anxious due to the situation they were in. They had time to talk and think. The students received answers to some of the tasks, after their responses were collected, if they wanted to have them. The purpose was to make it possible for the students to use the tasks in their learning and allow them to benefit from the study. The validity (Kilpatrick, 1993; Sierpinska, 1993) of the results relies on the many students participating in the study and on the different methods used in the data collection. Much of the data in this thesis is visible, making it possible for others to understand how I have been working. The methods are also described in the thesis for possible reproduction of the study. Tasks and questions were tested with other students before being used in the study and colleagues did some categorisation to further ensure the validity of the study. It is impossible to be completely objective (Kilpatrick, 1993; Sierpinska, 1993) in a study where interaction with other individuals is the dominant source of data. The goal in the processes of collecting and analysing data has been to be as open-minded as possible to what the data could reveal. Relations were established between myself, as a researcher, and the students, especially the participants in the interviews. I had this in mind and made an effort to act the same with all the students. Foreign researchers have presented the reactions of students to similar types of tasks and theory, as some of the tasks and theories I have used. This has given me the possibility to predict (Kilpatrick, 1993; Sierpinska,
33 1993) how the students in my study would respond to my tasks and questions, even though some things did not turn out as expected. The considerable problems caused by algebra (Juter, 2006b) and the responses to the attitudinal tasks (Juter, 2005b) are two such examples.
Discussion Contributions to the field of mathematics education are highlighted in this section, as are the practical implications for universities and upper secondary schools. The section ends with suggestions for further research.
New Results for the Field Several researchers have investigated students learning limits of functions (for example Bezuidenhout, 2001; Cornu, 1991; Przenioslo, 2004; Szydlik, 2000; Tall, 1980; and Williams, 1991). The present study has contributed to the existing results in a number of areas, some already mentioned. One significant new result is the report on the students’ developments of learning limits of functions over a period of time, in this case a semester (Juter, 2003; 2005a; 2006a). Learning takes time and it is therefore important to follow the students during an extended period when limits are taught. Results from this kind of study can place results from other types of studies in new, broader perspectives. The students’ developments have been compared to the historical development of limits (Juter, 2006a) with the aim to find developmental similarities and differences. The historical development was used as a tool to measure with rather than something to strive for in modern classrooms. One outcome of this comparison was that high achieving students were more inclined than low achieving students to follow the historical development. I argue that this notable fact is due to their abilities to perform abstraction. The students’ attitudes were compared to their achievements in solving limit tasks (Juter, 2005b). Attitudes, whether the individual is conscious of them or not, affect learning and is hence a factor to consider. Many students were unjustifiably confident about their control over the notion of limits. Their lack of abilities to solve problems or explain theoretical issues in the study did not affect their confidence. This is a serious result that I have not seen from other research studies, and its effect is that students do not try to adjust their concept images because nothing to adjust is perceived. The learning situation of Swedish students on the topic of limits of functions has not been investigated elsewhere in the context reported in this thesis (Juter, 2002; 2003; 2004a; 2004b; 2005a; 2005b; 2005c; 2006a; 2006b; Juter & Grevholm, 2004; 2006). This study can therefore provide
34 important results to all involved in mathematics teaching and learning. To consider additional aspects, the students’ achievements were studied in the context of curricula and textbooks (Juter & Grevholm, 2006). The mathematics mastered by students when they enter university can make a significant difference to the results of their future mathematics studies. Hence, the development of curricula and textbooks used at upper secondary school was studied to reveal if any major changes had occurred. If several small changes, each one individually not significantly impacting the next level, are done in an unfortunate manner they can be the growing grounds for serious gaps in the transition from upper secondary school to university. I have not found any investigation of this kind about limits in Sweden, but Bremler (2003) did a licentiate thesis about Swedish mathematics textbooks with a focus on the development of derivatives at upper secondary school from 1967 to 2002. These results strongly impact teaching and learning as illustrated in the following section.
Implications for Teaching and Learning It appears as if the students’ concept images of limits are not challenged enough in their learning environment today. If the confusion that appeared in the present research study can be highlighted, the students then have something to start working with. A task that is too difficult might cause a student to give up rather than provoke him or her to persevere and work it out. If the students have misinterpretations or no interpretations at all, then a task requiring understanding in a deeper sense than just knowing what algorithms or motions to go through can enlighten the students of the deficiencies in their concept images. This awareness will hopefully inspire the students to alter or complete their concept images to make sense. This is a time-consuming process; without enough time, the students are forced to select what they judge to be the most important parts of the course. The questionnaires and interviews in my study and in that by Mohammad Yusof and Tall (1994) imply that it is probably typical exam problems the students work with to have any possibility of passing an exam. One idea is to make more room for questions about the nature of, for example, the formal definition of limits, since it is so difficult to understand. The multifaceted difficulties readily lead to questions about what consequences small changes in the formulation have on the meaning of the limit definition. Such discussions were rather easy to initiate with the students in the interviews, because there were so many aspects to consider. Even if the discussions did not make them aware of what effects the changes had on the meaning of the definition right then and there, they got a chance to
35 become aware of their own point of view. This type of task can be used to start a discussion among a group of students about the critical and typical features of a notion. Svege (1997) suggested a humanistic approach to mathematics education, e.g. through discussions about various solutions. The students can then practise their ability to analyse and evaluate instead of just learning rules by rote. Some students actually said that they would benefit from the tasks, such as the non-routine tasks of the study, which open up for discussion (Juter, 2005a). The students’ problems to connect theory to problem solving and other critical issues can be identified and dealt with. McLeod (1992) proposed changes in the curriculum to avoid such beliefs that mathematical problems should be solved in less than five minutes. If students think that a mathematical problem is supposed to be solved quickly, then he or she might not be willing to spend an hour or more on more extensive problems that can lead to deeper, more enduring, knowledge. Students create knowledge of mathematical concepts and ideas differently, not only from each other, but sometimes also from what is presumed by mathematicians (Even & Tirosh, 2002). It is important for teachers to know as much as possible about the variety of their students’ mathematical conceptions to be able to design a constructive learning environment for them. Even and Tirosh mentioned students’ theory building and students’ misconceptions and the consequences of these factors on learning as parts of the meaning of ‘students’ mathematical learning’. The present study, whose results can be used as an example of how a group of students work with limits of functions through the duration of a semester, deals with such issues. The gaps mentioned in the results section between upper secondary school and university, limits as infinite and finite, intuitive and formal, and object and process imply too many and too large discontinuities for the students to understand the full extent of limits. Students are expected to handle limits in the formal world, since textbook and classroom presentations are on a formal level, but the students themselves are only mature enough to handle limits in the proceptual world. Teachers at upper secondary school and university need to be well aware of what is learned at the other’s level and how it is taught to be able to prepare upper secondary school students for university studies and to consider the pre-knowledge of new university students.
36 Further Research A natural continuation of research would be to implement and assess changes suggested from the results of this study. Björkqvist (2003) mentioned the importance of research on teaching methods in mathematics and emphasized the lack of such research in Sweden, especially in large heterogeneously composed student groups. Research on methods can provide teachers with important insights into teaching strategies to meet the learning of students. There is, as we have seen, a need to take action to help students make sense of parts of the university mathematics they learn. Lars-Erik Persson, Barbro Grevholm and I recently initiated a new study of lecturers’ views regarding the concept of limits of functions and how it should be taught as a complement to the present study. Björkqvist (2005) argued that research into the influence of teachers is underrepresented and needed. The results may illuminate the problem of bridging theory to everyday problem solving. I have investigated students’ attitudes to mathematics mainly as a complement to the rest of the study (Juter, 2005b). I plan to work further with attitudes at some point. If a topic is perceived as difficult, boring or useless, then there is a barrier to overcome before the mathematics is even considered. These kinds of attitudes can be formed at very early ages, but they can also appear very late at the university level, such as one example in the second study. A woman in her early-20s, who had studied mathematics to become an upper secondary school mathematics teacher, took part in the interviews. She told me that she had always loved mathematics until she entered university. She did not like the way formality dominated over the problem solving, which she had enjoyed at upper secondary school, and did not want to take further courses in mathematics. I met individuals with quite the opposite experience of university mathematics as well. A man about the same age as the woman above spoke in almost poetic words about the beauty of mathematics in its rigorous appearance. He celebrated the formality, which the woman rejected. It would be interesting to find out more about what the students actually think about mathematics in general, and limits in particular, and the effects it has on their learning. The influence of gender on the learning of limits of functions is not investigated in this study because of time constraints. However, the data is sorted with the men and women separated, so comparisons are possible in future analyses.
37 Conclusions The sub-questions posed in the introduction are addressed in this section, followed by some broader conclusions drawn from the work with the sub- questions combined.
x The developments of the concept of limits of functions by most students during a semester did not reach the formal world (Tall, 2004). The formal limit definition was not integrated in their concept images and was therefore not available to the students as a tool in their further work with mathematics. x The development of high achieving students resembled the historical development of the concept of limits, whereas that of low achieving students did not. Students who were unable to formally handle limits did not possess rich or strong enough concept images for the students to handle the required abstraction levels. x The students readily solved routine tasks, though as soon as the tasks became unfamiliar to them, the students became confused about what they should do. The limit definition was often the reason for errors because it was wrongly interpreted. Connected concepts also caused problems in the students’ problem solving, e.g. algebra and infinity. x Students with strong self-confidence in mathematics were better problem solvers than students with weak self-confidence. On the other hand, skilled problem solvers had a stronger self-confidence in mathematics than unskilled problem solvers. It is not possible to determine if the students’ confidence improves their problem solving abilities or if the students’ problem solving abilities improve their confidence, only that there is a connection.
The combined results imply that the abilities to perform abstractions of many students’ are insufficiently developed for them to reach the required level of understanding of limits of functions after a basic course in mathematics. One advantage of an investigation over a period of time, such as this study, is that the students’ late reactions to the teaching become visible. Many students complete basic courses in calculus without ever understanding the notion of limits, or even understanding that they do not understand limits. Several disconnections in their learning developments have been detected in the present study, for example between intuitive and formal perceptions, procedural and static perceptions, and the finite and the infinite, which all are cognitively demanding to link together. The lack of interaction between students and lecturers or teachers may be a reason for
38 the students’ struggles to see connections of various kinds, for example between everyday problem solving and the formal definition. Added to the disconnections are the students’ inclinations to misjudge their own abilities or the accuracies of their mental representations. Students who are convinced that their perceptions of limits are coherent and correct do not feel any urge to recreate mental representations of the concept. Students need to become aware of their misrepresentations to adjust them appropriately. For that to happen, they must be in a situation where incoherent parts of their concept images are evoked simultaneously and the inconsistencies become apparent. Carefully selected problems to work with or discuss help students to acquire such awareness. Connectedness and continuity are essential features of teaching and learning limits to prevent students from failing.
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43 Sierpinska, A. (1993). Criteria for scientific quality and relevance in the didactics of mathematics. In G. Nissen, & M. Blomhøj (Eds.), Criteria for scientific quality and relevance in the didactics of mathematics (pp. 35-75). Denmark: Roskilde university, IMFUFA. Skolverket, (2005). Kursinfo 2004/2005, (Course information 2004/2005). Available at http://www3.skolverket.se (2005-04-26) Smith, J. P., diSessa, A. A., & Rochelle, J. (1993). Misconceptions reconceived: A constructivist analysis of knowledge in transition. The Journal of the Learning Sciences 3(2), 115-163. Star, J. (2000). On the relationship between knowing and doing in procedural learning. In B. Fishman, & S. O’Connor-Divelbiss (Eds.), Fourth International Conference of the Learning Sciences (pp. 80- 86). Mahwah, NJ: Erlbaum. Strauss, A., & Corbin, J. (1998). Basics of qualitative research, techniques and procedures for developing grounded theory. Thousand Oaks: Sage Publications Inc. Svege, E. (1997). Studenters forestillinger, holdninger og følelser overfor matematikk (Students’ beliefs, attitudes and feelings about mathematics). Nordisk Matematikkdidaktikk 2, 25-53. Svenning, C. (1999). Metodboken, samhällsvetenskaplig metod och metodutveckling. Staffanstorp: Lorentz förlag. Szydlik, J. (2000). Mathematical beliefs and conceptual understanding of the limit of a function. Journal for Research in Mathematics Education 31(3), 258-276. Tall, D. (1980). Mathematical intuition, with special reference to limiting processes. In R. Karplus (Ed.), Proceedings of the Fourth International Conference for the Psychology of Mathematics Education (pp. 170-176). Berkeley, CA: PME. Tall, D. (1991). The psychology of advanced mathematical thinking. In D. Tall (Ed.), Advanced mathematical thinking (pp. 3-21). Dordrecht: Kluwer Academic Publishers. Tall, D. (1992). The transition to advanced mathematical thinking: Functions, limits, infinity and proof. In D. A. Grouws (Ed.), Handbook of research on mathematics teaching and learning (pp. 495-511). New York: Macmillan. Tall, D. (1993). Students’ difficulties in calculus. Proceedings of working group 3 on students’ difficulties in calculus, ICME-7, Québec, Canada, 13-28.
44 Tall, D. (2004). Thinking through three worlds of mathematics. In M. Johnsen Høines, & A. B. Fugelstad (Eds.), Proceedings of the 28:th conference of the International Group for the Psychology of Mathematics Education (pp. 281-288). Norway: Bergen University College. Tall, D. (2005). The transition from embodied thought experiment and symbolic manipulation to formal proof. Article written for the Delta Conference. Frazer Island, Australia. Tall, D., & Vinner, S. (1981). Concept image and concept definition in mathematics with particular reference to limits and continuity. Educational Studies in Mathematics 12, 151-169. Vinner, S. (1991). The role of definitions in the teaching and learning of mathematics. In D. Tall (Ed.), Advanced mathematical thinking (pp. 65-81). Dordrecht: Kluwer Academic Publishers. Williams, S. (1991). Models of limit held by college calculus students. Journal for Research in Mathematics Education 22(3), 219-236.
45 46 Appendix This appendix comprises the articles as listed in the list of publications on page 9. The formats of the articles are in some cases adjusted to agree with this presentation. The adjustments are, for example, larger text or altered margins. There is a co-author to the last article (Juter & Grevholm, 2006). The work was divided so that Juter did most of the writing and data collections. Grevholm particularly contributed with texts on textbooks and curricula. Both authors worked with the overall impression of the article. Juter presented the article at the conference.
47
Article I
Limits of functions – how do students handle them?
Kristina Juter
Kristianstad University College, Sweden Email: [email protected] This paper aims at formulating and analysing the development of 15 students in their creations of mental representations of limits of functions during a basic mathematics course at a Swedish university. Their concept images are sought via questionnaires and interviews. How do students respond to limits of functions? The data indicate that some students have incoherent representations, but they do not recognise it themselves.
Introduction In this paper I will discuss the The notion of limits of functions is an development at the first university level important part of calculus. It is the of 15 Swedish students’ learning about foundation of many other important limits. The results are however relevant concepts. If students do not understand for other countries as well since the what limits are about, how can they difficulties encountered by students are understand concepts as, for example, similar. A browse through some South derivatives and integrals? African universities’ course descriptions Students can experience cognitive indicates that courses at basic level difficulties when they are learning about comprise mainly the same topics as the limits of functions, for example Swedish courses do. The questions understanding the formal definition or posed in the paper are: How do the the rules for deciding limit values students’ mental representations of (Cornu, 1991; Szydlik, 2000; Tall & limits of functions develop during their Vinner, 1981; Williams, 1991). It can first semester of mathematics? How do be hard for them to grasp the full the representations change, if they meaning of the compact formulation change, during this time? Are the with which they are faced. It is mental representations of the high important for teachers to know the achieving students different from the different, and perhaps unexpected, rest of the students? Answers to these results students can get from taking a questions provide an image of the course in calculus. Students in the same students’ development of the notion of class can form totally different mental limits of functions. representations from the same books and lectures. If we know how these Theoretical framework different representations are formed, In this section I present a theoretical then we are better equipped to provide a framework for the results of the study. rewarding learning environment for the It starts with a discussion about concept students. formation followed by a presentation of
Pythagoras 61, June, 2005, pp. 11-20 Limits of functions – how do students handle them? results from similar studies from other concept images that are evoked at countries. different times. After some time, two or more concept image cells can merge Concept image when the student understands the Tall (1991) states that there are many relation between the representations. kinds of minds. Different kinds of Time can also have the opposite effect, minds are needed in the development of the student hangs on to one concept mathematics and no one kind of mind is image and forgets about the others or always superior to another. One way to only gets access to some images in distinguish these different minds is to certain situations. When a problem is at look at the way the learners are thinking hand and the aim is to solve it, there of a concept and its formal definition. should be some kind of contact with the Tall and Vinner (1981) introduce two definition. It can be that the person first notions called concept image and consults the concept image and then the concept definition. The concept image definition to solve the problem, or there is the total cognitive structure can be several changes between the associated with a concept. It can be a cells. Another scenario is when only visualisation of the concept or one cell is consulted. If it is the image experiences of it or both. The concept cell that is the only one, it might be that image is individual and in different the person’s everyday life experience is contexts the same concept name can misleading. In most cases the problem evoke different concept images. For this can be solved, but in an unfamiliar the term evoked concept image is used situation it might not be enough. Then (Tall & Vinner, 1981). This is not the lack of the formality of the automatically all that an individual definition can be the reason for failure. knows about a concept. The concept definition is a form of words used by Abstraction, objects and processes the learner to define the concept. The Reflective abstraction (Dubinsky, 1991) concept definition can be learnt by heart is defined as the construction of mental from a book or it can be a personal re- objects and actions upon them. These construction of a definition to fit in with actions become processes by the person’s mental structure. A interiorisation. The objects are created personal concept definition can differ by encapsulation of processes. A from the formal concept definition, concept can be thought of both as an which is a definition accepted by object and as a process depending on mathematicians in general. the current context (Dubinsky, 1991; Vinner (1991) describes a model with Sfard, 1991). A scheme is a network of two “cells”. One contains the processes and objects. The schemas definitions of a concept and the other overlap and form a large complex web contains the concept image. If a concept in the individual’s mind. According to definition is memorised without Dubinsky, there are five different ways meaning, the concept image cell is of construction in reflective abstraction, empty. A concept can have several two have already been mentioned: Kristina Juter
Interiorisation: Construction of important pieces of the synthesising mental processes in order to process are forgotten and the individual understand a perceived takes the product for granted, that is, the phenomenon. understanding of the notion. Coordination: Construction of a There are cognitively different kinds process by coordination of two of generalisations to do. Tall (1991) or more other processes. describes three of them. Expansive Encapsulation: Construction of generalisation is when existing mental an object through a process. representations are unaltered and the Generalisation: Ability to apply new knowledge is attached as a an existing schema on a greater complement. Reconstructive range of phenomena. generalisation is when existing mental Reversal: The individual is able representations have to be changed in to think of an existing internal order to make sense. Disjunctive process in reverse to construct a generalisation is when the individual is new process. rote learning pieces that he or she Generalisation is not as cognitively hard should learn and just adds it to what is as abstraction since abstraction often already in the mind. No integration means that the individual has to make a occurs. It becomes impossible to get an re-construction of the mental overview of the notion at hand. representation (Dreyfus, 1991). Crucial properties of the object at hand must be Learning and understanding recognised and isolated from the object When a process or an object is so that the properties are applicable in mentioned we refer to it by a mental other situations. If visualisation is representation (Dreyfus, 1991). Some possible it can be a great help. The learners prefer visual thinking and have images can make the important pictures in their mental representations structures and relations clear in a global while others use symbols or examples manner. When an individual is to be able to think of notions. One generalising, he or she has a foundation concept can be represented in more than of examples and experience from which one way and there can even be to build. It is an extension of what is conflicting representations that are already there. Abstraction includes the evoked at different times depending on possibility for both synthesis and the context. If the representations are generalisation. To synthesise is to create not contradicting they can merge into a whole from parts. When this is done, one when the individual is able to see it often becomes more than its parts the connections. A coordination or a together. Many small disjoint parts get synthesis can be the result. If they are linked together and suddenly more incoherent a conflict may arise. The things fall into place. This is a more connections between the mental rewarding feeling and once an representations, the better the individual individual has passed this process, he or understands the concept. Then he or she she can not undo it. Soon all the little Limits of functions – how do students handle them? can go from one to another depending net and of the number of accurate on the demands of the task. connections. Knowledge of a concept is, according Vinner (1991) states that to to Dubinsky (1991), the individual’s understand a concept is not the same as tendency to bring to mind a scheme in to be able to form its definition order to be able to handle, organise or formally, but if a person has a concept make sense of a problem situation. It image he or she can understand the can be hard to keep mathematical concept. In this paper I will regard knowledge apart from mathematical understanding and knowledge of a construction. In an attempt to do so, we concept as Hiebert and Carpenter can observe individuals solving (1992) and Dubinsky (1991) do. problems. Such observations will not explicitly reveal the objects, processes The limit concept and schemas, but can indicate how The limit concept causes difficulties in knowledge is created. A part of learning teaching and learning (Cornu, 1991). It is, according to Dubinsky, applying is a complex concept but there is also a reflective abstraction to existing possible cognitive problem in the schemas to create new ones that provide distinction between the definition and an understanding of the concepts in the actual concept. Students can question. Does this imply that learning remember the definition but it does not cannot take place until at least very necessarily mean that they understand simplistic schemas are made? the concept. Vinner (1991) claims that Otherwise the result can be that the mathematical definitions represent the students only know how to solve conflict between the structure of routine tasks and are unaware of the mathematics and attainment of the actual range of a concept. They can also concepts via cognitive processes. create schemas that are incoherent and Mathematics is deduced from axioms wrong. Periods of confusion are and definitions. New concepts arise necessary when new knowledge is to be from previous ones in a logical manner. implemented with existing knowledge This is not always the way mathematics and this confusion is a part of the is created though. In the beginning learning process (Cornu, 1991). It is the students can meet the notion of limit in confusion that creates a need for order an informal intuitive way where the and the students start to adjust their tasks involve situations where they can schema. easily see the outcome. This creates a James Hiebert and Thomas P. feeling of control and the students think Carpenter (1992) define understanding they know what the concept is about of a mathematical concept to be even if they could not solve a more something an individual has achieved demanding task where they would have when he or she can handle the concept to master the full meaning of the as a part of a mental network. The definition (Cornu, 1991). One problem degree of understanding is decided by is the quantifiers “for every” and “there the strength of the connections in the exists” in the H – G definition: Kristina Juter
“For every H > 0 there exists a G rigor caused by the notion of infinity > 0 such that |f(x) – A| < H for (Cornu, 1991; Tall, 1992). every x in the domain with 0 < If the obstacles are located, the |x – a| < G ” teaching can be altered accordingly, not In everyday language the quantifiers by excluding the difficult passages but can have a slightly different meaning by supporting and helping the students compared to the ones used in to overcome them. Tall (1991) mathematics. If the students have such describes the generic extension conceptual obstacles they can get into principle. It is when an individual is in a trouble later on. The conceptions of an situation where he or she only meets idea before any teaching occurs are examples with a specific property. If no called spontaneous conceptions by counter-examples are present, the mind Cornu (1991) and they can be hard to assumes that the property is valid in get rid of even after teaching. One every context even if it is not explicitly reason for this is the fact that when stated. One example is if students only students solve problems, they tend to go see examples of convergent sequences to natural or spontaneous reasoning that do not reach their limits, then they rather than scientific theories. It is as in might assume that convergent Vinner’s (1991) model with the cells, sequences never do. where the individual only refers to the These results indicate that there are image cell. The obstacles start a process several aspects to consider in a learning of re-constructing existing knowledge situation about limits. The study in this that is based on spontaneous paper is conducted to reveal how conceptions and this can lead to Swedish students cope with such misunderstandings. situations. Monaghan (1991) found that the students in his study saw the word limit The study as more specific than, for example, The students and the course are approaches and other verbs. The described, followed by the methods and students had problems with the instruments used to answer the vagueness of some expressions and this questions: How do the students’ mental led to confusion. representations of limits of functions Cornu (1991) talks about four develop during their first semester of epistemological obstacles in the history mathematics? How do the of the limit concept. Two of them are representations change, if they change, the notions of infinitely large and during this time? Are the mental infinitely small respectively and the representations of the high achieving question whether the limit is attainable students different from the rest of the or not. There are also metaphysical students? obstacles that students of today struggle with, such as the abstractness of the The sample limit concept and the feeling of lack of 112 students participated in the study and 29% of them were female. They Limits of functions – how do students handle them? were aged 19 and up. They were After limits had been taught in the enrolled in a first level university course course, the students got a second in mathematics. The course was divided questionnaire at stage B, with more into two sub-courses. Both of them limit tasks at different levels of dealt with calculus and algebra. The difficulty. The aim was for the students courses were given over 20 weeks, full to reveal their habits of calculating, time (10 weeks for each course). The their ability to explain what they did students had two lectures and two and their attitudes in some areas. The sessions for task solving three days per students were asked if they were willing week. Each lecture and session lasted to participate in two individual 45 minutes. Thus the total teaching time interviews later that semester. 38 for each course was 90 hours. The students agreed to do so. 18 of these notion of limits of functions was students were selected for two presented in the first course before individual interviews each. The derivatives. It was taught again in the selection was done with respect to the second course in different settings such students’ responses to the as integrals and series. The first course questionnaires so that the sample would had a written examination and the as much as possible resemble the whole second had a written examination group. The gender composition of the followed by an oral one. The marks whole group was also considered in the awarded were IG for not passing, G for choices. passing and VG for passing with a good The first session of interviews was margin. I neither taught the students at held at the beginning of the second any stage nor did I know any of them. course, at stage C. Each interview lasted about 45 minutes. The students were Methods asked about definitions of limits, both Different methods were used to collect the formal one from their textbook and different types of data. The students their individual ways of defining a limit were confronted with tasks in different of a function. They also solved limit ways at five times during the semester, tasks of various types with the purpose called stage A to stage E. of revealing their perceptions of limits The students got a questionnaire at and they commented on their own stage A at the beginning of the solutions from the questionnaires to semester. It contained easy tasks about clarify their written responses where it limits and some attitudinal queries. The was needed. scope of these and subsequent tasks is The students got a third questionnaire described in the instruments section. at the end of the semester, at stage D. It The students were also asked about the contained just one task. Two fictional situations in which they had met the students’ discussion about a problem concept before they started their was described. One reasoned incorrectly university studies. The attitudinal data and the other one objected and proposed is not presented in this paper. an argument to the objection. The Kristina Juter students in the study were asked to x 3 2 c) Can the function f(x) = decide who is right and why. x 3 1 A second interview was carried out at attain the limit value in 2a? stage E after the examinations. Each interview lasted for about 20 minutes. d) Why? The students commented on the last questionnaire and in connection to that, Example 2 is what I regard to be a the definition was scrutinised again. routine task. There were also non- The quantifiers “for every” and “there routine tasks. A solution to a task was exists” in the H – G definition were presented to the students. It could be discussed thoroughly. 15 of the 18 incomplete or wrong and the students students were interviewed at this point. were to make it complete and correct. Field notes were taken during the There were two such tasks. students’ task solving sessions and at At stage C, which was the first set the lectures when limits were treated to of interviews, the students were give a sense of how the concept was asked to comment on statements very presented to the students and how the similar to those used by Williams students responded to it. Tasks and (1991) in a study about students’ results from other parts of the study are models of limits. The statements the described in more detail in other papers students commented on are the (Juter, 2003; Juter, 2004). following (translation from Swedish): 1. A limit value describes how Instruments a function moves as x tends At stage A the students solved some to a certain point. easy tasks about limits of functions. 2. A limit value is a number or x 2 a point beyond which a Example 1: f(x) = . x 2 1 function cannot attain What happens with f(x) if x values. tends to infinity? 3. A limit value is a number The tasks did not mention limits per which y-values of a se, but were designed to explore if function can get arbitrarily the students could investigate close through restrictions on functions with respect to limits. the x-values. At stage B the tasks were more 4. A limit value is a number or demanding. Some of the tasks were a point which the function influenced by Szydlik (2000) and Tall approaches but never and Vinner (1981). Three tasks had the reaches. following structure: 5. A limit value is an Example 2: approximation which can be x 3 2 as accurate as desired. a) Decide the limit: lim . xof x 3 1 6. A limit value is decided by b) Explanation. inserting numbers closer and closer to a given number Limits of functions – how do students handle them?
until the limit value is summaries of fuller ones. Martin, reached. Tommy and Frank had similar The reason for having these statements responses, Filip, Leo and Dan had was to get to know the students’ similar responses, Louise, Mikael, perceptions about the ability of Dennis and Oliver had similar responses functions to attain limit values and other and Julia and Emma had similar characteristics of limits. The students responses. One student’s description were given the statements to have from each group of similar responses is something to compare to their own presented in Table 1. The number in thoughts. There were other tasks brackets after each name in the table designed to make the students consider indicates the number of students with the formal definition to clarify what it similar responses. Anna, John and really says and tasks about attainability, David had responses which did not for example: match any of the above. Pseudonyms Example 3: Is it the same thing to say have been used to retain student “For every G > 0 there exists an H > 0 anonymity. The digits at stage C such that |f(x) - A| < H for every x in the indicate the students’ preferred choices domain with 0 < |x – a| < G” as “For from the six statements. The bold and every H > 0 there exists a G > 0 such that larger digits are the students’ choices of |f(x) - A| < H for every x in the domain statements most similar to their own perceptions of limits. The first points at with 0 < |x – a| < G”? What is the difference if any? stage E are the students’ explanations of As indicated before, at stage D the what a limit is and the last points are students got a task with a description of about Example 3 where the students two students arguing over a solution to explain the difference between the a task. The problem argued about was correct and incorrect definition 2 connected to the task from stage D. cos(x ) whether the limit value lim Table 2 presents the students’ marks. xo0 10000 The first letter or letters is the mark for exists or not. The task for the students the first course and the second is the in the study was to decide which mark for the second course. The argument, if any, was correct and to numbers after the marks are the explain what was wrong in the numbers of times the students needed to erroneous argumentation. take the examination to pass. At stage E, the second interview, the The students’ connections of limits to students’ written responses to the task at other concepts are dominated by stage D were discussed. Example 3 was derivatives and various applications in also brought up again in connection areas other than mathematics at the with the task at stage D. beginning of the course. In the middle the connections concern derivatives and Results functions and at the end Taylor Four of the 15 students’ responses are expansions, integrals and series. described at the five stages A, B, C, D and E above. The descriptions are Kristina Juter
Table 1. Students’ developments through the course.
Time Tommy (3) Leo (3) Mikael (4) Julia (2) A -Links to derivatives -Links to nutrition, -Links to prior -Links to prior -Solves easy tasks physics and biology studies, problem studies well -Solves easy tasks solving, physics -Solves easy tasks well -Unable to solve well easy tasks B -Limits are -Limits not -Limits are not -Limits are attainable attainable in attainable in attainable in problem in problem solving problem solving problem solving solving -Can state a -Cannot state a -Cannot state a -Cannot state a definition definition definition definition -Solves tasks and -Solves routine -Solves routine -Solves tasks and explains fairly well tasks and explains tasks and explains explains well C -Limits are -Limits are not -Limits are attainable -Limits are attainable attainable in attainable in in problem solving in problem solving problem solving problem solving -Limits are not -Limits are attainable -Limits are not (hesitates) attainable in theory in theory attainable in theory -Limits are not -Thinks logically -It comes closer and -A limit of a function attainable in theory rather than explicitly closer to A as x is how the limit -Thinks of limits in defining comes closer and stands with respect pictures -2, 3, 4, 5 closer to a to another function, -3, 4 -Cannot state but -1, 3, 5 no motion -Cannot state or can identify the -Cannot state but -3, 4, 5, 6 identify the definition after can identify the -Cannot state or definition investigation definition identify the -Solves tasks well -Problems in solving (claims that both def definition -Links to derivatives tasks in ex 3 state the -Not a real limit if and number -Links to prior same, H and G come attainable by sequences studies and graphs in pairs) function -Claims to master -Claims to master -Solves tasks well -Problems in the notion of limits the notion of limits in -Links to derivatives, solving tasks fairly well problem solving but graphs and number -Links to derivatives not the definition sequences and curves -Claims to master -Claims to master the notion of limits the notion of limits D -Identifies the error -Identifies the error, -Identifies the error, -Identifies the error makes other error makes other error E -A limit is a point to -A function tends to -If the x-value tends -If the function goes tend to but never to a value as the to a value then the closer and closer to reach, motion variable controlling function tends to a a number as x goes -Links to integrals the function value connected to closer and closer to -Cannot identify the changes, it never the x-value a value definition reaches the limit -Links to Taylor -Links to continuity -Does not -No links expansions, series -Can identify the understand the -Cannot identify the and number definition after quantifiers’ roles definition sequences investigation -Does not -Can identify the -Can explain the understand the definition difference between quantifiers’ roles -Can explain the the definitions in ex difference between 3 fairly well the definitions in ex 3 T Limits of functions – how do students handle them?
The results reflect the content of the the words in the task to something that course quite well. did not say much. The six statements at stage C revealed that among students with Table 2. Students’ marks marks G and IG the most frequently Name Mark Name Mark selected statement as most similar to Filip G2/IG Dan VG1/G1 their own perception was number four, Martin G1/G2 Mikael VG1/G1 which says that limits are never Tommy G1/G2 David G1/VG1 attained. Statement three was the most Anna G1/G2 Julia VG1/VG1 John G1/G1 Dennis VG1/VG1 frequently selected statement among the Frank G1/G1 Emma VG1/VG1 other students. Statement three was not Louise G1/G1 Oliver VG1/VG1 selected as most similar to their own Leo VG1/G1 perception by any of the students with marks G and IG. The students seem Table 1 indicates that Tommy is able confused about functions’ abilities to to solve routine tasks but not non- attain limit values. Most students’ routine tasks at stage B. By this I mean concept images are incoherent at this that he solved all the routine tasks point. Theoretical situations are treated correctly but did not present any correct differently from problem solving solutions to the non-routine tasks. Most situations. students were able to handle the easy Despite their lack of capability to tasks at stage A at least partially. At state a definition that describes the limit stage B, where the students got the non- of a function, many of the students still routine tasks, there was a clearer felt as if they had control over the distinction between high achievers and concept. Three of the students, all the rest of the students. At stage C, without the mark VG, thought that they there is no longer a clear distinction. did not have control over the concept of The two women with highest marks limits at stage C. The other students managed the problems while the two were rather certain that they had control men with highest marks did not. The over the concept, despite the fact that low achievers did not show such a almost none of them could formulate or gender distribution and it is probably in some cases even recognise the just a coincident that the high achievers definition. Only Emma is able to did. There is overall a higher rate of formulate the formal definition. From correctly solved problems of all kinds the seven students with marks G or IG it among the high achievers as could be was only Anna who recognised the expected. correct definition in Example 3. Among The students had a hard time the eight others there were five who formulating a definition of a limit of a recognised the correct definition, but function depending on x as x tends to a among them, Emma and Julia stated number at stage B. Only Julia managed that they both said the same thing. Dan, to do this in a meaningful manner. Most who could not recognise the definition, of the other students just reformulated thought so too. Kristina Juter
David had also indicated that the two components used in the definitions in definitions in Example 3 meant the connection to each other. same thing at stage E, but now both The question whether limits are Julia and Emma were able to explain attainable seems to be confusing, as the difference of the definitions and other researchers have also established thereby also the meaning of the (Cornu, 1991; Williams, 1991). Most of quantifiers. Emma explained her the students’ responses to questions and solution to Example 3 like this: tasks about attainability presented in “…in the real definition, G this paper are incoherent. This study depends on H.” shows that the students interpret the “…it is the independent variable definition as stating that the limits can which so to speak forces the not be reached by the function. When function value in and not the they solved problems on the other hand, other way around.” they could see that limits sometimes are Many students did not show such attainable for functions. security in their explanations. The quantifiers were still a mystery to them. Discussion Dan reasoned about Example 3 like this: The three questions posed by this study “…in most cases I think you had are here answered with the results and been able to do it this way too, it theoretical framework presented. The does not seem totally students’ developments are discussed, unreasonable.” [About the followed by the changes in their wrong definition] concept images. The question whether “…you can go the other way, high achievers’ results differ from low but you have to in a way check or average achievers’ results is as you go along.” [Check if it is discussed along with the two first possible to make H arbitrarily questions. small for the G chosen in the wrong definition] The students’ developments Students with marks G or IG had The students seem to be very pragmatic trouble understanding the difference when they learn about limits. They between the two definitions through the focus on the problem solving and not so course. It was not easy for them to see much on the theory, with the result that connections between the formal the theory is somewhat vaguely definition and the example with the represented in their minds. Yet they are fictitious students arguing. Most almost all confident about their ability students with higher marks were, after to grasp the concept. This lack of some thinking, able to tell the difference awareness can perhaps be made explicit at stage E. They showed a better for them if the students are put in understanding (Dubinsky, 1991; Hiebert challenging and explorative situations & Carpenter, 1992) in that they could explain the roles of the different Limits of functions – how do students handle them? more often.1 It is impossible to know The students do not seem to focus on what the students are actually thinking theory in the first place since so few can (Asiala et al., 1996). All I can do is to explain what the definition says see what they can accomplish and what throughout the semester. Many have a they say and do. After reading results split concept image which is from other studies (Cornu, 1991; particularly obvious in the attainability Monaghan, 1991; Tall, 1991; Tall, question. The incoherence in the limits’ 1992; Vinner, 1991) I was prepared to attainability shows that the see students having difficulties, not representation of the definition is not least with the definition, but I did not evoked in the problem situation (Tall & expect to see so much confusion Vinner, 1981; Vinner, 1991). A reason through the course. for this can be that since the students Looking at the students as a group thought that there is no point in dealing there is a positive development in their with limits if the limits are attainable, achievements. They get better at they did not consider them to be limits problem solving and understanding the if they were. It can be a case of the theory, but some of them do not get as generic extension principle (Tall, 1991). far as to fully synthesise (Dreyfus, The functions in most of the examples 1991) the concept during the course. the students meet do not attain their Parts of the concept and its applications limits. are known, but not enough for it to Many of the students think that the become an entity. definition says that functions can not Many students could briefly explain attain limit values. There is a big what a limit is, but when it came to difference in how high achievers explaining the partial transposition of H perceive this part of the definition from and G in Example 3 there were serious how low and average achievers do. The problems in most cases. Some students results from the six statements in the claimed that the transposition did not instruments section show that the latter make any difference since the H and group of students is more likely to G come in pairs. This view is perceive limits as unattainable. independent of the students’ marks. It is Cottrill, et al. (1996) argue that the hard for them to understand the role of problem of learning limits can be the quantifiers as Cornu (1991) also dependent on difficulties in creating found. Dan, for example, does not take mental schemas rather than the formal the words “for every” into account definition. The results of this study when he says that you can check if there rather imply that the students have is an H which can be made arbitrarily problems creating mental schemas small for every choice of G in the wrong because of the formal definition. definition. Changes The representations that the students
1 form change through the duration of the Some students actually told me that tasks like the non-routine tasks in the study would be good to have in the course to stimulate course. Some changes are rearranging discussion. Kristina Juter entire parts of the concept image. Some since they all can be true. But they alter changes involve just smaller the way the concept is thought of. modifications of, or attachments to, the Perhaps the mental representations are concept images. Then there are parts not changed, but have stimuli that evoke that even though they can be wrong, are different parts of the concept image left unaltered. In some cases the (Tall & Vinner, 1981). students do not give up their first Despite the problems, most students representations so readily. One example still had a sense of control over the is the students who think that limits are concept of limits. The sense of control unattainable for functions and stick by probably comes from successful this apprehension through the entire problem solving in the course (Cornu, course despite counter examples. 1991). The students can have a concept Williams (1991) mentions one possible image that works in the situations they reason for this. Suppose a student is in a have been in so far. When they are put situation where his or her experience so in a different kind of situation their far has been consistent with the theory concept images can be altered. As long as he or she perceives it. Then the as the students’ concept images are student encounters an example or challenged in some way, they will still something else that does not fit in with have the opportunity to alter, refine and the theory. Then, instead of adjusting expand them. If they had a good his or her interpretation of the theory, concept definition compatible with the the student might just regard the formal definition, the adjustments example as a minor exception. would probably be successful (Vinner, It can be hard to change a part of a 1991). Anna is one example of this. At mental representation. It can affect the stage C she was solving a problem rest of the representation in a way that where she had a graph of a function and requires a further modification. This can she had to decide a limit value at a be a reason for leaving parts as they are discontinuous point. Right and left limit even if the individual is aware of the values were both three but the functions error. But changes did come to pass in value at the point of discontinuity was the study. Tommy for example stated at two. Anna correctly stated the right and stage C that there is no motion in limits. left limit values, but hesitated when she At stage E he had changed his mind and had to give the total limit value. She altered his concept image. He had also first said two but was uncomfortable gone from perceiving a limit as a with it since she felt as if she had comparison between functions to an contradicted herself. Her concept image unreachable point. John had also and concept definition were in conflict changed his concept image. He went with each other. Anna’s concept from focusing on the distance between definition, saying that functions never the function and the limit to seeing the reach their limits, made her say that the limit as a border. Those changes need right and left limits are three. Her not make any difference in the rest of concept image, saying that the functions the students’ mental representations value at the point should be the limit Limits of functions – how do students handle them? value if it is attainable, made her want Mathematical Thinking”, in Tall, D., to say it is two. After a while she got to ed., Advanced Mathematical look at the formal definition and then Thinking, pp 95-126, Dordrecht: she was able to correctly solve all parts Kluwer Academic Publishers of the task. HIEBERT, J. & CARPENTER, T. P., The material presented here has been 1992, “Learning and teaching with further analysed and more results have understanding”, in Grouws, D. A., been published (Juter, 2003; Juter, ed., Handbook of Research on 2004). Among the results is the impact Mathematics Teaching and Learning, of algebra on the students’ learning of pp 65-97, New York: National limits. The more we know about the Council of Teachers of Mathematics students’ abilities and perceptions, the JUTER, K., 2003, Learning limits of better we can do our jobs as function, University students’ mathematics teachers. development during a basic course in mathematics, licentiate thesis: Luleå References University of Technology, ASIALA, M., BROWN, A., DEVRIES, Department of Mathematics D., DUBINSKY, E., MATHEWS, D. JUTER, K., 2004, “Limits of functions & THOMAS, K., 1996, “A – how students solve tasks”, in framework for research and Bergsten, C., ed., Proceedings of curriculum development in MADIF 4, the 4th Swedish undergraduate mathematics Mathematics Education Research education”, CBMS Issues in Seminar, pp 146-156, Malmö, Mathematics Education 6, 1-32 Sweden CORNU, B., 1991, “Limits”, in Tall, MONAGHAN, J., 1991, “Problems D., ed., Advanced Mathematical with the language of limits”, For the Thinking, pp 153-166, Dordrecht: Learning of Mathematics 11, 3, pp Kluwer Academic Publishers 20-24 COTTRILL, J., DUBINSKY, E., SFARD, A., 1991, “On the dual nature NICHOLS D., SCHWINGENDORF, of mathematical conceptions: K. & VIDAKOVIC, D., 1996, reflections on processes and objects “Understanding the limit concept: as different sides of the same coin”, Beginning with a coordinated process Educational Studies in Mathematics scheme”, Journal of Mathematical 22, pp 1-36 Behaviour 15, pp 167-192 SZYDLIK, J., 2000, “Mathematical DREYFUS, T., 1991, “Advanced beliefs and conceptual understanding Mathematical Thinking Processes”, of the limit of a function”, Journal in Tall, D., ed., Advanced for Research in Mathematics Mathematical Thinking, pp 25-41, Education 31, 3, pp 258-276. Dordrecht: Kluwer Academic TALL, D., 1991, “The psychology of Publishers advanced mathematical thinking”, in DUBINSKY, E., 1991, “Reflective Tall, D., ed., Advanced Mathematical Abstraction in Advanced Kristina Juter
Thinking, pp 3-24, Dordrecht: continuity”, Educational Studies in Kluwer Academic Publishers Mathematics 12, pp 151-169 TALL, D., 1992, “Students’ Difficulties VINNER, S., 1991, “The role of in Calculus”, in Proceedings of definitions in the teaching and Working Group 3, The 7th learning of mathematics”, in Tall, D., International Congress on ed., Advanced Mathematical Mathematical Education, pp 13-28, Thinking, pp 65-81, Dordrecht: Québec, Canada Kluwer Academic Publishers TALL, D. & VINNER, S., 1981, WILLIAMS, S., 1991, “Models of limit “Concept image and concept held by college calculus students”, definition in mathematics with Journal for Research in Mathematics particular reference to limits and Education 22, 3, pp 219-236
Article II
,IMITS OF FUNCTIONS 4RACES OF STUDENTS CONCEPT IMAGES
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3TUDENTS AT A 3WEDISH UNIVERSITY WERE SUBJECTS IN A STUDY ABOUT LEARNING LIMITS OF FUNCTIONS 4HE STUDENTS PERCEPTIONS WERE INVESTIGATED IN TERMS OF TRACES OF CONCEPT IMAGES THROUGH INTERVIEWS AND PROBLEM SOLVING 4HE RESULTS IMPLY THAT MOST STU DENTS FOUNDATIONS WERE NOT SUFlCIENTLY STRONG FOR THEM TO UNDERSTAND THE CONCEPT OF LIMIT WELL ENOUGH TO BE ABLE TO FORM COHERENT CONCEPT IMAGES 4HE TRACES OF THE STU DENTS CONCEPT IMAGES REVEAL CONFUSION ABOUT DIFFERENT FEATURES OF THE LIMIT CONCEPT
3TUDENTS STUDYING BASIC UNIVERSITY COURSES IN MATHEMATICS PERCEIVE THE CONCEPT OF LIMITS DIFFERENTLY IN DIFFERENT SITUATIONS *UTER 3TU DENTS ARE IN A SITUATION WHERE THEY HAVE TO INTEGRATE A VAST AMOUNT OF NEW INFORMATION WITH THEIR MENTAL REPRESENTATIONS OF WHAT THEY ALREADY KNOW $URING A PERIOD OF SUCH IMPLEMENTATION THERE WILL PROBABLY BE TIMES OF CONFUSION AND AMBIGUITY IN THE REPRESENTATIONS $IFFERENT REPRESENTA TIONS CAN BE INCOMPATIBLE WITHOUT THE STUDENTS TRYING TO RESOLVE THE CON TRADICTIONS )T CAN BE THAT A STUDENT ACTS AS IF ONE RULE APPLIES IN THEORETI CAL DISCUSSIONS AND A DIFFERENT ONE IN PROBLEM SOLVING SITUATIONS 4HE CONCEPT OF LIMIT OF A FUNCTION HAS BEEN SHOWN TO BE DIFlCULT TO UN DERSTAND DUE TO VARIOUS CHARACTERISTICS OF THE CONCEPT /NE EXAMPLE IS THE DUAL NATURE OF THE CONCEPT WHICH CAN BE THOUGHT OF AS AN OBJECT AND AN INlNITE PROCESS AT THE SAME TIME ANOTHER IS THE FORMAL DElNITION WHICH TAKES TIME AND EFFORT TO UNDERSTAND #ORNU *UTER 7ILLIAMS 4HE INmUENCE OF THE ABSTRACT CONCEPT OF INlNITY IS ALSO SOMETHING STUDENTS HAVE TO WORK WITH 4ALL 4HERE ARE CONSEQUENTLY SEVERAL POSSIBLE CONmICT SITUATIONS IN THE WAYS STUDENTS CAN PERCEIVE LIMITS
+RISTINA *UTER +RISTIANSTAD 5NIVERSITY #OLLEGE
.ORDIC 3TUDIES IN -ATHEMATICS %DUCATION .O +2)34).! *54%2
) HAVE DONE A STUDY TO lND OUT HOW 3WEDISH UNIVERSITY STUDENTS LEARN LIMITS OF FUNCTIONS DURING THEIR lRST MATHEMATICS SEMESTER 4HIS ARTI CLE FOCUSES ON ONE STAGE IN THE STUDENTS DEVELOPMENT OF THE CONCEPT OF LIMITS OF FUNCTIONS 4HE AIM IS TO lND OUT HOW THEY ARE REASONING AND IF THEY SHOW INCONSISTENCIES IN THEIR REPRESENTATIONS AT THAT SPECIlC TIME 4HE RESEARCH QUESTIONS ADDRESSED ARE THE FOLLOWING (OW DO STUDENTS PERCEIVE LIMITS OF FUNCTIONS (OW ARE THE CRITICAL FEATURES OF LIMITS OF FUNCTIONS WHICH STUDENTS DESCRIBE CONNECTED THAT IS DO THEY FORM A COHERENT PICTURE $O STUDENTS WITH HIGH GRADES REASON DIFFERENTLY FROM STUDENTS WITH AVERAGE GRADES
4HEORETICAL FRAMEWORK 4HEORIES WITH PARTICULAR RELEVANCE TO THE TOPIC OF THIS PAPER ARE PRE SENTED IN THIS SECTION
4RACES OF CONCEPT IMAGES 7HEN A PROCESS OR AN OBJECT IS MENTIONED IT IS REFERRED TO BY MENTAL REP RESENTATIONS $REYFUS ! CONCEPT IMAGE 4A L L A ND 6I N NER CA N BE USED AS A MODEL OF THIS MENTAL REPRESENTATION OF A CONCEPT 4HE CON CEPT IMAGE CONSISTS OF EVERYTHING THE INDIVIDUAL CONNECTS WITH THE CON CEPT 4HIS INCLUDES FOR EXAMPLE EXPERIENCES AND RELATIONS TO OTHER CON CEPTS )T IS NOT POSSIBLE TO SEE ANOTHER PERSONS CONCEPT IMAGE BUT HIS OR HER ACTIONS ARE DERIVED FROM THE PERSONS CONCEPT IMAGE AND CAN THERE FORE BE REGARDED AS A TRACE OF THE CONCEPT IMAGE 7HAT THE STUDENTS DO IS REGULATED BY THEIR ABILITIES AMONG OTHER FACTORS 3TAR 4HE TRACE OF A CONCEPT IMAGE IS INmUENCED BY THE SITUATION IN WHICH THE STUDENTS ARE AT THE TIME WHEN THE TRACE IS BEING CONSIDERED )NmUENCES CAN COME FROM PEERS OR TEACHERS BUT ALSO FROM THE STUDENTS OWN MINDS WHEN THEY THINK AND ARGUE FOR THEIR CONJECTURES 3FARD AND #OTTRILL $UBINSKY .ICHOLS 3CHWINGENDORF 4HOMAS AND 6IDAKOVIC HAVE DESCRIBED THEORIES OF OBJECT FORMATION OF PROCESSES 4HESE OBJECTS AND PROCESSES FORM MENTAL SCHEMAS TO REPRE SENT THE CONCEPT AND CONNECTIONS TO IT 3UCH SCHEMAS ARE PART OF INDI VIDUALS CONCEPT IMAGES #OTTRILL ET AL CONCLUDE THAT THE FORMAL CONCEPT OF LIMIT IS AN INTRICATE DYNAMICAL SCHEMA AND NOT A STATIC ONE )T IS IMPORTANT TO HAVE A STRONG DYNAMICAL CONCEPTION OF THE NOTION BEFORE IT IS POSSIBLE TO FULLY EMBRACE A MORE FORMAL INTERPRETATION )N THE DEVEL OPMENT OF RElNING THE SCHEMA OF LIMITS THERE ARE LEAPS BETWEEN SEEING A LIMIT AS A PROCESS AND AS AN OBJECT 3FARD DElNES REIlCATION AS AN
.ORDIC 3TUDIES IN -ATHEMATICS %DUCATION .O ,IMITS OF FUNCTIONS TRACES OF STUDENTS CONCEPT IMAGES
vONTOLOGICAL SHIFTv PAGE WHICH IS ACHIEVED WHEN A PROCESS CAN BE SEEN AS AN OBJECT SOMETHING FAMILIAR BECOMES CLEAR FROM A COMPLETELY DIFFER ENT POINT OF VIEW )T IS OFTEN COGNITIVELY DEMANDING TO REACH THIS STAGE OF PERCEPTION "LOMHJ 4HERE IS A CONNECTION BETWEEN INTERIORISA TION AT A HIGHER LEVEL WITH REIlCATION AT A LOWER LEVEL IN THE DEVELOPMENT OF MATHEMATICS WHICH SOMETIMES LEADS TO ATTEMPTS TO REACH THE REIlCATION STAGE PREMATURELY )N THIS DUAL SITUATION THERE IS A DANGER OF CONFUSION )F A STUDENT HAS A SET OF CONCEPTIONS OF LIMITS AND WANTS TO SOLVE A PROBLEM IT IS NOT CERTAIN THAT WHAT IS CONSULTED IN THE MIND IS THE MOST APPROPRI ATE CONCEPTION $AVIS AND 6INNER 4HE MOST DOMINANT PART OF A CONCEPT IMAGE CAN CAST A SHADOW OVER OTHER PARTS FOR DIFFERENT REASONS )T CAN BE FAMILIARITY DIFlCULTY OR HOW LONG THE CONCEPTION HAS BEEN AVAIL ABLE TO THE INDIVIDUAL THAT INmUENCE WHAT WILL BE DRAWN FROM THE CONCEP TIONS (IGH ACHIEVING STUDENTS CREATE MORE COMPLEX REPRESENTATIONS OF CONCEPTS THAN OTHER STUDENTS DO ACCORDING TO #HINNAPPAN WITH THE EFFECT THAT THE HIGH ACHIEVING STUDENTS CAN lND NEW SOLUTIONS TO PROB LEMS THAT THE LOW ACHIEVING STUDENTS ARE UNABLE TO lND 4HE HIGH ACHIEV ING STUDENTS ARE MORE LIKELY TO DRAW THE MOST SUITABLE SCHEMAS FROM THEIR REPRESENTATIONS TO SOLVE THE PROBLEMS THEY ARE DEALING WITH
5NDERSTANDING AND KNOWLEDGE ! CONCEPT CAN BE REPRESENTED IN MORE THAN ONE WAY AND THERE CAN EVEN BE CONmICTING REPRESENTATIONS THAT ARE EVOKED AT DIFFERENT TIMES DEPEND ING ON THE CONTEXT )F THE REPRESENTATIONS ARE NOT CONTRADICTORY THEY CAN MERGE INTO ONE WHEN THE INDIVIDUAL IS ABLE TO SEE THE CONNECTIONS $REY FUS )F THEY ARE INCOHERENT IN ANY ASPECT A CONmICT MAY ARISE 4HE INCOHERENCY CAN FOR EXAMPLE CONCERN INTERPRETATIONS OF RULES OR PER CEPTIONS OF DElNITIONS IN DIFFERENT CONTEXTS 4HE MORE APPROPRIATE THE CONNECTIONS BETWEEN THE MENTAL REPRESENTATIONS ARE THE BETTER THE INDI VIDUAL UNDERSTANDS THE CONCEPT $REYFUS (IEBERT AND #ARPENTER (IEBERT AND #ARPENTER DElNE UNDERSTANDING OF A MATHE MATICAL CONCEPT TO BE SOMETHING AN INDIVIDUAL HAS ACHIEVED WHEN HE OR SHE CAN HANDLE THE CONCEPT AS A PART OF A MENTAL NETWORK + NOWLEDGE OF A CONCEPT IS ACCORDING TO $UBINSKY THE INDIVID UALS TENDENCY TO BRING TO MIND A SCHEME IN ORDER TO BE ABLE TO HANDLE ORGANIZE OR MAKE SENSE OF A PROBLEM SITUATION )N THIS PAPER ) WILL REGARD UNDERSTANDING AND KNOWLEDGE OF A CONCEPT AS (IEBERT AND #ARPENTER AND $UBINSKY DO
.ORDIC 3TUDIES IN -ATHEMATICS %DUCATION .O +2)34).! *54%2
0ERCEPTIONS OF LIMITS OF FUNCTIONS )N THE BEGINNING STUDENTS CAN MEET THE NOTION OF LIMIT IN AN INFORMAL IN TUITIVE WAY WHERE THE TASKS ARE FROM SITUATIONS WHERE THEY CAN EASILY SEE THE OUTCOME 4HIS CREATES A FEELING OF CONTROL AND THE STUDENTS THINK THEY KNOW WHAT THE CONCEPT IS ABOUT EVEN IF THEY COULD NOT SOLVE A MORE DE MANDING TASK WHERE THEY WOULD HAVE TO MASTER THE FULL MEANING OF THE DElNITION #ORNU *UTER 4HIS FEELING OF CONlDENCE CONCEA L S THE NEED FOR FURTHER SOPHISTICATION OF THE CONCEPT IMAGE ,IMITS OF FUNC TIONS CAN BE THOUGHT OF IN VERY DIFFERENT WAYS DEPENDING ON WHAT SITUA TION THE LIMITS OCCUR IN #RITICAL FEATURES OF A CONCEPT DEPEND ON WHICH PERSPECTIVE THE STUDENTS HAVE ON THEIR APPROACH TO IT )T TAKES DIFFERENT EFFORTS TO BE ABLE TO UNDERSTAND OR DESCRIBE THE CONCEPT OF LIMITS THAN IT TAKES TO BE ABLE TO CALCULATE LIMIT VALUES %VERYDAY LANGUAGE CAN HAVE A SLIGHTLY DIFFERENT MEANING COMPARED TO THE LANGUAGE USED IN MATHEMATICS WHICH CAN HAVE AN EFFECT ON STUDENTS PERCEPTION OF THE CONCEPT -ONAGHAN 7E USE WORDS OR PHRASES AS CONVERGENCE BORDER ARBITRARILY CLOSE TEND TO AND LIMIT WHEN WE WORK WITH LIMITS OF FUNCTIONS 4HE EVERYDAY MEANINGS OF THE WORDS CAN IN mUENCE STUDENTS PERCEPTIONS OF THE WORDS IN A MATHEMATICAL CONTEXT 4HERE IS AN AMBIGUITY IN THE WAY THE CONCEPT OF LIMITS CAN BE PERCEIVED /NE CAN FOCUS ON THE PROCESS OF APPROACHING THE LIMIT AND HENCE CON SIDER IT A NEVER ENDING PROCEDURE "UT ONE CAN ALSO THINK OF THE LIMIT AS A STATIC ENTITY TO WHICH FUNCTIONS CAN BE COMPARED 4HIS KIND OF AMBI GUITY IS EXEMPLIlED IN A STUDY BY 7ILLIAMS WHERE TWO STUDENTS CONCEPTIONS OF LIMITS WERE EXAMINED "OTH OF THE STUDENTS UNDERSTOOD THAT CONTINUOUS FUNCTIONS REACH THEIR LIMIT VALUES BUT THEY WERE NOT SURE IF IT WAS CORRECT TO SAY THAT THE LIMIT WAS REACHED IN THE LIMIT PROC ESS 4HE STUDENTS APPEAR AS ) SEE IT TO BE CONFUSED BY THE CONTEXT OF THE TASK 4HEY KNOW THAT CONTINUOUS FUNCTIONS CAN REACH THEIR LIMIT VALUES BUT THEIR CONCEPT IMAGES OF LIMIT PROCESSES INTERFERE WITH THEIR CONCEPT IMAGES OF CONTINUOUS FUNCTIONS CAUSING THIS HESITATION 7ILLIAMS MEN TIONS THE COMPLEXITY OF THE NOTION OF INlNITY AS A SOURCE FOR THE PROBLEM IN THIS SITUATION BUT IT CAN ALSO BE THAT THE STUDENTS DO NOT ACCEPT THE DElNITION OF LIMITS )F THEY DO NOT SEE ANY USE FOR THE DElNITION OR IF THE DElNITION IS TOO HARD TO IMPLEMENT TO THE STUDENTS CONCEPT IMAGES THE STUDENTS CAN CHOSE NOT TO ACCEPT THE DElNITION )NSTEAD THEY CAN CREATE AN ALTERNATIVE DElNITION MORE SUITABLE TO THEIR CONCEPT IMAGES )F SUCH STUDENTS HAVE PROBLEMS UNDERSTANDING THE DElNITION IT IS MOST LIKELY THAT THE ALTERNATIVE DElNITION HAS FOCUS ON PROCESSES RATHER THAN OBJECTS "LOMHJ #OTTRILL ET AL 3FARD 0RZENIOSLO HAS DONE A STUDY ABOUT LIMITS OF FUNCTIONS WITH UNIVERSITY STUDENTS IN YEARS THREE TO lVE 3HE USED TASKS SIMILAR TO 4ASK AND 4ASK PRESENTED LATER
.ORDIC 3TUDIES IN -ATHEMATICS %DUCATION .O ,IMITS OF FUNCTIONS TRACES OF STUDENTS CONCEPT IMAGES
ON IN THIS STUDY 3HE FOUND THAT STUDENTS CONCEPT IMAGES DIFFERING FROM THE FORMAL CONCEPT DElNITION COULD HAVE THEIR ORIGIN IN AN INTUITION THAT LIMITS DO NOT EXIST IF THE FUNCTION IS NOT CONTINUOUS 4HE INTUITIVE VIEW COMES FROM SECONDARY SCHOOL WORK ACCORDING TO 0RZENIOSLO /PPOSED TO THIS INTUITIVE VIEW EARLIER RESULTS FROM THE STUDY PRESENTED IN THIS PAPER SHOW STUDENTS STATING THAT A VALUE IS NOT A LIMIT VALUE IF THE FUNCTION CAN ATTAIN IT *UTER 4RACES OF CONCEPT IMAGES FROM STUDENTS ARE DESCRIBED AND ANALYSED IN ORDER TO lND OUT MORE ABOUT THEIR LEARNING OF LIMITS OF FUNCTIONS
4HE STUDY 4HE AIM OF THE WHOLE STUDY AS WELL AS THE PART PRESENTED IN THE PRESENT PAPER IS STATED IN THIS SECTION 4HE STUDENT SAMPLE AND THE COURSE ARE DE SCRIBED FOLLOWED BY A PRESENTATION OF THE METHODS AND INSTRUMENTS USED TO ANSWER THE QUESTIONS POSED 4HE METHODS OF THE WHOLE STUDY ARE OUT LINED BRIEmY TO FRAME THIS PART OF IT
!IM 4HE GENERAL AIM OF THE STUDY WAS TO lND OUT AS MUCH AS POSSIBLE ABOUT THE STUDENTS DEVELOPMENT IN THEIR LEARNING OF LIMITS OF FUNCTIONS .O SUCH INVESTIGATION HAD BEEN DONE IN 3WEDEN BEFORE 4HE AIM OF THIS PAPER IN PARTICULAR IS TO LOOK AT THE STUDENTS CONCEPTIONS AT ONE SPECIlC TIME 4HE QUESTIONS INVESTIGATED ARE (OW DO STUDENTS PERCEIVE LIMITS OF FUNCTIONS (OW ARE THE CRITICAL FEATURES OF LIMITS OF FUNCTIONS WHICH STUDENTS DESCRIBE CONNECTED THAT IS DO THEY FORM A COHERENT PICTURE $O STUDENTS WITH HIGH GRADES REASON DIFFERENTLY FROM STUDENTS WITH AVERAGE GRADES
4HE SAMPLE /F THE STUDENTS THAT PARTICIPATED IN THE STUDY WERE FEMALE 4HE STUDENTS WERE AGED AND ABOVE 4HEY WERE STUDYING MATHEMATICS AT THE BASIC UNIVERSITY LEVEL FOR TWENTY WEEKS FULL TIME DIVIDED IN TWO TEN WEEK COURSES CALLED THE _ COURSE AND THE ` COURSE RESPECTIVELY BOTH DEALING WITH ALGEBRA AND CALCULUS .OT ALL STUDENTS TOOK BOTH COURSES 4HE STU DENTS HAD LECTURES TOGETHER IN THE WHOLE GROUP WITH TWO MINUTES LEC TURES PER DAY FOR THREE DAYS PER WEEK !FTER THE LECTURES THEY HAD TWO
.ORDIC 3TUDIES IN -ATHEMATICS %DUCATION .O +2)34).! *54%2
MINUTES TASK SOLVING SESSIONS IN SMALLER GROUPS WITH ABOUT STUDENTS IN EACH "OTH COURSES WERE ASSESSED WITH AN INDIVIDUAL WRITTEN EXAM !T THE END OF THE SECOND COURSE THE STUDENTS ALSO HAD AN INDIVIDUAL ORAL EXAM 4HE ORAL EXAMS WERE MAINLY ABOUT THEORY AND THE WRITTEN EXAMS HAD A FOCUS ON TASK SOLVING ) WAS NEITHER TEACHING THE STUDENTS AT ANY STAGE NOR DID ) KNOW ANY OF THEM
1 1 ) 1 ) \_ LIMITS \ ` \EXAMS
&IGURE 4IMELINE
-ETHODS
$IFFERENT METHODS WERE USED TO COLLECT DIFFERENT TYPES OF DATA 4 HE T IME LINE &IGURE SHOWS WHEN THE SETS OF DATA WERE COLLECTED 4 HE S T UDENT S GOT A QUES T ION NA I RE 1 I N T HE BEG I N N I NG OF T HE _ COURSE )T CONTAINED EASY TASKS ABOUT LIMITS AND SOME ATTITUDINAL QUERIES !FTER LIMITS HAD BEEN TAUGHT IN THE _ COURSE THE STUDENTS GOT A SECOND QUES TIONNAIRE 1 WITH MORE LIMIT TASKS AT DIFFERENT LEVELS OF DIFlCULTY 4HE STUDENTS WERE ASKED IF THEY WERE WILLING TO PARTICIPATE IN TWO INTERVIEWS LATER THAT SEMESTER 4HIRTY EIGHT STUDENTS AGREED TO DO SO %IGHTEEN OF THESE STUDENTS WERE SELECTED FOR TWO INDIVIDUAL INTERVIEWS EACH 4HE SE LECTION WAS DONE WITH RESPECT TO THE STUDENTS RESPONSES TO THE QUESTION NAIRES SO THAT THE SAMPLE WOULD AS MUCH AS POSSIBLE RESEMBLE THE WHOLE GROUP 4HE lRST SESSION OF INTERVIEWS ) WAS HELD IN THE BEGINNING OF THE ` COURSE 4HE INTERVIEWS WERE SEMI STRUCTURAL WITH A SET OF QUESTIONS AND TASKS WHICH WERE FOLLOWED BY QUESTIONS BASED ON THE RESPONSES FROM THE S T UDENT S %ACH I NTER V IEW LA S TED ABOUT M I NUTES A ND WA S AUD IO RECORDED 4HE STUDENTS WERE ASKED ABOUT DElNITIONS OF LIMITS BOTH THE FORMAL ONE FROM THEIR TEXTBOOK AND THEIR INDIVIDUAL WAYS TO DElNE A LIMIT OF A FUNC TION 4HEY ALSO SOLVED LIMIT TASKS OF VARIOUS TYPES WITH THE PURPOSE TO REVEAL THEIR PERCEPTIONS OF LIMITS 4HIS INTERVIEW IS THE STAGE IN THE STU DENTS DEVELOPMENTS ON WHICH THIS PAPER IS FOCUSED 4HE INSTRUMENTS USED WILL BE FURTHER DESCRIBED BELOW 4HE STUDENTS GOT A THIRD QUESTION NAIRE 1 AT THE END OF THE SEMESTER AND A SECOND INTERVIEW ) WAS CAR RIED THROUGH AF TER THE EXAMS &IELD NOTES WERE TAKEN DURING THE STUDENTS TASK SOLVING SESSIONS AND AT THE LECTURES WHEN LIMITS WERE TREATED TO GIVE A SENSE OF HOW THE CONCEPT WAS PRESENTED TO THE STUDENTS AND HOW THE STUDENTS REACTED TO IT 4ASKS AND RESULTS FROM OTHER PARTS OF THE STUDY
.ORDIC 3TUDIES IN -ATHEMATICS %DUCATION .O ,IMITS OF FUNCTIONS TRACES OF STUDENTS CONCEPT IMAGES
ARE DESCRIBED IN MORE DETAIL IN OTHER PAPERS *UTER *UTER )N THIS PAPER ) WILL DISCUSS THE RESULTS OF THE lRST SET OF INTERVIEWS WITH RESPECT TO THE STUDENTS CONCEPTIONS OF LIMITS OF FUNCTIONS
)NSTRUMENTS !T THE TIME OF THE lRST INTERVIEW THE STUDENTS WERE ASKED TO COMMENT ON STATEMENTS VERY SIMILAR TO THOSE USED BY 7ILLIAMS IN A STUDY ABOUT STUDENTS MODELS OF LIMITS 4HE STATEMENTS THE STUDENTS COMMENTED ON ARE THE FOLLOWING TRANSLATION FROM 3WEDISH ! LIMIT VALUE DESCRIBES HOW A FUNCTION MOVES AS X TENDS TO A CERTAIN POINT ! LIMIT VALUE IS A NUMBER OR A POINT BEYOND WHICH A FUNCTION CANNOT ATTAIN VALUES ! LIMIT VALUE IS A NUMBER WHICH Y VALUES OF A FUNCTION CAN GET ARBITRARILY CLOSE TO THROUGH RESTRICTIONS ON THE X VALUES ! LIMIT VALUE IS A NUMBER OR A POINT WHICH THE FUNCTION APPROACHES BUT NEVER REACHES ! LIMIT VALUE IS AN APPROXIMATION WHICH CAN BE AS ACCURATE AS DESIRED ! LIMIT VALUE IS DECIDED BY INSERTING NUMBERS CLOSER AND CLOSER TO A GIVEN NUMBER UNTIL THE LIMIT VALUE IS REACHED
4HE REASON FOR HAVING THESE STATEMENTS WAS TO GET TO KNOW THE STUDENTS PERCEPTIONS ABOUT FUNCTIONS ABILITIES TO ATTAIN LIMIT VALUES AND OTHER CHARACTERISTICS OF LIMITS 4HE STUDENTS WERE GIVEN THE STATEMENTS TO HAVE SOMETHING TO COMPARE TO THEIR OWN THOUGHTS 4HERE WERE OTHER TASKS DE SIG NED TO MA KE T HE S T UDENT S CON SIDER T HE FOR MA L DE l N IT ION TO CLARIFY WHAT IT REALLY SAYS AND TASKS ABOUT ATTAINABILITY *UTER *UTER 4WO TASKS DEALT WITH ATTAINABILITY AND WHAT HAPPENS NEAR AND AT A LIMIT VALUE THEY WILL BE REFERRED TO AS 4ASK AND 4ASK 4HE AIM WAS TO SEE HOW THE STUDENTS WERE GOING TO INTERPRET THE DElNITION AND ATTAINABIL ITY OF LIMITS 4HE STUDENTS GOT TWO GRAPHS OF FUNCTIONS &IGURE CORRE SPONDING TO 4ASK AND &IGURE CORRESPONDING TO 4ASK 4HE GRAPHS ARE PRESENTED IN THE SAME WAY AS THEY WERE TO THE STUDENTS IN HANDMADE PICTURES 4HE TASKS FOR THE STUDENTS WERE TO DETERMINE RIGHT AND LEFT LIMIT VALUES AND IF THE FUNCTIONS HAD A LIMIT VALUE AS X TENDED TO THE POINTS A AND B
.ORDIC 3TUDIES IN -ATHEMATICS %DUCATION .O +2)34).! *54%2
RESPECTIVELY MARKED IN THE GRAPHS 4HEY WERE ALSO ASKED TO DETERMINE THE VALUES OF THE FUNCTIONS AT THESE POINTS
$ATA ANALYSIS 4HE INTERVIEWS WERE TRANSCRIBED AND THE STUDENTS ANSWERS AND STATE MENTS WERE CATEGORIZED WITH THE AID OF THE COMPUTER PROGRAM .5$ )34 . 4HE PROGRAM MAKES IT POSSIBLE TO CATEGORIZE PARTS OF A TEXT SUCH AS A TRANSCRIPT OF AN INTERVIEW 0ARTS OF THE TEXT CONTAINING TRACES OF A CERTAIN PERCEPTION WERE SELECTED AND SAVED IN A CATEGORY 4HE CATEGORI ZATION IS THEREBY SOMEWHAT SUBJECTIVE IN CHARACTER SINCE TRACES OF PERCEP TIONS OCCUR IN VARIOUS FORMS AND WITH VARYING CLARITY 4HE CATEGORIZATION WAS DONE SEVERAL TIMES TO MAKE THE JUDGEMENTS AS ACCURATE AS POSSIBLE WITH RESPECT TO THE ISSUES CHOSEN FOR THE STUDY FOR EXAMPLE ATTAINABILITY OR LIMITS AS APPROXIMATIONS 4HE KEY ISSUES CODED WERE THE HEADINGS OF TABLE 4 HE PROGRAM MA KES T HE CATEGOR IZAT ION EASY TO SU R VEY AND CHANGE 4HE CATEGORIES AT THIS PART OF THE STUDY WERE PRIMARILY CHOSEN TO REVEAL THE STUDENTS TRACES OF CONCEPT IMAGES AT CRITICAL PARTS 4HE SIX STATEMENTS FROM THE INTERVIEW SERVED AS A BASIS IN THE CATEGORY SELECTION PROCESS !T TAINABILITY IN THEORETICAL CONTEXTS AND PROBLEM SOLVING CONTEXTS BECAME TWO DIFFERENT CATEGORIES SINCE THERE WERE INTERESTING lNDINGS RELATED TO THIS SEPARATION /NE AIM WITH THE CATEGORIES WAS TO DISCERN CONCEPTIONS OF LIMITS AS OBJECTS FROM CONCEPTIONS OF LIMITS AS PROCESSES 4HE COM PLEXITY OF THE CONCEPT MADE IT HARD TO DO THIS DISCERNMENT WITHOUT INFER RING TOO MUCH 4HE RESULTS ARE THEREFORE PRESENTED WITH ALL THE LAST FOUR
&IGURE 'RAPH CORRESPONDING TO 4ASK
.ORDIC 3TUDIES IN -ATHEMATICS %DUCATION .O ,IMITS OF FUNCTIONS TRACES OF STUDENTS CONCEPT IMAGES
CATEGORIES IN TABLE INSTEAD OF JUST TWO CATEGORIES IN TERMS OF OBJECTS AND PROCESSES 4HE NUMBERS IN TABLE INDICATE HOW MANY TIMES EACH PERCEP TION WAS CONlRMED DURING THE INTERVIEW
2ESULTS 4HE RESULTS OF THE INTERVIEWS ARE PRESENTED IN TABLE AND TABLE 4ABLE CONTAINS SEVEN CATEGORIES EACH OF WHICH IS IDENTIlED WITH A PHRASE OR A WORD 3OME STUDENT EXAMPLES FROM EACH CATEGORY ARE PROVIDED 4HE STU DENTS ARE LISTED IN ORDER OF THEIR GRADES FROM THE LOWEST TO THE HIGHEST GRADES 4HE LETTERS AFTER THE NAMES INDICATE THE RESULTS OF THE EXAMINA TIONS !N vAv MEANS THE HIGHEST GRADE ON BOTH EXAMS AT THE lRST ATTEMPT vBv IS ONE EXAM PASSED WITH THE HIGHEST GRADE AND THE OTHER PASSED BOTH AT THE lRST ATTEMPT vCv MEANS BOTH PASSED AT THE lRST ATTEMPT NOT WITH THE HIGHEST GRADE vDv MEANS ONE PASSED AT THE lRST ATTEMPT AND ONE AT THE SECOND AND vEv MEANS ONE PASSED AND ONE NOT PASSED 4HE ORAL EXAM AND THE SECOND WRITTEN EXAM ARE HERE TOGETHER CONSIDERED TO BE THE SECOND EXAM 4HE lNDINGS DISPLAYED IN TABLE REVEAL THAT THERE IS A VARIETY OF COM BINATIONS OF THE PERCEPTIONS IN THE CATEGORIES 3EVERAL OF THEM ARE INCO HERENT AT DIFFERENT POINTS 4HERE IS NO OBVIOUS DISTINCTION BETWEEN HIGH AND AVERAGE ACHIEVERS
&IGURE 'RAPH CORRESPONDING TO 4ASK
.ORDIC 3TUDIES IN -ATHEMATICS %DUCATION .O +2)34).! *54%2
4ABLE 3TUDENTS CONCEPTIONS OF LIMITS OF FUNCTIONS
!TTAIN !TTAIN $O NOT $ISTANCE "ORDER !PPROX %XACT LIMIT LIMIT ATTAIN BETWEEN IMATION VALUE 4 VALUE 0 LIMIT FUNC VALUE TION AND LIMIT &ILIP E -ARTIN D 4OMMY D !NNA D *OHN C &RANK C ,OUISE C ,EO B $AN B -IKAEL B $AVID B *ULIA A $ENNIS A %MMA A /LIVER A
.OTE 4HE NUMBERS SHOW HOW MANY TIMES EACH VIEW WAS CONlRMED DURING THE INTERVIEWS
%XAMPLES OF STUDENTS STATEMENTS IN THE DIFFERENT CATEGORIES OF TABLE
!TTAIN LIMIT VALUE 4 &UNCTIONS CAN ATTAIN LIMIT VALUES IN THEORY %MMA THE DElNITION OF LIMIT VALUES IS LIKE THAT IT IS POSSIBLE MOST OR IT DE PENDS THERE ARE LIMIT VALUES WHICH ARE ATTAINABLE *ULIA THE FUNCTION CAN ATTAIN VALUES BEYOND THE LIMIT VALUE IF IT IF X IS NOT WELL FOR OTHER X SO TO SPEAK 9ES IT IS OKAY THAT IT REACHES ITS LIMIT VALUE THAT IS NO PROBLEM
!TTAIN LIMIT VALUE 0 &UNCTIONS CAN ATTAIN LIMIT VALUES IN PROBLEM SOLV ING SITUATIONS )NT )F IT ;THE VARIABLE= HAD TENDED TO MINUS ONE INSTEAD AND THE SAME FUNCTION WHAT WOULD THAT BE ,EO -M IT BECOMES )NT -M CAN IT ATTAIN THAT VALUE ,EO 9ES
.ORDIC 3TUDIES IN -ATHEMATICS %DUCATION .O ,IMITS OF FUNCTIONS TRACES OF STUDENTS CONCEPT IMAGES
)NT #AN THIS FUNCTION EVER BE EQUAL TO TWO -ARTIN 9ES WHEN IT IS ONE THEN IT BECOMES TWO
$O NOT ATTAIN LIMIT VALUE &UNCTIONS CANNOT ATTAIN LIMIT VALUES -ARTIN YOU NEVER REACH THE LIMIT VALUE YOU APPROACH THE LIMIT VALUE %MMA 6ISUALLY IT FEELS LIKE ;= IF ONE CONTINUES LONG ENOUGH IT WILL REACH ITS LIMIT VALUE BUT IT DOES NOT SINCE IT DOES NOT REACH THE LIMIT VALUE IT IS PART OF THE DElNITION BUT
$ISTANCE BETWEEN FUNCTION AND LIMIT 3PEAKS OF DISTANCE BETWEEN LIMIT VALUE AND FUNCTION VALUES $AVID 9ES THE DIFFERENCE BETWEEN FX AND ! IS LESS OR EQUAL TO EPSILON
"ORDER ,IMITS AS BORDERS /LIVER 9OU GET CLOSE TO ANOTHER VALUE AS SOME KIND OF BORDER YES ;= !S A PRINCIPLE WHEN IT COMES TO LIMIT VALUES THERE CANNOT BE ANYTHING BEYOND *OHN 9OU THINK OF SOME BORDER OF SOME KIND THAT YOU COME TO A BORDER AND BEYOND IT YOU HAVE DECIDED SOMETHING OR ) THINK A LOT ABOUT THE BODY WHAT YOU CAN DO PHYSICALLY OR SOMETHING LIKE THAT
!PPROXIMATION ,IMITS AS APPROXIMATIONS OF FUNCTION VALUES -IKAEL 7E CAN DO IT AS ACCURATE AS WE WANT BUT WE CAN NEVER GET IT EXACT
%XACT ,IMITS AS EXACT VALUES $AN APPROXIMATIONS CAN BE DONE IN SEVERAL WAYS BUT A LIMIT VALUE IS UNIQUELY DETERMINED
!TTAINABILITY IS CLEARLY AN ISSUE IN NEED OF ATTENTION SINCE A CLEAR MAJORITY OF THE STUDENTS HAD CONCEPTIONS FROM BOTH ONE OF THE TWO lRST CATEGORIES AND THE THIRD CATEGORY 4HE LAST TWO CATEGORIES ON THE OTHER HAND NEATLY COMPLEMENT EACH OTHER 4HERE ARE NO CASES OF INCONSISTENCIES REGARDING LIMITS AS APPROXIMATIONS OR AS EXACT VALUES .OT ALL STUDENTS THOUGHT OF LIMITS AS EITHER AN EXACT OR AN APPROXIMATE VALUE 3IX OF THE STUDENTS STATED THAT LIMITS ARE UNATTAINABLE AND THAT LIMITS ARE ATTAINABLE IN PROBLEM SOLVING SITUATIONS BUT NOT IN THEORETICAL CON TEXTS 4HEORY AND PROBLEM SOLVING SEEM TO BE SEPARATE ISSUES FOR THESE STUDENTS 4HREE STUDENTS REVEALED AN OPPOSITE TRACE TO THE SIX STUDENTS
.ORDIC 3TUDIES IN -ATHEMATICS %DUCATION .O +2)34).! *54%2
WHEN THEY CLAIMED THAT LIMITS ARE NOT ATTAINABLE AND THAT LIMITS ARE AT TAINABLE IN THEORY BUT THEY DID NOT SAY THAT LIMITS ARE ATTAINABLE IN PROB LEM SOLVING 4HE THEORY USED APPEARS TO BE UNCLEAR FOR THE STUDENTS %MMA AND -ARTIN AS TWO EXAMPLES CONTRADICT THEMSELVES IN THE EXAM PLES GIVEN PREVIOUSLY 4HE DElNITION IS CAUSING %MMAS DIVIDED PERCEP TION AND -ARTIN SEEMS TO EVOKE DIFFERENT PARTS OF HIS CONCEPT IMAGE IN THE DIFFERENT SITUATIONS 3OME STUDENTS THOUGHT OF LIMITS AS OBJECTS AS SOMETHING A FUNCTIONS VA LUES CA N APPROACH 3UCH STATEMENT S BELONG TO T HE CATEGOR Y %XACT 3OME STUDENTS VIEWED LIMITS AS PROCESSES WITH A FOCUS ON THE FUNCTIONS VALUES
4ABLE 3TUDENTS SOLUTIONS TO 4ASK AND
4ASK 4ASK 2IGHT ,EFT ,IMIT &UNC 2IGHT ,EFT ,IMIT &UNC LIMIT LIMIT TION LIMIT LIMIT TION VALUE VALUE &ILIP E#### #### -ARTIN D# #&6## #&6# 4OMMY D##&6.$##.$ !NNA D # # #7 # # # #7 # *OHN C# #&6## #&6# &RANK C#### #### ,OUISE C#### ### ,EO B###7##### $AN B#### ###7# -IKAEL B&6&6&6,6#### $AVID B#### #### *ULIA A#### #### $ENNIS A# #&6## #&6# %MMA A#### #### /LIVER A##.$#####
.OTES #ORRECT ANSWERS # #ORRECT ANSWER #7 7RONG ANSWER FOLLOWED BY AN ADJUSTMENT TO A CORRECT ONE 4HE ERROR WAS THAT THE STUDENTS POINTED OUT THE FUNCTION VALUE IN ALL CASES 7RONG ANSWERS &6 #HOSE THE FUNCTION VALUE AT THE POINT INSTEAD OF THE LIMIT VALUE ,6 #HOSE THE LIMIT VALUE AT THE POINT INSTEAD OF THE FUNCTION VALUE .$ !NSWERED v.OT DElNEDv 4HE STUDENT WROTE 4HE STUDENT WROTE
.ORDIC 3TUDIES IN -ATHEMATICS %DUCATION .O ,IMITS OF FUNCTIONS TRACES OF STUDENTS CONCEPT IMAGES
GETTING CLOSER TO THE LIMIT VALUE 4HE CATEGORY !PPROXIMATION CONTAINS S T ATEMENT S OF T H I S K I ND 4 HE CATEGOR IES $ISTANCE BETWEEN FUNCTION AND LIMIT AND "ORDER CAN BE PERCEIVED BOTH AS PROCESS CATEGORIES AND OBJECT CATEGO RIES SINCE IT IS NOT CLEAR WHETHER THE STUDENTS CONSIDERED THE DISTANCE AND MOTION OR THE LIMIT AS THE MAIN OBJECT 4ABLE SHOWS THAT MOST STUDENTS SAW LIMITS AS BOTH OBJECTS AND PROCESSES &OUR STUDENTS HAD TRACES FROM ONLY ONE OR LESS OF THE LAST FOUR CATEGORIES /NLY TWO STUDENTS *ULIA AND $ENNIS SHOWED TRACES OF CONCEPT IMAGES WHICH ARE COHERENT 4HEY SHOWED NO TRACES OF PROBLEMS WITH FUNCTIONS ATTAINING LIMIT VALUES AND THEY BOTH EXPLICITLY STATED THAT FUNCTIONS CAN ATTAIN LIMIT VALUES 4HE REST OF THEIR CONCEPTIONS DID NOT MATCH EACH OTHER BUT THERE WERE NO CONTRADICTIONS EITHER 3TUDENTS WITH HIGH GRADES DID NOT HAVE A VERY DIFFERENT STRUCTURE IN THEIR PERCEPTIONS COMPARED TO THE OTHER STUDENTS 4HE ONE DIFFERENCE IS THAT THE TWO STUDENTS WITH COHERENT TRACES OF THEIR CONCEPT IMAGES WERE AMONG THE STUDENTS WITH THE HIGHEST GRADES 4ABLE SHOWS THE OUTCOME OF THE STUDENTS SOLUTIONS TO THE TASKS WITH THE GRAPHS 4ASK AND 4ASK 4HERE IS AN UNCERTAINTY ABOUT THE LIMIT VALUE IN THE RESULTS OF BOTH TASKS &IVE STUDENTS GAVE ALL ANSWERS CORRECTLY !LMOST ALL STUDENTS WERE ABLE TO DETERMINE RIGHT AND LEFT LIMIT VALUES 4HE POINT OF DISCONTINUITY CAUSED DIFlCULTIES FOR THE STUDENTS WHEN THEY WERE ASKED TO DETERMINE THE LIMIT VALUE (ALF OF THE GROUP OF STUDENTS MANAGED TO GIVE AN ACCURATE ANSWER AT THEIR lRST ATTEMPT -OST ERRORS WERE CONNECTED TO THE VALUE OF THE FUNCTION AT THE POINT IN BOTH TASKS 4HE RESULTS IN TABLE SHOW THAT THE NUMBER OF ERRORS IS SOMEWHAT HIGHER AMONG THE LOW AND AVERAGE PERFORMING STUDENTS AT 4ASK
$ISCUSSION 4HE RESULTS OF THE STUDY ARE DISCUSSED IN THIS SECTION AND COMPARED WITH THE LITERATURE PRESENTED EARLIER 4HE THREE RESEARCH QUESTIONS ARE ADDRESSED IN THE ORDER OF THE PRIOR PRESENTATION
(OW THE STUDENTS PERCEIVED LIMITS OF FUNCTIONS 4HE CURRENT STUDY SHOWS THAT STUDENTS PERCEIVE LIMITS AS OBJECTS AND AS PROCESSES .ONE OF THE PERCEPTIONS DOMINATE AND THEY SEEM INTERTWINED IN THE STUDENTS RESPONSES TO TASKS AND QUESTIONS !LMOST ALL STUDENTS STRONGLY STATED THAT LIMITS ARE UNATTAINABLE FOR FUNCTIONS BUT IN ANOTHER CONTEXT THEY ARE ATTAINABLE ! MAJORITY OF THE STUDENTS CONNECTED LIMITS TO DISTANCE WHICH THE FORMAL DElNITION IN THEIR TEXTBOOK ALSO DOES BUT
.ORDIC 3TUDIES IN -ATHEMATICS %DUCATION .O +2)34).! *54%2
THE HESITATION ABOUT THE DISCONTINUOUS POINTS IN 4ASK AND 4ASK AND THE RESULTS IN TABLE SHOW THAT THE DElNITION IN MOST CASES IS NOT REALLY CLEAR TO THE STUDENTS %IGHT OF THE STUDENTS CONSIDERED A LIMIT TO BE A BORDER 4HIS VIEW CAN CONSTRAIN THEIR THOUGHTS ABOUT FUNCTIONS AND PREVENT THEM FROM UN DERSTANDING THAT FUNCTIONS CAN FOR EXAMPLE OSCILLATE OVER AND UNDER THE LIMIT VALUE AND STILL TEND TO THAT LIMIT ! COMMON MISINTERPRETATION OF THE LIMIT DElNITION AMONG THESE STUDENTS IS THAT IT SAYS THAT LIMITS ARE NOT ATTAINABLE *UTER AND THAT WOULD MEAN THAT A FUNCTION CANNOT OSCILLATE OVER AND UNDER THE LIMIT VALUE 4HE IMPACT OF EVERYDAY LAN GUAGE -ONAGHAN IS PARTICULARLY OBVIOUS FROM THE EXAMPLES BELOW TABLE FOR THE CATEGORY v"ORDERv 4HE 3WEDISH WORD FOR LIMIT IS A SYNO NYM FOR THE WORD BORDER AND THERE IS HENCE A NATURAL TRANSITION BETWEEN THE WORDS ,IMITS BECOME CONNECTED TO BOUNDARIES FOR EXAMPLE PHYSI CAL LIMITATIONS WHICH ARE NOT SUPPOSED TO BE CROSSED )T IS CRUCIAL THAT STUDENTS DEVELOP THEIR MATHEMATICAL LANGUAGE WELL ENOUGH TO BE ABLE TO SEPARATE IT FROM EVERYDAY LANGUAGE -ATHEMATICAL LANGUAGE DEVELOPMENT HAS BEEN PROVEN TO TAKE MORE TIME THAN MOST STUDENTS HAVE IN A COURSE AT THE BASIC LEVEL 'REVHOLM ,IMITS OF FUNCTIONS HAVE BOTH PROCEDURAL AND STATIC FEATURES #OTTRILL ET AL 3FARD 4HE DUAL NATURE OF THE CONCEPT MAKES IT HARDER FOR THE STUDENTS TO EMBRACE IT )T IS NECESSARY TO PASS A STAGE WHERE LIMITS ARE SEEN AS PROCESSES AND OBJECTS AT THE SAME TIME TO REACH THE REIlCATION STAGE 3FARD -OST OF THE STUDENTS IN THIS STUDY HAD AT THE TIME NOT YET COME SO FAR IN THEIR DEVELOPMENT THAT THEY CLEARLY COULD COMPRE HEND THE FACETS OF THE NOTION AND GO BETWEEN LIMITS AS OBJECTS AND LIMITS AS PROCESSES 3EVERAL OF THE STUDENTS CONFUSED THE PROCESS OF APPROACHING A LIMIT VALUE WITH THE ACTUAL LIMIT VALUE AS THE OBJECT TO APPROACH
4HE INCOHERENCE OF THE TRACES OF CONCEPT IMAGES 4HE RESULTS OF THIS STUDY SHOW THE STUDENTS MOVEMENT BETWEEN DIFFER ENT WAYS OF PERCEIVING LIMITS OF FUNCTIONS 4HE COGNITIVE WAY TO REACH THE NOTION OF LIMIT IS VERY DIFFERENT FROM THE MATHEMATICAL WAY TO REACH IT 7ILLIAMS 4HE MATHEMATICAL MANNER IS STRICTLY FORMAL WITH DE DUCTIVE REASONING FROM AXIOMS AND DElNITIONS TO PROVE THEOREMS WHILE THE COGNITIVE WAY HAS VARIOUS STAGES OF ABSTRACTION FOR THE INDIVIDUAL TO GO THROUGH AND BACK AGAIN AS DESCRIBED BY 3FARD AND #OTTRILL ET AL 4HE SOMETIMES VERY DIFFERENT CHARACTERISTICS OF THE TWO WAYS TO ADVANCE TO A MATHEMATICAL CONCEPT CAN BE A REASON FOR THE INCOHER ENCE IN THE RESULTS
.ORDIC 3TUDIES IN -ATHEMATICS %DUCATION .O ,IMITS OF FUNCTIONS TRACES OF STUDENTS CONCEPT IMAGES
4HE STUDENTS WHO CONSIDERED LIMITS TO BE UNATTAINABLE EXCEPT WHEN THEY SOLVED PROBLEMS SHOW TWO SETTINGS OF CONCEPTIONS ONE FOR THEORY AND ONE FOR PROBLEM SOLVING 4HEY WERE NOT EVOKED AT THE SAME TIME WHICH GIVES THE INCOMPATIBLE RESULTS IN TABLE -ANY PROBLEM SOLVING SITUATIONS WERE MANAGEABLE FOR THE STUDENTS AND THEY COULD EXPLICITLY SEE THAT LIMITS ARE ATTAINABLE IN SOME CASES )N THEORETICAL DISCUSSIONS HOWEVER THEY SE LECTED THE DOMINANT AND IN THIS CASE NOT MOST SUITABLE PART OF THEIR CON CEPT IMAGES $AVIS AND 6INNER WHICH STATES THAT LIMITS ARE NOT ATTAINABLE 4HE IMPRESSION THAT LIMITS ARE UNATTAINABLE OFTEN COMES FROM THE DElNITION WITH STRICT INEQUALITIES *UTER 4HE LIMIT DElNI TION IS HARD FOR MANY STUDENTS TO UNDERSTAND #ORNU *UTER *UTER )F IT IS NOT IMPLEMENTED IN THE STUDENTS MENTAL NETWORKS SO THAT THEY UNDERSTAND THE CONCEPT (IEBERT AND #ARPENTER IT BE COMES ALMOST IMPOSSIBLE TO CONNECT THEORY AND PROBLEM SOLVING LEAVING THE STUDENTS WITH AN EVER PRESENT FEELING OF UNCERTAINTY ABOUT THEIR CAL CULATIONS 4HERE ARE CONTRADICTORY PERCEPTIONS WITHIN SEVERAL STUDENTS MINDS BUT SERIOUS INCONSISTENCIES BETWEEN DIFFERENT STUDENTS PERCEP TIONS ALSO OCCUR 4HE STUDENTS CONFUSION ABOUT WHAT HAPPENS AT THE APPROACHED POINTS SEEMS TO BE LINKED TO THE STUDENTS LACK OF CLARITY IN THEIR OWN PERCEP TIONS OF THE DElNITION !S WE HAVE SEEN IN TABLE MANY STUDENTS WERE NOT SURE WHETHER FUNCTIONS CAN ATTAIN LIMIT VALUES OR NOT 4HE DIFFERENT RESULTS FROM 0RZENIOSLO AND *UTER WHERE STUDENTS IN THE FORMER STUDY HAD A PERCEPTION THAT ONLY CONTINUOUS FUNCTIONS HAVE LIMITS AND STUDENTS IN THE LATTER ARGUED THAT A VALUE CANNOT BE A LIMIT VALUE IF THE FUNCTION CAN ATTAIN IT REVEAL QUITE OPPOSITE CONCEPTIONS IN STUDENTS CONCEPT IMAGES 7ILLIAMS INVESTIGATION WITH TWO STUDENTS ALSO SHOWED UNCERTAINTY ABOUT WHAT HAPPENS AT THE VALUE APPROACHED #ON TINUOUS FUNCTIONS REACH THEIR LIMIT VALUES BUT THE STUDENTS WERE HESITANT TO SAY THAT THE LIMIT WAS REACHED IN THE LIMIT PROCESS 4HESE FUNDAMEN TALLY DIFFERENT WAYS TO COMPREHEND THE NOTION OF LIMITS INDICATE THAT STU DENTS NEED MORE GUIDANCE FROM LECTURERS OR PEERS IN THEIR WORK 4HEIR CONCEPT IMAGES NEED TO BE CONFRONTED AND TESTED IN VARIOUS SITUATIONS TO MAKE IT CLEAR FOR THE STUDENTS IF THEIR MENTAL REPRESENTATIONS ARE VALID IN GENERAL OR JUST IN SPECIAL CASES
#OMPARISON OF STUDENTS WITH DIFFERENT GRADES 4HE STUDENTS ARE LISTED IN ORDER OF THEIR GRADES WITH THE STUDENTS WITH BEST GRADES LAST 3TUDENTS FROM *ULIA TO /LIVER HAVE THE HIGHEST GRADES 4HE ONLY TWO STUDENTS WITH COHERENT TRACES OF CONCEPT IMAGES WERE AMONG THESE HIGH ACHIEVERS *ULIA AND $ENNIS WHICH CONlRMS THE RESULTS OF
.ORDIC 3TUDIES IN -ATHEMATICS %DUCATION .O +2)34).! *54%2
#HINNAPPANS STUDY THAT HIGH ACHIEVERS CONSTRUCT REPRESENTATIONS OF HIGHER QUALITY /THER THAN THIS THERE ARE NO CLEAR PATTERNS SUGGESTING RELATIONS BETWEEN GRADES AND THE OUTCOME OF THE STUDY &ILIP THE ONLY ONE HAVING THE LOWEST GRADE HE DID NOT PASS THE SECOND COURSE SOLVED 4ASK AND 4ASK CORRECTLY /LIVER AND $ENNIS WHO BOTH BELONGED TO THOSE HAVING THE HIGHEST GRADES MADE MISTAKES SOLVING THE TASKS %VEN THOUGH $ENNIS HAD A COHERENT CONCEPT IMAGE HE WAS NOT ABLE TO SOLVE 4ASK AND 4ASK (E CONFUSED THE LIMIT VALUE WITH THE FUNCTION VALUE *ULIA SOLVED BOTH TASKS CORRECTLY 4HE HIGH ACHIEVERS HAD A BETTER RESULT AT SOLVING 4ASK 4HE DIFFERENT PERCEPTIONS OF LIMITS THAT ALMOST ALL STU DENTS SHOWED AND THE PROBLEMS WITH THE TASKS IN THIS STUDY INDICATE THAT THE STUDENTS STILL HAD CONCEPT IMAGES WITH MORE THAN ONE REPRESENTATION FOR THE SAME PART OF THE CONCEPT WITHOUT REALISING IT 4HE MERGE OF THE REPRESENTATIONS $REYFUS HAD NOT YET COME TO PASS
#ONCLUSIONS %ARLIER RESULTS FOUND IN LITERATURE AND THE DATA PRESENTED IN THIS PAPER SUGGEST THAT STUDENTS PERCEIVE LIMITS IN VERY DIFFERENT WAYS 4HE TRACES OF THE STUDENTS CONCEPT IMAGES WERE NOT COHERENT IN MOST CASES IN THIS STUDY NOT INDIVIDUALLY NOR AS A GROUP 4HE RESULTS IMPLY THAT THE STUDENTS NEEDED TO WORK FURTHER TO RELATE THE CONCEPT OF LIMIT IN PROBLEM SOLV ING SITUATIONS TO THE THEORY THEY LEARN )F THEY HAD HAD MORE OPPORTUNI TIES TO BECOME AWARE OF THE INCONSISTENCIES OF THEIR MENTAL REPRESENTA TIONS AND EXPERIENCED AN URGE TO MAKE THEM ACCURATE THE RESULTS OF THIS STUDY WOULD PROBABLY HAVE BEEN MORE POSITIVE -OST COURSES HAVE A TIGHT SCHEDULE AND THERE ARE MANY CONCEPTS TO HANDLE SO CREATING SUCH OPPOR TUNITIES IS NOT AN EASY THING TO DO BUT EVEN THE SMALLEST EFFORT TO PRO VOKE INCONSISTENCIES MIGHT GET SOME STUDENTS TO START QUESTIONING THEIR REPRESENTATIONS
2EFERENCES "LOMHJ - 6ANSKLIGHEDER VED TILEGNELSE AF MATEMATIK )N # "ERGSTEN %D $OKUMENTATION AV E MATEMATIKBIENNALEN ;0ROCEEDINGS OF E -ATEMATIKBIENNALEN= PP ,INKÚPING 5NIVERSITY #HINNAPPAN - 2ESTRUCTURING CONCEPTUAL AND PROCEDURAL KNOWLEDGE FOR PROBLEM REPRESENTATION )N ! /LIVIER %D 0ROCEEDINGS OF THE ND #ONFERENCE OF THE )NTERNATIONAL 'ROUP FOR THE 0SYCHOLOGY OF -ATHEMATICS %DUCATION 6OL PP 5NIVERSITY OF 3TELLENBOSCH 3OUTH !FRICA #ORNU " ,IMITS )N $ 4ALL %D !DVANCED -ATHEMATICAL 4HINKING PP $ORDRECHT +LUWER !CADEMIC 0UBLISHERS
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.ORDIC 3TUDIES IN -ATHEMATICS %DUCATION .O Article III
Mathematics Education Research Journal 2005, Vol. 17, No. 2, 91–110
Students’ Attitudes to Mathematics and Performance in Limits of Functions
Kristina Juter Kristianstad University College, Sweden
The main aim of this article is to discuss the attitudes to mathematics of students taking a basic mathematics course at a Swedish university, and to explore possible links between how well such students manage to solve tasks about limits of functions and their attitudes. Two groups, each of about a hundred students, were investigated using questionnaires, field notes and interviews. From the results presented a connection can be inferred between students’ attitudes to mathematics and their ability to solve limit tasks. Students with positive attitudes perform better in solving limit problems. The educational implications of these findings are also discussed.
Introduction When students learn a new mathematical concept or learn something further about an already known concept, they have to draw on parts of what they have already learnt or perhaps draw on related concepts. It is not only the mathematics per se that is recalled but also feelings and attitudes attached to the concept. The students’ mathematical beliefs serve as a filter that influences mathematical thoughts and actions (Pehkonen, 2001). Thus in a learning situation students have to grapple not only with the new mathematics, but also with their own experiences, confidence, common sense and prior learning. Limits of functions are for many students difficult to understand because of the nature of the concept. Students often confuse limit values with function values, the notion of infinity is sometimes not understood correctly, the limit definition is difficult to understand and there is a possible confusion between limits as processes and as static objects (Cornu, 1991; Davis & Vinner, 1986; Tall, 1980, 2001; Vinner, 1991). The multitude of challenges in learning about limits affect students in a number of possible ways, the most obvious one being that students who manage to overcome such barriers reinforce their positive attitudes and vice versa. But there are other possible scenarios, for example where students’ perseverance drives them to work despite the influence of negative attitudes and prior failure or where students have positive attitudes to mathematics and still do not manage to learn about limits of functions because of other factors. The purpose of this article is to identify students’ attitudes to mathematics in general and to limits in particular as they progressed through a basic mathematics course, and to compare these attitudes to the students’ abilities to solve limit tasks. The following questions will be addressed: 92 Juter
• What are the students’ attitudes to mathematics? • What are the relations between these attitudes and the learning of limits of functions? Two studies about students’ development of the concept of limits of functions were conducted in 2002 (Juter, 2003). I wanted to determine the students’ attitudes to mathematics first and then see how their conceptions of limits developed over a semester. The studies were conducted at a Swedish university. No such study has previously been conducted in Sweden. Theoretical background Attitudes and beliefs Beliefs, attitudes and other related words are often used synonymously in the field of mathematics education (Leder & Forgasz, 2002). Some researchers’ choices and definitions of such words are presented below to exemplify their diverse use. Leder and Forgasz argued that meaningful research can still be done without a single, universally accepted definition. McLeod (1992) speaks of stability in students’ affective responses. Beliefs and attitudes are often considered to be relatively stable while emotions change more readily. He distinguished between beliefs, attitudes and emotions. Beliefs are described as the ways in which the students perceive mathematics or their own ability. One example is that “mathematics is about solving problems”. Attitudes are, for example, dislikes of specific types of tasks. Emotions include the anxiety experienced in working through a proof. Mohammad Yusof and Tall (1994,1996) used “attitude” for all three words given by McLeod in their investigation of students’ attitudes before and after a university course with an emphasis on cooperative problem solving and reflection on the thoughts of the problem solving process. The students, aged 18 to 21 years, were enrolled in different courses and constituted a mixture of third to fifth year undergraduates. They were asked to complete a two-part attitudinal questionnaire before and after the course and also after completing a traditional course in mathematics. The first part of the questionnaire dealt with attitudes to mathematics and the other with attitudes to problem solving. Mohammad Yusof and Tall reported that the students’ attitudes were different before and after they had completed the more innovative course, but almost the same attitudes were displayed at the time of the first questionnaire and after the traditional course. These results suggest that students’ attitudes are directly dependent on input from the surrounding environment, but any changes produced do not necessarily remain after the input causing the changes has stopped. However, the results in Mohammad Yusof and Tall’s study may have been affected by the fact that the students were asked to respond to the same statements three times. Students’ attitudes that mathematics merely requires facts and procedures to be remembered emerged from the work of Mohammad Yusof and Tall (1994, 1996) as well as from Svege’s (1997) study about students’ Students’ Attitudes to Mathematics and Performance in Limits of Functions 93 beliefs, attitudes, and emotions concerning mathematics. Novak (1998), influenced by Ausubel, postulated a model of learning styles ranging from rote learning where there are no relations to existing knowledge, to meaningful learning where new knowledge is intentionally integrated in cognitive structures. Ausubel (2000) describes meaningful learning as “new symbolically expressed ideas (the learning task) […] related in a non arbitrary, and non verbatim fashion, to what the learner already knows” (p. 67). Students who rely on rote learning in learning mathematics are typically more anxious about mathematical work than other students (Entwistle, 1998). Anxiety lowers the students’ self confidence and the whole experience becomes unpleasant. Students who use rote learning are more likely to see the parts of mathematics they have learned as disjoint units of methods or rules, with the result that they can not see connections between different concepts or underlying crucial properties. Their mathematical understanding becomes weak and incomplete and it takes a lot of effort to sustain their mathematics studies as the pressure to remember rules, methods, theorems, and definitions mounts. Such a dysfunctional pattern is difficult to reverse since the students’ foundational mathematical knowledge has gaps, which have to be filled, at least partially. If the gaps remain, there is, in some areas, nothing in the students’ mental representations to which new knowledge can be connected. In such a case the new knowledge has no meaning for the students. They develop no sense of what is true and what is not. An investigation (Szydlik, 2000) on students’ beliefs and conceptual understanding revealed a connection between students’ beliefs about how mathematical truth is established and the understanding of limits. Students who preferred authorities such as lecturers and textbooks to establish mathematical truth held inadequate definitions and misconceptions of limits to a higher degree than students who relied on intuition, logic and empirical evidence. McLeod (1992) pointed to two different developments of attitudes. One comes from repeated emotional reactions to mathematics that become automatic, such as certain attitudes to a specific mathematical activity. The intensity of the emotional reaction will be weaker each time the situation occurs. The student’s reaction is triggered by the situation rather than by actual emotions. The second development comes from assigning an existing attitude to a new, similar situation. A positive attitude to proofs in analysis can, for example, spread to a positive attitude to proofs in algebra. Attitudes and achievement have a relation that is complex and can not be described as one depending on the other (McLeod, 1992). Attitudes and confidence depend, on the other hand, strongly upon one another. Attitudes to mathematics indicate whether a student is confident working with mathematics. The confidence emerges from various personal experiences, derived from emotions, accomplishment, social settings and all other inner and outer stimuli. Therefore, it is important that all students experience success in their mathematics education. Students’ own knowledge, or their 94 Juter perceptions of their own knowledge, is hence an important factor in attitudinal discussions. The nature of knowledge as objective and subjective is discussed in Pehkonen (2001). Objective knowledge is the generally accepted structure within the mathematics community, while subjective knowledge is individual and based on personal experience and attainment. These types of knowledge are linked together and influence each other. Students taking a course in analysis simultaneously develop their mathematics and their attitudes towards the subject. Their subjective knowledge, affected by their attitudes, can influence the development in mathematics either in a positive way or in a negative way. If students think mathematics is all about formulas and counting, then it can be difficult for them to solve problems later on. Pehkonen compared systems of knowledge with systems of beliefs. Knowledge is often structured in a logical manner while beliefs have a tendency to occur in a quasi logical structure. This means that individuals use their own axioms and logic rules. These are not always the same as those used by mathematicians. Belief systems have two other distinctive features beside quasi logic. One is the psychological importance, which is the strength of the beliefs. The other feature is the beliefs’ cluster structure, which means that the beliefs are gathered in clusters that do not have to be linked together directly. The structures of knowledge and beliefs, however, are linked together in mental representations. Mental representations and understanding A concept image is the total cognitive representation of a notion that an individual has in his or her mind (Tall & Vinner, 1981). A concept can be represented in more than one way and there can even be conflicting representations that are evoked at different times, depending on the context. If the representations are not contradictory they can merge into one when the individual is able to see the connections (Dreyfus, 1991). If they are incoherent in any aspect, a conflict may arise. The incoherence can, for example, relate to interpretations of rules or perceptions of definitions in different contexts. Mathematical representations can be compared to webs of mathematical concepts with connections between them (Novak, 1998). Attitudes influence the constructions of mental networks. If a specific concept is stimulating for a student, he or she is more likely to devote time and effort to work with it and compare it to other concepts. Then more and stronger links are made and that part of the concept image is more likely to make sense to the student. Even if not everything about the concept is clear, the time spent on working and trying to make sense of it strengthens the concept image. The influence goes in the opposite direction too, as constructions influence attitudes. The construction of the cognitive representations involves pattern recognition, categorisation and association (Hannula, 2002) and students’ attitudes are dependent on how well they think they manage to come through the Students’ Attitudes to Mathematics and Performance in Limits of Functions 95 cognitive processes. The attitudes are also dependent on emotions, to a greater degree than on cognition (Hannula, 2002). Attitudes are hence an important factor in students’ constructions of mental representations. Hiebert and Carpenter (1992) define understanding of a mathematical concept to be something an individual has achieved when he or she can handle the concept as a part of a mental network, which is similar to Novak’s (1998) and Ausubel’s (2000) descriptions of meaningful learning. The more connections there are between mental representations, the better the individual understands the concept (Dreyfus, 1991; Hiebert & Carpenter, 1992). If a concept is complicated in its nature, the connections become harder to make. The dual nature of limits of functions There is an ambiguity in the way the concept of limits can be perceived. One can focus on the process of approaching the limit and hence consider it a never ending procedure. But one can also think of the limit as a static entity, or object, to which functions can be compared. Sfard (1991) and Cottrill et al. (1996) have described theories of object formation from processes. A notion is first regarded as a process and when the characteristics of the notion become clear the notion can be seen as an object upon which other processes can be applied. These objects and processes form mental schemas to represent the concept and connections to it. Such schemas constitute individuals’ concept images. Cottrill et al. (1996) concluded that the formal concept of a limit is an intricate dynamical schema and not a static one. It is important to have a strong dynamical conception of the notion before it is possible to embrace fully a more formal interpretation where the limit is seen as an object. In the development of refining the schema of limits there are leaps between seeing a limit as a process and as an object. Limits are taught at upper secondary school in Sweden, but only in an informal intuitive manner with the focus on limits as processes. Therefore, the students have some conceptions of limits when they come to university. These conceptions can be difficult to change since they have been sufficient until the students started at the university. Hence these students do not feel a need to learn the formal definition (Williams, 1991) or to work further to understand the dual nature described above. Tsamir and Tirosh (2002) described intuitive beliefs as enduring self evident cognition forms. The strong feelings of certainty can cause a false sense of understanding. The study The questions explored in this study, as already indicated in the introduction, are: • What are the students’ attitudes to mathematics? • What are the relations between these attitudes and the learning of limits of functions? 96 Juter
The methods and instruments used in the study were both quantitative and qualitative, but the data presented in this article are mainly quantitative to give a clearer picture of possible patterns in the students’ responses to the questions and tasks. In the data collection, students were variously considered as a group, and as individuals, to elicit as much information as possible about their attitudes to mathematics and knowledge of limits of functions. Details of the sample, methods and instruments are presented and discussed in this section. The sample The study was conducted in two stages with two different groups. The first group comprised 143 students who took part in the first part of the study in the spring semester of 2002. The following semester, autumn of 2002, 112 students participated in the second part of the study. About one third of the students were women, 36% in the first part and 29% in the second. All students were enrolled in a 20-week, full-time, basic level mathematics course covering algebra and analysis. Each group of students was taught by different teachers, one in algebra and one in analysis. The lectures were held three days per week with two lectures per day to the whole group. After the lectures, the students were separated into smaller groups for a teacher led problem solving session. I was not teaching any of the students and I did not know any of them. The groups were slightly differently composed as the spring group had a larger number of students whose main interests were in biology, chemistry or some other non-mathematical topic. Students who study mathematics because their non-mathematical main interests require it are not likely to have attitudes as positive to mathematics as students with mathematics as their main topic of study. The differences in composition of the groups affected the study as we will see later in this article. Methods and data analysis The data were collected at three stages – stage A, stage B, and stage C – with each group of students. There were 132 students in the spring study and 99 students in the autumn study who responded to a questionnaire at stage A in the second week of the course. The questionnaire was designed to determine the students’ attitudes to mathematics through seven attitudinal statements, and their knowledge about limits from upper secondary school through some easy limit tasks. Only the attitudinal part is described in this article to show the students’ attitudes before limits were taught. The instruments used are discussed more fully in the next section. The students’ responses were sorted, with data from the two groups kept separate. The students were individually categorised with respect to their responses to the attitudinal part of the questionnaire. The last four of the seven statements (see the instruments section) were used to categorise the students as “confident” or “unconfident”. A typical “confident” student agreed with Students’ Attitudes to Mathematics and Performance in Limits of Functions 97 statements 5 and 7 and disagreed with statements 4 and 6. An “unconfident” student responded in the opposite way. Categorisation patterns were defined by at least three out of four consistent responses to these four statements. Students with some neutral responses to some of the statements but otherwise following one of the categorisation patterns were added to the relevant category. The aim with the categorisation was to be able to compare confident students’ abilities to solve limit tasks with unconfident students’ abilities to solve limit tasks. This categorisation of students does not take different degrees of confidence into account but, as will be shown further on, a majority of the students could be categorised by this method. Limits were introduced to the students between stage A and stage B. At stage B, in the fifth week of the course, 106 of the students in the spring study responded to a second questionnaire with limit tasks and attitudinal questions. The attitudinal part, as described in the instruments section, included four open questions about what the students found most interesting, stimulating and difficult in the course. From the answers to these four questions, the most commonly mentioned concepts were listed to show which concepts the students thought of in the given contexts. The instruments used in the two groups were not identical. One reason for this was that some tasks were trialled in the spring study. The students in the autumn course were not asked the four questions because of time constraints. This was the only difference in the instruments used in the study described in this article. However, the responses to the four items are relevant for the questions posed in this article; hence they are included in the data. The students in both groups responded to a questionnaire at stage C in the seventh week of the course. The questionnaire comprised two tasks about limits with given solutions, which are described in the instruments section. Solutions were given by 87 students from the spring study and 78 students from the autumn study. The tasks are called Task 1 and Task 2 in a later section of the article. The students’ solutions to the tasks were transcribed and put into a computer program for categorisation. In this article I will only use the categories representing complete and correct solutions to measure the students’ abilities to solve limit tasks. Examples of student solutions and expected solutions are provided in later sections. The number of students in the groups and the methods used at the three stages are summarised in Table 1. 98 Juter
Table 1 Number of students enrolled, instruments used and time at each stage
Spring group Autumn group Instruments Time
Stage A 132 99 Attitudinal tasks Second week Stage B 106 – Attitudinal tasks Fifth week Stage C 87 78 Limit tasks Seventh week
Field notes were taken for both groups to shed some light on the students’ activities during lectures and in the classroom where tasks were solved. Students were together for the lectures but were divided into smaller groups with 20 to 30 students for the task solving sessions. I followed one such group and almost all the lectures, as an observer, when they were dealing with limits. The field notes were organised so that each student was represented by a square in a grid representing the classroom at the group sessions, and all actions such as asking questions, solving tasks at the black board or commenting on something were written down to give a picture of the activities during each lesson. The students were less active during the lectures and there were too many students to consider them individually. Therefore, the data collection at the lectures consisted of notes of what the lecturer presented and questions and comments from the students. Fifteen of the students in the autumn study were interviewed on two different occasions to capture as many individual experiences as possible (Juter, 2003). The results of the interviews are not presented in this article.
Instruments The students responded to a questionnaire in the beginning of the semesters, at stage A. The following statements, based on Mohammad Yusof and Tall (1994), were part of the questionnaire. 1. Mathematics is a collection of facts and processes to be remembered. 2. Mathematics is about solving problems. 3. Mathematics is about coming up with new ideas. 4. I learn mathematics through rote learning. 5. I usually understand a mathematical idea quickly. 6. I have to work very hard to understand mathematics. 7. I can connect mathematical ideas that I have learned. Students’ Attitudes to Mathematics and Performance in Limits of Functions 99
The students had five possible responses for each given proposition. They indicated with: • Y if they agreed strongly, • y if they agreed, • – if they neither agreed nor disagreed, • n if they disagreed, and • N if they strongly disagreed. The questionnaires also contained questions about limits of functions. The students in the spring course were asked the following questions at stage B: 1. What has been the most important thing you have learned in the analysis part of the course to date? 2. What has most stimulated you in the analysis part of the course to date? 3. What has been hardest to understand? 4. Write what you still do not understand. The questions were designed to allow the students to express their priorities and experiences of mathematics in general. The overall purpose was to see how the students mentioned limits of functions among other mathematical concepts. The questionnaire at stage C contained tasks about limits of functions for the students to solve. The questionnaire comprised tasks with presented solutions and tasks without solutions. The solutions were given to challenge the students to react, i.e., either to agree or to come up with alternative solutions. If a task was too difficult at first sight and there was no pressure to produce a solution, the students could leave it unsolved. But an attempt at a solution or a whole solution might stimulate some sort of response from the students. The two following tasks with solutions were given to both groups. 2 lim x + x Task 1: Problem: Decide the following limit value: ➔ . x 1 x2 – 1 x 2 + x x(x + 1) x Solution: = =➔ ∞ when x ➔ 1. x 2 – 1 (x – 1) (x + 1) x – 1 Adjustments (What will change or be completed and why). 2 lim x sin . Task 2: Problem: Decide the following limit value: x ➔ ∞ ( x ) 2 sin 2 ( x ) 2 1 Solution: xsin= . We know that➔ 0 when x ➔ ∞ and ➔ 0 ( x ) 1 x x 2 x sin sinx ( x ) when x ➔ ∞. The limit value lim = 1 implies that ➔ 1 x ➔ ∞ x 1 x when x ➔ ∞. Adjustments (What will change or be completed and why). 100 Juter
In Task 1 the students were expected to calculate left and right limit values and to see that they are not the same. The given solution to Task 2 does not compensate for the constant 2 and the students were expected to rewrite 2 2 sin sin ( x ) ( x ) the function,= . 2, and then use the standard limit value 1 2 x x in the given solution. Results The attitudinal statements at stage A The statements from the questionnaires and the results of the students’ responses are presented in Table 2. The numbers do not add up to 100%, because not all the students responded to all the statements.
Table 2 Percentage responses to the attitudinal questions: Spring study 2002 and Autumn study 2002.
Statement Y y – n N
1. Mathematics is a collection of facts Spring 10 43 15 21 10 and processes to be remembered. Autumn 3 34 16 28 16
2. Mathematics is about solving Spring 57 40 1.5 1.5 0 problems. Autumn 45 42 7 2 2
3. Mathematics is about coming up Spring 17 39 27 15 1.5 with new ideas. Autumn 17 45 26 6 3
4. I learn mathematics through rote Spring 3.1 21 16 37 21 learning. Autumn 0 19 15 46 18
5. I usually understand a mathematical Spring 10 30 27 28 3.8 idea quickly. Autumn 6 39 23 24 4
6. I have to work very hard to Spring 10 35 18 27 9 understand mathematics. Autumn 10 25 29 23 11
7. I can connect mathematical ideas Spring 14 50 25 10 0.8 that I have learned. Autumn 10 62 22 5 0 Students’ Attitudes to Mathematics and Performance in Limits of Functions 101
The two groups of students had very similar responses to the questionnaire, though there are small variations. A larger proportion of the students in the spring study agreed with statements 1, 2, 4 and 6 than of the students in the autumn study. A larger proportion of the students in the autumn study agreed with statements 3, 5 and 7. According to these numbers, the students in the autumn study were slightly more confident about their own abilities in mathematics, which is not surprising since a number of students in the spring group did not have mathematics as their main field of study. About 45% of the students considered mathematics to be facts and processes to be remembered, as in the studies by Mohammad Yusof and Tall (1994) and Svege (1997). More than 90% of the students thought that mathematics was about solving problems and 60% of them agreed with the statement that mathematics is about coming up with new ideas (27% of the students were neutral on this statement). Around 60% of the students stated that they do not learn mathematics through rote learning but over 20% claimed to do so and 15% were neutral. In contrast to the results of this study, 75% of the students in Mohammad Yusof and Tall’s (1994) study claimed to learn by rote. There were nearly as many students who claimed to understand a mathematical idea quickly (statement 5) as there were in agreement with the opposite statement. The same goes for the number of students who had to work hard to understand mathematics (statement 6). Most students stated that they could connect mathematical ideas that they have learned (statement 7). The four questions at stage B The questions were answered by 106 students, and the most frequently mentioned concepts are listed in Table 3.
Table 3 Students’ responses to the four questions in the spring group. Number and (%).
1. Most 2. Most 3. Hardest to 4. Still do not Concepts important stimulating understand understand
Limits 25 (24) 8 (8) 58 (55) 35 (33) Trigonometry 18 (17) 14 (13) 13 (12) 5 (5) Derivatives 5 (5) 1 (1) 2 (2) 1 (1) Functions 4 (4) 2 (2) 7 (7) 4 (4) Proofs 7 (7) 4 (4) 11 (10) 5 (5) Diophantic equations – 8 (8) 2 (2) 3 (3) Complex numbers 4 (4) 4 (4) 1 (1) 2 (2) 102 Juter
The data presented in Table 3 indicate the students’ perceptions of the mathematics in the course. Limits are considered to be the most important and hardest to understand but not the most stimulating by this group of students. The last two concepts in the table were taught in the algebra part of the course and not in the analysis part, but the students mentioned them here anyway. Task 1 and Task 2 at stage C Table 4 displays the percentages of the students in each group who were able to solve the tasks completely and correctly. There were 87 students in the spring group and 78 in the autumn group involved in this part of the study.
Table 4 Percentage of the students solving the tasks completely and correctly.
Spring group Autumn group
Task 1 21 36 Task 2 11 21
As expected, the students in the autumn study show better results in solving these two tasks. One common error, which 17% of the spring group and 15% of the autumn group made, was to use a method which works for rational functions when x tends to infinity but not when x tends to one as in Task 1. An example of a student solution of this type is the following:
2 1 x 2 + x x (1 + x ) 1 + 1 = ➔ ➔ ∞ x 2 – 1 1 1 – 1 x 2 1 – ( x 2 )
The limit process is taken in two steps where the problem with different left and right limit values is ignored. As is indicated by the data in Table 4, Task 2 was more difficult for the students to solve. The most commonly used method was to reason in terms of infinity, often locally, which means that the students did not regard the features of the whole function as x tends to infinity. For example:
1 ➔ ➔ ∞ ➔ ∞ x 0 when x so the denominator has to go and the expression ➔ ∞ . Students’ Attitudes to Mathematics and Performance in Limits of Functions 103
The results so far have focussed on the groups as such and not on the students as individuals. It is interesting to see how students strongly confident in their own capability coped with the tasks compared to students with weak self-confidence. The categorisation described in the method section gave the result that 61 (47%) students from the spring study and 54 (55%) students from the autumn study were categorised as “confident” and 28 (21%) students from the spring study and 16 (16%) students from the autumn study were categorised as “unconfident”. This shows again that the autumn students were more confident. Not all students solved the tasks. There were 35 from the 61 “confident” spring students, 17 from the 28 “unconfident” spring students, 36 from the 54 “confident” autumn students and 9 from the 16 “unconfident” autumn students who did. Table 5 shows the results of the students’ success at solving the tasks with respect to their overall confidence in mathematics.
Table 5 Number and (%) of correct solutions to Task 1 and Task 2 by students who responded to both statements and tasks.
Confident: Unconfident: Confident: Unconfident: Task Spring Spring Autumn Autumn
Task 1 5 (14) 2 (12) 13 (36) 2 (22) Task 2 6 (17) 1 (6) 9 (25) 0 (0) Total number of students 35 17 36 9
The “confident” students in the autumn study had a higher rate of correct answers than did the “confident” students in the spring study. The results of the “unconfident” students’ task solving are not very clear, which may be explained by having too few students in these categories. The field notes The field notes revealed that the students were not very active during lectures or in class. There were not many questions or remarks from the students during the lectures that I observed. The task solving lessons were planned for the students to solve problems at the blackboard, with a teacher helping if necessary, but the students’ unwillingness to do that often left the teacher solving the students’ problems. In the autumn study the teacher at the task solving sessions asked the students about the limit tasks. The first question to the students was whether they found the limit value tasks harder than the other tasks. The answer from the class was yes. They said that they had not met epsilons and deltas before and this was hard for them to understand. The second time was two days 104 Juter later when the teacher asked if the students found a specific kind of task harder than tasks that involved only calculation. The students said that they did and that they preferred to use standard limit values. The specific task x + 7 was a function, , that was supposed to be proved, with the limit value x – 3 definition, to be continuous at x = 2. Not all students participated in the discussion. Discussion The results from the previous section, together with the theoretical background, are used to address the research questions: What are the students’ attitudes to mathematics? What are the relations between these attitudes and the learning of limits of functions? The questions are discussed under three headings relating to students’ abilities to solve limit problems, understanding of the limit concept, and attitudinal changes. Educational implications are linked to the discussion under each heading. Students’ abilities to solve problems The results of this study suggest that there is a connection between students’ positive or confident attitudes to mathematics and their ability to solve limit tasks successfully. Which one depends on the other is not possible to tell from this study and perhaps not at all (McLeod, 1992). The results from the groups presented in Table 4, as well as from the students as individuals in Table 5, imply that students with positive confidence perform better in solving problems about limits of functions, or that students who are capable of solving problems have a positive attitude to mathematics. Mohammad Yusof and Tall (1994, 1996) also reported that students participating in a course with an emphasis on problem solving processes showed more positive attitudes to mathematics and problem solving. Thus when teaching limits of functions in particular, it seems important to ensure that the students’ learning environment offers varied opportunities for discussion and problem solving. In that way students can consolidate their current knowledge and be stretched to enhance their skills. There are several students who claimed that they had to work very hard to understand mathematics (statement 6 in Table 2), and that they did not understand a mathematical idea quickly (statement 5). Examples of this were found in the field notes. One student had solved a task at the blackboard during a problem solving session. He used the limit definition. When he was finished, he asked the teacher: “What have I proved?”. Other students in the group agreed with him and they thought it was hard to know when something was proved with the limit definition. So although the students worked with the suggested tasks, they did not always understand how they used the mathematics. On the other hand, there were almost no students who stated that they could not connect mathematical ideas they had learned (statement Students’ Attitudes to Mathematics and Performance in Limits of Functions 105
7). The responses to these three statements (statements 5, 6 and 7) are both encouraging and dispiriting. The result of the first two of them suggests that if students have time to spend on mathematics, then they can learn it. But the third statement (statement 7) can also be true. If an individual does not have the time or inclination to work, then there can not be a successful connection between the new and the existing mathematical parts in the concept image (Tall & Vinner, 1981). Perhaps students do not see the new parts as learned if they are not fully integrated with the mental networks of mathematics. Aspects of understanding The fact that so many students in the present study, as well as in Mohammad Yusof and Tall’s (1994) and Svege’s (1997) studies, see mathematics as something to be remembered indicates that understanding is not the goal of the students’ learning processes or perhaps of the teaching. This is emphasised by the statement that mathematics is about solving problems. It seems that the students’ efforts are focused on the problems and solution methods rather than on the theory. This situation is an example of Pehkonen’s (2001) description of the influence of beliefs on the learning of mathematics, where the students’ subjective knowledge interferes with their objective knowledge. But the students’ ideas of what constitutes a mathematical problem are not clear. It can be any task or something that is not trivial for the students to solve (Schoenfeld, 1992). If students think that mathematical problems are not only to find solutions to tasks, but also to justify conjectures or to understand proofs, then I would not say that the theory part is neglected. The field notes show that most students did not want to solve problems in class. Apart from the tasks being too difficult for the students, there can be other reasons for this, for example doubt about their own mathematical ability or shyness. These problem solving sessions also affect students’ attitudes. Confident and successful students who solve tasks in front of the class strengthen their positive attitudes, while inactive students with low mathematical self-confidence do not get, or take, the opportunity to feel success through interactions with peers. The responses to statement 3 (Table 2) stand in contrast to a shallow view of mathematics, as quite a few students thought that mathematics was about coming up with new ideas. If students only work with standard tasks and do not consider the underlying theory, then they will probably find it hard to discover new things. There is a better chance for them to get new ideas if they are bold and explore the features of the processes and objects on which they are working rather than just rely on texts and lecturers’ words. Szydlik (2000) claimed bold students are more likely to have accurate conceptions of limits of functions. The students are certainly aware that they should be prepared to explore new approaches and perhaps want to work in that way, but understanding often takes time, and time has a tendency to run out. A strategic selection of what to study can favour routine-tasks and rote learning (Mohammad Yusof & Tall, 1994). Courses are often assessed with 106 Juter similar tasks, so the choice to skip theory is not surprising. Theorems and proofs can be learned by heart as well; in this way it might seem as if a student has understood a concept even if that is not the case. About 60% of the students in this study claimed not to learn by rote, which implies that there is a rather large number of students who do. If a student is learning limits of functions by rote, what are the specific features that he or she will remember? Many students find limits of functions to be difficult to understand as we have shown before. Selecting the appropriate method to use when dealing with limits is often not trivial. The formalistic formulation of the definition, and for many students the new way of thinking, compound the difficulties. If students are unable to see and understand the concept’s critical features, then they do not know what to learn by heart. They see that several methods are present in textbooks and at lectures and feel pressured to make sense of them all, which possibly adds to their anxiety (Entwistle, 1998). There are methods that are systematically used, which students learn and remember, for example to use the dominant term when they work with rational functions where the independent variable tends to infinity. The students in the autumn study had learned that method for rational functions and then they got a rational function where the independent variable tended to one. Many of the students started to divide by the dominant term, which is the term that would have dominated if x tended to infinity, even though it did not help them to solve the problem. They had simply recognised a rational function as a surface property, and they did not remember why they used the method with the dominant term. This illustrates the danger of learning a method without fully understanding it. Attitudinal changes The notion of limits of functions is such that it takes time and effort for most students to understand fully the meaning of the concise formalised definition and to integrate it with their existing mental representations (Cornu, 1991; Davis & Vinner, 1986; Juter, 2003, 2004; Szydlik, 2000; Tall & Vinner, 1981; Williams, 1991). Students have to devote a lot of energy and, if they are excited and curious about the subject, there is something that drives them to make sense of what they work with. If their first experiences with limits are positive, then the attitudinal development triggered by emotional reactions (McLeod, 1992) has a chance to grow strong (Pehkonen, 2001) and be sustained during the demanding nature of the learning process. One outcome of Mohammad Yusof and Tall’s (1994, 1996) study was that even though attitudinal changes did occur, the attitudes tended to change back again after some time. The information summarised in Table 3 indicates that limits of functions are considered to be important, but very hard to understand, so these attitudes have a strong psychological importance (Pehkonen, 2001). Hence there has to be strong positive experiences for the students to change such attitudes and maintain the changes. Students’ Attitudes to Mathematics and Performance in Limits of Functions 107
Intuitive beliefs (Tsamir & Tirosh, 2002) that are inconsistent with the formal concept definition are possible causes for misplaced high self- confidence. Intuitive beliefs need to be challenged to bring students to understand that there is much more about limits to investigate and make sense of. This in turn can get the students to lose their confidence, not only about limits but about their total ability to work with mathematics. This is, however, a necessary risk to be taken if students are to get a chance to create true and rich concept images (Tall & Vinner, 1981), and to understand their meaning (Dreyfus, 1991; Hiebert & Carpenter, 1992). As I see it, the teacher’s role involves giving a balanced challenge so that students experience the need to modify their concept images intertwined with moments of success in order to maintain or develop positive attitudes towards mathematics. This study traced students’ attitudes and abilities measured at separate, distinct times. This approach may prevent changes that take place between periods of data collection being detected. If students successfully construct their cognitive representations over a period of time, then their attitudes to mathematics are likely to become more positive (Hannula, 2002). When they strike a problem, the positive attitudes become less positive, or more negative. There can then be several sets of attitudinal clusters or clusters of beliefs (Pehkonen, 2001) which are connected to the different contexts in which they were created. The clusters can be disjoint and, depending on the input from questions or elsewhere, the students evoke different clusters; this might confound the findings in the data collected. Therefore, it is important to keep in mind the different stages of the course when the data were collected. The data presented in Table 2 were gathered before the students had worked with limits in the course. The rest of the results presented are from the time after limits of functions had been presented and many of the students struggled to integrate this new material with what they already knew. The students’ attitudes described in this article are what lecturers and teachers have to consider in their classrooms. The students have developed these attitudes through many years of schooling and they are not easily changed over a semester. Most changes do not occur without effort. Therefore, for changes to occur, explicit attempts must be made by the teacher and the student. Conclusions To answer the first of the research questions (What are the students’ attitudes to mathematics?), a majority of the students considered mathematics to be facts and processes to remember and mathematics to be about solving problems but also about coming up with new ideas. Most students claimed not to learn by rote. The majority of the students stated that they could synthesise mathematical ideas that they had learned, but many students had to work very hard to understand mathematics. Limits of functions were regarded to be most important in the analysis course, but also most difficult 108 Juter to understand. This implies that most students in the study had to work hard to integrate the new knowledge to existing concept images. The fact that they thought limits were important probably encouraged them not to give up their attempts to learn limits. To address the second question (What are the relations between these attitudes and the learning of limits of functions?), the results of the study show that confident students perform better in solving limit problems than unconfident students do. The students’ attitudes were contradictory at some points. Students who claimed to be able to connect mathematical ideas they have learned, but who regard mathematics to be facts and processes to remember can have a meaningful learning approach if they see remembering as a synonym for learning. If they have a rote learning strategy instead, the connections to other mathematical ideas are likely to be weak and based on first sight similarities as exemplified in this study. The results from the present study provide teachers with evidence of students’ efforts to learn limits, and of their attitudes to mathematics. The relationship between attitudes and achievement in solving limit problems implies that time should be spent on enhancing positive attitudes. Since so many (55%) of the students in the spring study found limits to be the hardest part of the analysis course to understand, there is reason to work through limits thoroughly and use various problems at different levels to help students understand the concept’s features, and hence to be able to make strong and meaningful mental connections to other concepts.
Acknowledgement: I want to thank Professor Gilah Leder and several anonymous reviewers for their comments and helpful suggestions on earlier drafts of this article.
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Author Kristina Juter, Kristianstad University College, Department of Mathematics and Science, 291 88, Kristianstad, Sweden. Email:
Limits of Functions as They Developed Through Time and as Students Learn Them Today
Kristina Juter Kristianstad University College, Sweden
In this article, the ways in which first year university students’ develop the concept of limits are compared to the historical development of the concept. The aim is to find out if the students perceive the notion as mathematicians of the past did as understandings of the concept evolved. The results imply that there are some similarities, for example, the struggle with rigour and attainability. Knowledge of such critical areas can be used to improve the students’ opportunities of learning limits of functions. Some teaching aspects are also discussed.
Introduction Limits of functions have developed over many centuries to reach the current level of stringency (Katz, 1998). Mathematicians have struggled with the concept for various reasons. Some of the reasons are mentioned here to indicate the development of limits of functions through time. The historical development of limits shows some crucial points, for example the issue of rigour, which students also need to address as they learn mathematics. A comparison of students’ conceptual developments with the historical development can have many theoretical and methodological complications as history and psychology are two different domains (Furinghetti & Radford, 2002). The meaning of mathematics has changed over the years and it has also meant different things in different cultures (Grugnetti & Rogers, 2000), which further complicates comparisons. A comparison can, despite the possible complications, be useful and informative, for instance in finding critical learning passages. The historical development can also, as Bartolini Bussi and Sierpinska (2000) and Grugnetti and Rogers (2000) argued, inspire students to learn mathematics. Today students learn limits of functions at upper secondary schools and universities. They, too, have some obstacles to overcome. The
1 definition causes problems, the concept of infinity makes limits unclear, and there are other potential obstacles when learning limits of functions (Cornu, 1991; Davis & Vinner, 1986; Juter, 2003; Tall, 1980; Tall, 2001; Vinner, 1991). Different universities and schools, both nationally and internationally, teach limits in different ways. The sample of this article comprises students from a Swedish university. They are taught the formal H - G definition in their first calculus course. Some of the students have already seen the definition at upper secondary school and all of them are supposed to have had an intuitive introduction at upper secondary school. The aim of this article is to compare, with proper caution, the historical development of limits of functions with the way 15 Swedish university students learned this topic. The questions investigated are: What similarities do the students’ developments have with the historical development? How, historically, has rigour of limits been handled, and how do the students cope with rigour now? The article concludes with a consideration of some teaching and learning issues.
Limits, Evanescent Quantities and Other Elusive Concepts The use of limits of functions did not require a strict formal definition, in our modern sense, from the start. Initially, an implicit treatment was considered sufficient. One example from about four centuries before Christ 2 A1 d1 was given by Hippocrates of Chios who proved that 2 where A2 d 2
A1 , A2 , d1 and d 2 are areas and diameters of two circles (Edwards, 1979). According to Edwards, Hippocrates probably inscribed regular polygons and let the number of sides tend to infinity to fill the circles, or exhaust them. In this limit process he presumably regarded infinitesimals as something to discard as zero, even though he did not have a strict enough limit concept to describe the infinitesimals. This type of geometrical use of limits was not causing problems with the implicit notion of limits. Archimedes worked with foundations of calculus but he did not introduce limits because of his fear of the infinite (Edwards, 1979). Instead he used a method similar to Hippocrates’s, for example, filling a sphere with a sufficient number of pyramids with the sphere’s radius as height (Thiele, 2003). This way of proving geometrical properties was the beginning of indivisibles. In the 17th century Cavalieri was also reluctant to use limits (Edwards, 1979). He thought that rigor was a philosophical matter. Cavalieri introduced the concept of indivisibles in 1635 (van Maanen, 2003), but he did not explain what he meant by indivisibles (Boyer, 1949). He used infinitesimal volumes or areas, and called them indivisibles, to show that two geometrical objects have the same size. He hid the limit concept in his computations and did not address it (Edwards, 1979).
2 Fermat, like Hippocrates, regarded infinitesimals as zero in his work with maximum and minimum problems (Edwards, 1979). He first created a pseudo equality where he added an e so that f(x+e) | f(x). Then he divided by e and finally discarded e. He used, for example, f (x) x(b x) bx x 2 so he got the pseudo equality and calculations: b(x e) (x e) 2 bx be x 2 2xe e 2 | bx x 2
2x e | b
b x 2 Fermat provided only partial explanations for the logic of his method so researchers of today are not certain of his intentions. The missing explanations could have been expressed elsewhere and this is an example of the problems of describing a historical development. Leibniz and Newton worked with the foundations of calculus in different ways during twenty years from about 1660 (Guicciardini, 2003). Neither of them worked with functions. Instead they both used an intuitive geometric continuum to develop limits. Both men thought that infinitesimals did not actually exist but used them as symbols in proofs. They regarded infinitesimals as quantities as they were tending to zero. Despite these similarities in their work, there were some differences in their approaches to limits. Leibniz had no problem to accept infinitesimals even though he regarded them as not really existing. He thought of limits as the foundations of calculus and settled with the belief that limits made the creation of calculus theoretically possible (Guicciardini, 2003). He defined limits as follows: “If any continuous transition is proposed terminating in a certain limit, then it is possible to form a general reasoning, which covers also the final limit” (Katz, 1998, p. 708). Newton worked with limits of ratios of geometrical quantities. He explicitly created a theory of limits in which he tried to discard infinitesimals (Guicciardini, 2003). He stated the following definition in his Principia: “Quantities and the ratios of quantities, which in any finite time converge continually to equality, and before the end of that time approach nearer to each other than by any given difference, become ultimately equal” (Edwards, 1979, p. 225). Newton used infinitesimals in Principia in a way that was misunderstood by other mathematicians. He used the words prime and ultimate ratio and evanescent quantity in his presentation without defining what he meant by them, causing confusion for other mathematicians (Boyer, 1949). Leibniz’s presentations also caused confusion and mathematicians after him interpreted differentials as infinitesimals or as zero.
3 In the 18th century, Berkeley came with criticism of earlier presentations of the calculus in an essay in 1734 called The analyst (Edwards, 1979; Katz, 1998). His criticism was aimed at Halley who was a disciple of Newton. Berkeley compared the foundations of mathematics with religious foundations. Berkeley thought that mathematicians using Leibniz’s or Newton’s methods did not themselves understand what they were doing and that they based their work on vague concepts and incompatible logic. Berkeley criticized the use of infinitesimals as zero sometimes and as something small sometimes, similarly to Fermat’s aforementioned method. A few years later, James Jurin and Benjamin Robins argued whether variables reach their limits or not (Boyer, 1949). Jurin thought that some variables could reach their limits and he accused Robins of having misunderstood Newton. In Robins’s definition of limits the variable does not reach its limit. He used a regular polygon inscribed in a circle with the circle as its limit to illustrate his reasoning. Arithmetical and geometrical conceptions of limits became mixed up and caused confusion. Such confusion was also connected to Newton’s and Leibniz’s reasoning and it remained a problem for about a hundred years. Cauchy emphasised rigor in his presentation of calculus and his was the first to present a fairly clear limit definition: “When the successively attributed values of one variable indefinitely approach a fixed value in such a way that they finally differ from it by as little as desired, then that fixed value is called the limit of all the others” (Belhoste, 1991, p. 66). The new definition enabled him to strengthen the rigor of infinitesimals (Edwards, 1979) and avoid the infinitely small fixed numbers used by earlier mathematicians such as Newton. Cauchy defined an infinitesimal to be a variable with limit value zero. Cauchy’s formulations lacked a clear definition of the concept of number, which made his formulations somewhat vague, with the concepts of number and limit depending on each other (Boyer, 1949). Up to now, the limit concept had been treated as a process in a continuous motion. Weierstrass perceived limits as static entities instead. His exposition was non-geometrical and founded on real numbers (Boyer, 1949). His definition of irrational numbers was independent of limits, so he did not rely on circular reasoning as Cauchy did. Weierstrass’s definition is the one we often use today with H and G : A is called the limit of f (x) as x o a , if for every H ! 0 there exists a G ! 0 such that f (x) A H for every x in the domain with 0 x a G . To summarise the historical development of limits, it began with an informal geometrical approach and computations were more important than rigour. Then the lack of rigour caused ambiguities in mathematicians’
4 perceptions of the concept, as they progressed in their use of limits, forcing them to work further with their expositions of limits. Issues like attainability and perceptions of the definition were not settled until centuries had passed. Many other mathematicians have contributed to the development of limits through textbooks, letters, and articles. The highlights given here capture some important views over time. A historical overview of a concept can not be seen as a linear development as there are many factors causing us to interpret historical documents differently. Our socio-cultural conceptions of modern time and of ancient times and our frameworks influence the way we perceive concepts (Radford, 1997). There are problems in comparing ontogenesis with phylogenesis for many reasons. Societies change and cultural differences bias the results of comparisons, which has to be recognised (Furinghetti & Radford, 2002; Radford, 2000). The purpose of this article is not to argue that ontogenesis recapitulates phylogenesis, but to compare the different developments of limits of functions. Historical documents are not always complete, making us draw our own conclusions of what aims and thoughts mathematicians had. The examples given here imply that we can not parallel ontogenesis and phylogenesis without caution. In the present article, important views of the phylogenic evolution of limits are compared to students’ developments of the concept within a theoretical frame, which is outlined in the next section.
Three Worlds of Mathematics Compared to History A concept image is the total cognitive representation of a concept that an individual has in his or her mind (Tall & Vinner, 1981). It might be partially evoked and different parts can be active in different situations leading to possible inconsistencies. An individual’s concept image might differ from the formal concept definition or the concept image in itself can be confusing or incoherent. Mental representations of concepts are created at different levels. Tall (2004) has introduced three worlds of mathematics to distinguish different modes of mathematical thinking, with the purpose to “gain an overview of the full range of mathematical cognitive development” (Tall, 2004, p. 287). The theory of the three worlds emphasizes the construction of mental representations of concepts and has emerged from several theories on concept development, such as Sfard’s (1991) work on encapsulation of processes to objects and Piaget’s abstraction theories (Tall, 2004). The three worlds are somewhat hierarchical in the sense that there is a development from just perceiving a concept through actions to formal comprehension of the concept. The first world is called the conceptual-embodied world and here individuals use their physical perceptions of the real world to perform mental experiments to build
5 mental conceptions of mathematical concepts. The mental experiments can be children’s categorisations of real-world objects, such as an odd number of items or, later, students’ explorations of intuitive perceptions of limits of functions. The second world is called the proceptual-symbolic world. Here individuals start with procedural actions on mental conceptions from the first world, as counting, which by using symbols become encapsulated as concepts. The symbols represent both processes and concepts, for example counting and number or addition and sum. The symbols, together with the processes and the concepts, are called procepts (Gray & Tall, 1994) and are used dually as processes and concepts depending on the context. The third world is called the formal-axiomatic world and here properties are expressed with formal definitions as axioms. There is a change from the second world with connections between objects and processes to axiomatic theories comprising formal proofs and deductions. Individuals go between the worlds as their needs and experiences change and mental representations of concepts are formed and altered. Students who are unable to encapsulate processes as objects or take the step from procepts to a strictly formalistic exposition can use rote learning as a substitute. Rote learning can be sufficient for routine situations but if the students are unable to create manageable mental entities, they will encounter even bigger problems learning new things. Students’ mathematical development can be observed in the context of these three worlds, as can the development of mathematical concepts. If we consider the development of limits as it started in a geometrical intuitive manner, then through symbolic manipulations to strictly formal representations, we can see that the phases are those of Tall’s (2004) three worlds. In the beginning of the existence of limits they were pragmatically used and not explicitly stated. Hippocrates’s and Archimedes’s (Edwards, 1979) methods of exhaustion, which came before the concept was strictly established, can be seen as mental experiments in the first world of mathematics, the embodied world. Geometrical situations resulted in a need for limits and they were successively starting to take form. Later Fermat, Newton, and Leibniz, to mention some, used calculations with symbols (Edwards, 1979; Guicciardini, 2003) when they dealt with limits. They had not yet a formal theory of limits to use, but they manipulated limits with the aid of symbols and words. The development at this stage had entered the second world of mathematics, the proceptual world. This phase altering between the two first worlds continued until Weierstrass came up with his theory of limits (Boyer, 1949) and extended the development of limits to the formal world of mathematics. The comparison of history and the three worlds is very rough, but it gives a picture of the development, which can be valuable in teaching limits.
6 Limits as They are Taught in Modern Time Students learn differently and hence there is not one teaching strategy superior to all others. Different countries and universities use different approaches to teach limits. In at least one Australian university, which I visited recently, students are not taught the formal definition until their second year of mathematics (School of Mathematics, 2004). In their first year they are intuitively introduced to the concept through examples, as opposed to the students in the present study who were taught Weierstrass’s formal definition in their first semester. There are advantages as well as drawbacks with both ways to introduce limits of functions, as argued further on. Sierpinska (1994) described a lesson from a study where a teacher introduced limits to a class. The teacher used three different forms of explanations: natural language, geometric representation, and examples. Despite the varying presentation there was not much evidence of the students understanding limits. All examples used in the lesson were of convergent sequences, so the students had nothing with which to compare the convergent sequences. Sierpinska argued that there are ambiguities in the use of the term limit. The definition can be formulated in different ways, for example, with absolute values or in terms of neighbourhoods, but limits are often taught in general terms with the aim that the students’ representations of the concept will mature during further studies in mathematics. Sierpinska also explored the use of metaphors in students’ mathematical learning. She claimed that understanding on the basis of a new metaphor causes reorganisation in the mental representations creating a ground for concept formation. The language of limits (e.g. tend to) has traces of metaphors used by mathematicians such as Newton. Stergiou and Patronis (2002) investigated how students learn rate of convergence. The students solved limit tasks that converge to zero as an introduction to infinitesimals. Some students perceived a mathematical subject, for example a finite number of terms representing a sequence, to be a given fact rather than to develop a meaningful mental representation of it. The students had problems separating a convergent sequence from a continuous function. Some of them reformulated problems posed in terms of sequences into terms of functions. Stergiou and Patronis claimed that the students’ problems of separating the concepts could be worse if the teaching of limits of sequences is left for the teaching of limits of functions. Todorov (2001) criticised the method of teaching limits with calculators and the H -G definition and argued that the theory is more accessible to students if taught in terms of infinitesimals. One reason is that the number of quantifiers in the formulation with infinitesimals is smaller than in the H -G formulation. The quantifiers are problematic for students to
7 understand (Cornu, 1991; Juter, 2003) but a substitute is not necessarily easier for them. Todorov claimed that another advantage of the infinitesimal approach is that the students do not need to guess the limit and then prove that it is a limit, as they have to if they use the H -G definition. The proof of existence and the calculation of the limit are done at the same time. The students will do more calculations and less proving. The method is similar to the Leibniz-Euler infinitesimal method (Todorov, 2001). Another method was used in a teaching experiment carried out in a numerical analysis course at another Australian university (Furina, 1994). Five students participated in a four week study, in which the students solved limit problems during 40 minutes once a week. The students used spreadsheets to solve the tasks individually for 20 minutes and then they discussed the tasks together. Furina concluded that there were students who neglected terms tending to zero and that they had an algebraic approach to limits, leaving them without a developed understanding of limits, thus making them unable to solve more demanding tasks. The developments of 15 Swedish students are presented and analysed with respect to the historical development of limits and the way limits are introduced to the students.
The Study The students and the course are described in this part of the article, along with the methods and instruments used to collect data.
The Sample and the Course There were 112 students participating in the study, of these, 29% were female. The students were aged 19 and up. They were enrolled in a first level university course in mathematics. The course was divided into two sub-courses. Both of them dealt with calculus and algebra. The courses were given over 20 weeks full time (10 weeks for each course). The students had two lectures and two sessions for task solving three days per week. Each lecture and session lasted 45 minutes. Thus the total teaching time for each course was 90 hours. The notion of limits of functions was presented in the first course before derivatives. The lectures and sessions dealing with limits are outlined here to describe the students’ first encounter with limits of functions at university level. On the first lecture on limits, the lecturer followed the textbook presenting formal definitions and theorems on indefinite and definite limits of functions and limits of monotonic functions as x tends to infinity (for functions depending on x). The textbook has an intuitive approach in the initial pages of the book, but the exposition becomes strictly formal after that. On the following task solving session, the students in the group were reluctant to go up to the black board to solve
8 tasks, and the teacher ended up solving seven of ten tasks for that session. Students tried to solve three of the seven tasks before the teacher solved them. The students said that absolute values confused them and that was also one of the problems with the tasks. The second lecture dealt with standard limit values and some proofs were rapidly presented (some comments on the speed were whispered among the students). The number e was introduced and so were H -G definitions as x tends to a number. Continuity was then presented with some following theorems. Parts of the proofs were omitted. The lecturer kept on following the textbook to help the students follow his reasoning. The second task solving session was very similar to the first where the teacher solved most of the tasks. The triangle inequality was discussed. A question of whether a function can have several limits was posed and answered. In the third lecture the lecturer continued to prove theorems from last lecture. Trigonometric formulas were repeated from lectures before limits were taught. Some theorems were made plausible through pictures. Derivatives were introduced. The lecturer said that derivatives and integrals were what the course is all about. On the following task solving session, there were many questions from the students about theorems and definitions and how to use the theories they had met. Continuity and monotonic functions were particularly discussed. The rest of the session was about derivatives. The following lectures and sessions dealt with derivatives and integrals. Limits were taught again in the second course in different settings such as integrals and series. The first course had a written exam and the second had a written exam followed by an oral one. The marks awarded were IG for not passing, G for passing and VG for passing with a good margin. I was neither teaching the students at any stage nor did I know any of them.
Methods Different methods were used to collect different types of data, such as students’ solutions to limit tasks and responses to attitudinal queries. The sets of data were collected at different stages in the students’ developments. The instruments used were designed to take those differences into account. The limit tasks were of increasing difficulty and the attitudinal part was mainly in the beginning of the semester. The students were confronted with tasks at five times during the semester, called stage A to stage E. The students got a questionnaire at stage A in the beginning of the semester. It contained easy tasks about limits and some attitudinal queries. The scope of these and subsequent tasks is described in the instruments section. The students were also asked about the situations in which they
9 had met the concept before they started their university studies. The attitudinal data are not presented in this article. After limits had been taught in the first course, as described in the former section, the students received a second questionnaire at stage B, with more limit tasks at different levels of difficulty. The aim was for the students to reveal their habits of calculating, their ability to explain what they did, and their attitudes in some areas. The students were asked if they were willing to participate in two individual interviews later that semester. Thirty-eight students agreed to do so; of these, 18 students were selected for two individual interviews each. The selection was done with respect to the students’ responses to the questionnaires so that the sample would as much as possible resemble the whole group. The gender composition of the whole group was also considered in the choices. The first session of interviews was held in the beginning of the second course, at stage C. Each interview lasted about 45 minutes. The students were asked about definitions of limits, both the formal one from their textbook and their individual ways to define a limit of a function. They also solved limit tasks of various types with the purpose to reveal their perceptions of limits and commented on their own solutions from the questionnaires to clarify their written responses where it was needed. The students received a third questionnaire at the end of the semester, at stage D. It contained just one task. Two fictional students’ discussion about a problem was described. One reasoned incorrectly and the other one objected and proposed an argument to the objection. The students in the study were asked to decide who was correct and why. A second interview was carried through at stage E after the exams. Each interview lasted for about 20 minutes. Of the 18 students, 15 were interviewed at this point. The remaining three students were unable to participate for various reasons. The students commented on the last questionnaire and, linked to that, the definition was scrutinized again. The quantifiers for every and there exists in the H - G definition were discussed thoroughly. Field notes were taken during the students’ task solving sessions and at the lectures when limits were treated to give a sense of how the concept was presented to the students and how the students responded to it. Tasks and results from other parts of the study are described in more detail in other articles (Juter, 2003; 2004; 2005a; 2005b).
Instruments At stage A the students solved some easy tasks about limits of functions. x 2 Example 1: f(x) = . What happens with f(x) if x tends to infinity? x 2 1
10 The tasks did not mention limits per se, but were designed as a means to explore if the students could investigate functions with respect to limits. At stage B the tasks were more demanding. Some of the tasks were influenced by Szydlik (2000) and Tall and Vinner (1981). Three tasks had the following structure: x3 2 Example 2: a) Decide the limit: lim . xof x 3 1 b) Explanation. x3 2 c) Can the function f(x) = attain the limit value in 2a? x 3 1 d) Why?
Example 2 is what I regard to be a routine task. There were also non- routine tasks. A solution to a task was presented to the students. It could be incomplete or wrong and the students were to make it complete and correct. There were two such tasks. The students were also asked to formulate a definition of a limit, not necessarily the one in their textbooks. At stage C, which was the first set of interviews, the students were asked to comment on statements very similar to those used by Williams (1991) in a study about students’ models of limits. The statements the students commented on are the following (translation from Swedish):
1. A limit value describes how a function moves as x tends to a certain point. 2. A limit value is a number or a point beyond which a function can not attain values. 3. A limit value is a number which y-values of a function can get arbitrarily close to through restrictions on the x-values. 4. A limit value is a number or a point which the function approaches but never reaches. 5. A limit value is an approximation, which can be as accurate as desired. 6. A limit value is decided by inserting numbers closer and closer to a given number until the limit value is reached.
The reason for having these statements was to get to know the students’ perceptions about the ability of functions to attain limit values and other characteristics of limits. The students were given the statements to have something to compare with their own thoughts. There were other tasks designed to make the students consider the formal definition to clarify what it really says, and tasks about attainability, for example:
11 Example 3: Is it the same thing to say ”For every G > 0 there exists an H > 0 such that |f(x) - A| < H for every x in the domain with 0 < |x – a| < G” as ”For every H > 0 there exists a G > 0 such that |f(x) - A| < H for every x in the domain with 0 < |x – a| < G”? What is the difference if any?
As indicated before, at stage D the students got a task with a description of two students arguing over a solution to a task. The problem cos(x 2 ) was to determine whether the limit value lim exists or not. The xo0 10000 task for the students in the study was to decide which argument, if any, was correct and to explain what was wrong in the erroneous argumentation. At stage E, the second interview, the students’ written responses to the task at stage D were discussed. Example 3 was also brought up again in connection with the task at stage D.
Results of the Study The 15 students’ representations are described at the five stages A, B, C, D and E previously discussed. The descriptions are summaries of fuller ones. Martin, Tommy and Frank had similar representations with procedural approaches to limits and problems with theory through the five stages. Filip’s, Leo’s and Dan’s representations were alike as they thought of limits procedural as well, but these students were better at solving tasks than the first group. Louise, Mikael, Dennis and Oliver all had representations with a clear progression in their theoretical understanding, from problems at the start to awareness at the end. Julia and Emma had comparable representations as they were able to solve tasks through all stages and had a clear progression in their theoretical development. The last row in Table 1 presents the students’ marks. The first letter or letters is the mark at the first course and the second is the mark at the second course. The numbers after the marks are the numbers of times the student needed to take the exam to pass, for example, G2 means that the student was awarded the mark G after failing the first exam and passing the second. Pseudonym names have been used to retain student anonymity. The digits at stage C indicate the students’ preferred choices from the six statements. The bold and larger digits are the students’ choices of statements most similar to their own perceptions of limits. Some students managed to solve tasks or explain “fairly well” as indicated in Table 1. “Fairly well” means that they made minor mistakes in their solutions or explanations.
12 Table 1. Students’ Developments Through the Course Stage Filip Martin Tommy Anna A -Links to -Links to process -Links to derivatives -Links to physics and derivatives, integrals technology -Solves easy tasks limes and limes -Solves easy tasks well -Solves easy tasks well -Solves easy tasks well fairly well B -Limits are -Limits are attainable -Limits are attainable -Limits are attainable attainable in in problem solving in problem solving in problem solving problem solving -Can not state a -Can not state a -Can not state a -Can not state a definition definition definition definition -Solves routine tasks -Solves routine tasks -Solves routine tasks -Solves tasks and and explains and explains and explains explains fairly well C -Limits are -Limits are attainable -Limits are attainable -Limits are attainable attainable in in problem solving in problem solving in problem solving problem solving -Limits are not -Limits are not -Limits are not -Unclear if limits attainable in theory attainable in theory attainable in theory are attainable in -Does not describe -A limit of a function -Has no own theory limit is how the limit stands description -Functions tend to -1, 4 with respect to -1, 3, 4 but never reaches a -Can not state or another function, no -Can not state but can value identify the definition motion identify the definition -3, 4 -Not a real limit if -3, 4, 5, 6 -Problems to solve -Can not state or attainable by function -Can not state or tasks identify the -Problems to solve identify the definition -Links to functions, definition tasks -Not a real limit if curves and number -Solves tasks fairly -Links to chemistry attainable by function series well -Claims hesitantly to -Problems to solve -Claims not to master -Links to master the notion of tasks -Links to the notion of limits derivatives, integrals limits derivatives and curves and graphs -Claims to master the -Claims not to notion of limits master the notion of limits
D -Identifies the error -Unclear -Identifies the error -Wrong answer E -Functions tend to -Functions tend to but -A limit is a point to -No description from but never reaches a do not attain a value tend to but never to the interview value -Links to sums and reach, motion -Links to series and -Links to series, integrals -Links to integrals integrals derivatives, -Unclear if he can -Can not identify the -Can identify the supremum and identify the definition definition definition infimum -Unclear if he -Does not understand -Unclear if she -Can not identify the understands the the quantifiers’ roles understands the definition quantifiers’ roles quantifiers’ roles -Unclear if he understands the quantifiers’ roles Mark G2/IG G1/G2 G1/G2 G1/G2
13 Stage John Frank Louise Leo A -Links to diet and -No links -Links to prior studies -Links to nutrition, athletics -Unable to solve easy -Solves easy tasks physics and biology -Unable to solve tasks fairly well -Solves easy tasks well easy tasks B -Limits are -Limits not attainable -Unclear if limits are -Limits not attainable attainable in in problem solving attainable in problem in problem solving problem solving -Can not state a solving -Can not state a -Can not state a definition -Can not state a definition definition -Problems to solve definition -Solves routine tasks -Solves tasks and tasks -Solves tasks and and explains explains fairly well - Sees infinitesimals explains fairly well -Sees infinitesimals as zero - Sees infinitesimals as zero as zero C -Limits are -Limits are attainable -Unclear if limits are -Limits are not attainable in in problem solving attainable in problem attainable in problem problem solving -Limits are not solving solving (hesitates) (hesitates) attainable in theory -Unclear if limits are -Limits are not -Unclear if limits are -Has no own attainable in theory attainable in theory attainable in theory description -Has no own -Thinks of limits in -The function comes -1, 4 description pictures so close that the -Can not state or -2, 3, 4, 6 -3, 4 distance is neglected identify the definition -Can not state or -Can not state or -2, 3, 4, 6 -Solves tasks well identify the definition identify the definition -Can not state or -Links to derivatives -Problems to solve -Solves tasks well identify the and graphs tasks -Links to -Links to derivatives definition -Claims to master the derivatives and curves and number sequences -No point if the limit notion of limits -Claims not to master -Claims to master the is attainable the notion of limits notion of limits fairly -Problems to solve well tasks -Links to derivatives and number sequences -Claims to master the notion of limits D -Identifies the error -Unclear -Identifies the error -Identifies the error, at stage E makes other error E -A limit is an upper -Where the function -Limit is what a -A function tends to a or lower bound goes as it tends to a function tends to as x value as the variable which a function certain value tends to infinity controlling the approaches -Links to Taylor -Links to Taylor’s function changes, it -Links to integrals, expansions formula never reaches the limit series, derivatives -Can not identify the -Can identify the -No links and convergence definition definition -Can not identify the -Can identify the -Does not understand -Can explain the definition definition the quantifiers’ roles difference between -Does not understand -Does not the definitions in ex 3 the quantifiers’ roles understand the fairly well quantifiers’ roles -Understands infinitesimals Mark G1/G1 G1/G1 G1/G1 VG1/G1
14 Stage Dan Mikael David Julia A -Links to derivatives -Links to prior -Links to derivatives -Links to prior studies -Solves easy tasks studies, problem -Solves easy tasks -Solves easy tasks well fairly well solving, physics well -Unable to solve easy tasks B -Limits are -Limits are not -Limits are attainable -Limits are attainable attainable in attainable in problem in problem solving in problem solving problem solving solving -Can not state a -Can state a definition -Can not state a -Can not state a definition -Solves tasks and definition definition -Problems to solve explains fairly well -Solves tasks and -Solves tasks and tasks explains well explains well C -Limits are -Limits are attainable -Limits are attainable -Limits are attainable attainable in in problem solving in problem solving in problem solving problem solving -Limits are not -Limits are not -Limits are attainable -Limits are not attainable in theory attainable in theory in theory attainable in theory -Thinks logically -There is always a Z -It comes closer and -The variation rather than explicitly so that x – a < H closer to A as x comes between the define closer and closer to a difference get -3, 4 smaller and smaller -2, 3, 4, 5 -Can not state but can -1, 3, 5 or smaller than a -Can not state but can identify the definition -Can not state but can small number identify the definition -Solves tasks well identify the definition -3, 4 after investigation -Links to number (claims that both def in -Can not state or -Problems to solve sequences and ex 3 state the same, H identify the tasks functions and G come in pairs) definition (claims -Links to prior studies -Claims to master the -Solves tasks well that both def in ex 3 and graphs notion of limits -Links to derivatives, state the same, H and -Claims to master the graphs and number G come in pairs) notion of limits in sequences -No point if the limit problem solving but -Claims to master the is attainable not the definition notion of limits -Solves tasks well -Links to derivatives, integrals and curves -Claims to master the notion of limits D -Identifies the error -Identifies the error, -Identifies the error -Identifies the error makes other error E -A limit is a number -If the x-value tends -Eventually it will be -If the function goes which the function to a value then the in a certain area, does closer and closer a will not reach function tends to a not reach the limit number as x goes -Links to series and value connected to the -Links to number closer and closer to a integrals x-value sequences and series value -Can identify the -Links to Taylor -Can identify the -Links to continuity definition expansions, series and definition (claims that -Can identify the -Does not number sequences both def in ex 3 state definition after understand the -Can identify the the same) investigation quantifiers’ roles definition -Unclear if he -Can explain the -Can explain the understands the difference between the difference between quantifiers’ roles definitions in ex 3 the definitions in ex 3 fairly well Mark VG1/G1 VG1/G1 G1/VG1 VG1/VG1
15 Stage Dennis Emma Oliver A -Links to integrals -Links to derivatives -Links to statistics and -Solves easy tasks -Solves easy tasks economics well well -Solves easy tasks well B -Limits are -Limits are attainable -Limits are attainable attainable in in problem solving in problem solving problem solving -Can not state a -Can not state a -Can not state a definition definition definition -Solves tasks and -Problems to solve -Solves tasks and explains well tasks explains well C -Limits are -Limits are attainable -Limits are attainable attainable in in problem solving in problem solving problem solving -Unclear if limits are -Limits are not -Limits are attainable in theory attainable in theory attainable in theory -When x comes close -A limit is something -The function value enough to a value the you tend to in the function of a two lines will -3, 4, 6 -2, 3, 5 practically emerge -Can not state or -Can not state but -2, 3, 4 identify the definition can identify the -Can state the -No point if the limit definition after definition (claims that is attainable investigation both def in ex 3 state -Problems to solve -Problems to solve the same) tasks -Links to tasks -No point if the limit number sequences -Links to derivatives is attainable except and series and functions with continuity -Claims not to master -Claims to master -Solves tasks well the notion of limits the notion of limits -Links to extreme fairly well values and functions -Unsure about her own ability to master the notion of limits D -Identifies the error -Identifies the error -Identifies the error E -You can get a value -The function tends to -As you tend to an which it tends to all the limit as x tends to unidentified point you the time when it a get a limit which is tends to a point -Limits are attainable unattainable -Links to series and in some cases and not -Links to Taylor integrals in others extensions, -Can identify the -Links to Taylor derivatives and definition extensions and extreme values -Can explain the number sequences -Can identify the difference between -Can identify the definition the definitions in ex definition -Can explain the 3 -Can explain the difference between difference between the definitions in ex 3 the definitions in ex 3 fairly well Mark VG1/VG1 VG1/VG1 VG1/VG1
16 There are many different ways the students perceived limits of functions. Many students changed their views between the different stages. Some changes show progression in the learning development following the historical development, for example Julia. At stage A she linked limits to her prior studies at upper secondary school, so she had a concept image which she used to successfully solve the easy tasks. Her approach to the tasks at stage B implies an intuitive perception of limits as the following solution to Example 2c, d reveals: “No“, “You can not insert x = infinity”. She had a similar explanation for the following task, but she was able to solve a task that required algebra and the ability to view limits as an entity. The task was presented to the students with an erroneous solution: “ § 2 · Problem: Decide the following limit value: lim xsin¨ ¸ . Solution: xof © x ¹ § 2 · sin¨ ¸ § 2 · © x ¹ 2 1 xsin¨ ¸ . We know that o 0 when x o f and o 0 when © x ¹ 1 x x x § 2 · sin¨ ¸ sin x © x ¹ x o f . The limit value lim 1 implies that o1 when x o f . xo0 x 1 x Adjustments (What will change or be completed and why)”. Julia’s § 2 · 2sin¨ ¸ § 2 · © x ¹ solution was: “ xsin¨ ¸ o 2 “. © x ¹ 2 x Julia displayed a concept image comprising both intuitive reasoning and a perception of limits as objects, which indicates that she had reached Tall’s second world, at least partially. At stage C she had problems with the formalism of the definition as she regarded both definitions in Example 3 to be equal. She explained: “Yes, I feel as if it is the same thing” … “ So it is, so to say, two corresponding to the number’s x-value and y-value …” [Interviewer: “… as pairs so to say?”], Julia: “Yes”. At stage E she was able to explain why the definitions did not say the same thing, so she understood the formal exposition. Other changes did not follow the historical development. Tommy, for example, said at stage C that a limit of a function is how the limit compares to another function and that there is no motion. At stage E he had a completely different description where he said that a limit is a point to tend to but never to reach and that there is motion. He was unable to state or even recognise the limit definition through the entire course, but he could solve easy tasks. The tasks at stage C were too hard for him. His concept image revealed inconsistencies regarding functions’ abilities to attain limit
17 values without improvement. Tommy’s development had not followed the historical development, as he seemed to float between the two first worlds, never entering the third. Development occurred at various stages and in different parts of the concept images. John, Frank and Louise, for example, regarded infinitesimals as zero at stage B, but at the last stage John knew how to deal with infinitesimals. The aim of the study was not explicitly to investigate infinitesimals but in their work with limits, these students revealed how they perceived infinitesimals. Many of the students appeared to have problems with theory. Martin, Tommy and Frank all had difficulties handling theory through the semester. They had trouble with task solving at different stages as well. Filip, Leo and Dan also showed difficulties with theory, but they were able to solve the tasks in the study. Louise, Mikael, Dennis and Oliver had theoretical problems in the start and middle of the semester, but they could correctly answer all the theoretical questions at the last stage. Julia and Emma managed both theory and task solving well through the whole semester with only some theoretical issues. The students’ grades are, if roughly regarded, increasing through this description of their accomplishments. It was hard for the students to solve one of the tasks where they were asked to formulate a definition for the limit of a function depending on x as x tends to a number. At stage B it was only Julia who could do this in a meaningful manner. Most of the other students just rearranged the words used to formulate the task and created a meaningless text. At stage C it was only Emma who was able to formulate the formal definition. From the seven students with marks G or IG it was only Anna who recognised the definition in Example 3. Five of the remaining eight students recognised the definition, but Julia and Emma thought that both definitions in Example 3 state the same thing. Dan, who could not recognise the definition, also thought so. At stage E, Julia and Emma could explain the difference between the correctly and the wrongly formulated definitions and they could hence understand the quantifiers’ roles, but now David thought that the definitions say the same thing. The quantifiers were hard for the students with marks G and IG to understand through the whole semester, while most students with higher marks were able to explain the meaning of the words at stage E. The six statements were brought up at stage C. Statement four, but not statement three, was selected most frequently among students with marks G or IG to be the statement most similar to their own perception of limits of functions. Statement three was the most common choice among students with higher marks. The statement most students in Williams’s (1991) study selected as best was statement four (36%) followed by statement one (30%) and statement three (19%). Statement four states that functions’ limits are never reached, while statement three says that a function’s y values can
18 come arbitrarily close to the limit value. There was great confusion about functions’ abilities to attain limit values. Many students treated limits as attainable when they solved tasks but unattainable in theoretical discussions. Three students, all without the mark VG, claimed that they did not have control over the notion of limits. The rest of the students were fairly certain about their control over the notion at stage C, despite the fact that almost none of them could formulate and, in some cases, even recognise the definition of limits of functions. To summarise the students’ developments, they started with a computational approach and some, mostly low achievers, kept this approach for the duration of the semester, while others, mostly high- achievers, got better at describing the theoretical parts of limits during the semester. Almost all students had ambiguous views about attainability at some point.
Discussion The students’ developments have been compared to the historical development of limits, which in turn was compared to Tall’s (2004) worlds of mathematics in the theory section of this article. The results from the present study are discussed with respect to results from other studies, as are implications for teaching.
Students’ Developments Through the Semester Many students in this study were able to solve easy tasks about limits, even though they were unable to explain the meaning of the definition. Many of the low or average achieving students did not progress very far in their learning of limits, despite the lecturer’s efforts with, for example, graphical “proofs” and following the textbook’s exposition. Both theory and problem solving were unclear to them. The implementation of the concept does not seem to have been successful even though many of the students felt confident about their control over the concept. One reason for the unsuccessful implementations could be that they learned limits as facts like the students in Stergiou’s and Patronis’s (2002) study did. Then it becomes impossible to explore the features of the concept. Such mental representations are rigid and shallow. Some of the students in Furina’s (1994) study had an algebraic perception of limits and they ended up with similar problems of poor concept images. Many students in the present study also had an algebraic approach to limits, where they used known procedures to calculate problems even if the chosen procedures were unsuitable for the problem at hand. The security of known methods seems to be dominating the uncertainty of exploring new features of the concept. If the concept images are too poor, there is not enough to explore.
19 High-achieving students obviously came further along in their developments. Students like Louise (average grades), Mikael, Dennis, and Oliver all had some theoretical problems in the beginning of the course, but they had overcome them at the end of the course. These students paralleled the historical development of the concept in the sense that they were able to solve tasks in the beginning before they could explain the theory, but with time the theory became clear to them. Limits were first treated in problem solving situations historically. With time through many mathematicians’ efforts, the theory became clear. One big difference is how much time was needed, and even though the students did not have to invent the mathematics, they still needed time to digest it. The fact that students have access to the theory can tempt them to learn difficult parts by rote (Tall, 2004), which was impossible for mathematicians inventing mathematics. The definition of limits, as one example of difficult parts, remained a problem through the semester for many of the students. The definitions by Leibniz (Katz, 1998), Newton (Edwards, 1979) and Cauchy (Belhoste, 1991) show just how hard it is to formulate a concise definition that is easy to understand. The three formulations are very different from each other, but they were all stated to describe the same thing. Leibniz’s and Newton’s formulations are quite vague and the criticism Berkeley had about mathematicians using their methods (Edwards, 1979), seems to capture the difficulties experienced by some of the students in this study as well. The students with problems explaining the limit definition appear to be mainly at the stage of Newton and Leibniz with a sense of what they do, but lacking the means to strictly formulate and explain the concept of limits. There is hence no evidence that the students had reached Tall’s (2004) third world of mathematics. The four students with the highest marks could explain the quantifiers’ roles in the definition with its formal exposition, so they seem to have reached the third world.
Rigour Then and Now The meanings of the quantifiers in the definition were hard for the students to understand, as Table 1 indicates. Almost only the high-achieving students managed to explain the quantifiers’ roles in the definition at the end of the course. Their conceptions of limits had matured during the second course, in the way Sierpinska (1994) argued teachers hope for. The other students did not come so far as to understand the definition with its quantifiers. This implies that the students are not ready to embrace the complexity of the concept in their first course. It took many years for the historical development of rigor in the formulation of limits to reach the current level and there were controversies during that time. The need for rigor became very clear from controversies such as Jurin’s and Robins’s dispute about whether limits can be reached or
20 not (Boyer, 1949). Students today have the same problem even though they have access to unambiguous definitions of limits. The rigor of the definition is hence not implemented in the students’ concept images (Tall & Vinner, 1981). It is important that students get examples that they can not solve without the sharpness of a strict definition, otherwise they see no reason to learn a strict definition. The task at stage D require an understanding of the limit definition, and as Table 1 indicates, a majority of the students could identify the error but not explain why it is wrong. At stage E, when asked to explain, only six students managed to do so. Students need to evaluate their perceptions repeatedly during mathematics education to adjust their mental representations. Tasks, discussions, and other stimulating activities help them to find critical parts of their concept images.
Teaching Limits of Functions Whether the students are taught limits through the formal definition or through infinitesimals (Todorov, 2001), there are still complex issues to deal with. Mathematicians worked with infinitesimals a long time before the concept of limits was formally stated by Weierstrass. There was a need for an unambiguous theory about limits to avoid the problems of misunderstanding that occurred in the past. This implies, despite the different conditions for mathematicians before Weierstrass and students today, that infinitesimals are not trivial to understand and that there is a danger of going back to this way of teaching the topic. One problem that has been hard to overcome historically is if, and when, infinitesimals can be discarded as zero. The study presented in this article did not explicitly investigate this issue, but the problem came up in some student responses anyway. Furina’s (1994) study addressed this issue and the students in her study regarded infinitesimals as zero like Fermat (Edwards, 1979) and other mathematicians did. Parts of mathematics at higher levels are often formulated in terms of the H -G definition, and students who are taking those courses need to have an accurate mental representation of limits in terms of H and G to be able to understand other concepts. If they first learn limits with the infinitesimal method, they would probably have a hard time to learn another way of perceiving limits. The benefits, as described by Todorov (2001), of fever quantifiers are then lost. If limits are taught metaphorically or intuitively (Sierpinska, 1994), they can be thought of as infinitesimals in cases like those in the study by Stergiou and Patronis (2002). The infinitesimals are then examples and not the foundation of the work with limits. Historical examples have helped students learn other mathematical concepts, for example linear dependence and independence (Radford, 2000) and algebra (Katz, Dorier, Bekken & Sierpinska, 2000). McGinn and Boote (2003) emphasized the importance for students to work
21 with different types of tasks to practice both routine skills and problem solving abilities in mathematics, and they argued that mathematics history provides a range of different kinds of problems suitable for such practice. Students learning limits can be helped to overcome the shift to abstract thinking if they work with historical problems. Abstract thinking is essential for students to understand how the definition can be used to solve non-routine problems such as the problem posed to them at stage D, or to understand why the definition does not state that limits are unattainable for functions. Such work requires historically competent teachers who are attentive to the students’ potential cognitive obstacles, both historical obstacles and obstacles that are specific for their current students (Katz et. al., 2000). An Australian way of teaching limits intuitively in the first year (School of Mathematics, 2004) differs from the Swedish way of teaching limits with the formal theory stated in the first year. The outcome, however, of the study of the Swedish students shows that even though the students have access to the formal theory, many of the students do not seem to use it. The Australian students did not get to see the H -G definition until their second year of mathematics. By then they had been working with limits and created an intuitive sense of the notion. One danger is that their everyday experiences disturb their perceptions of limits. Erroneous representations can become strongly implemented and after some time very hard to change (Cornu, 1991). The apparent benefit of waiting with the formal definition is that the students get time to get accustomed to the notion of limits, which the results of this study implies they need. The dangers of introducing limits informally might be worth facing to make the learning easier for the students. The price to pay for this more gentle introduction is that teachers have to be more aware of the kinds of problems the students can face later as they learn the formal definition, for example the influence of everyday perceptions and language (Sierpinska, 1994).
Conclusions The historical progression through the three worlds of mathematics (Tall, 2004) only seems to match the high-achieving students’ progressions. Among the other students there are not always progressions and the third world seems to be out of reach for some of them. Historically, clear and stringent formulations helped mathematicians to agree what the limit concept should be and high-achieving students might manage to handle such formality, but most students need guidance grounded in their prior experiences to be able to understand limits. The average student is in the first world of mathematics with an intuitive perception of the limit concept when he or she starts at university level, because that is how it is taught at
22 upper secondary schools in Sweden. The formalistic presentations of limits at lectures and in textbooks appear to be too far away from the students’ previous experiences causing the problems to integrate their new knowledge with existing knowledge. The definition, as it was taught in the course, was not encapsulated in most students’ concept images, with the result that they did not use it properly in problem solving. Students’ false sense of security, founded on success in routine problem solving, needs to be challenged. Tasks based on original texts can be used to help students to understand different ways to perceive limits, and also to make them explore limits in various historical formulations. There continues to be ambiguities in teaching limits of functions in different countries as the examples given indicate. Different teaching strategies come with different problems and there are no shortcuts. Students need to become aware of their own perceptions of limits in different contexts to be able to develop as accurate concept images as possible. Teachers can help them with explorative tasks, which expose the need for rigour as well as help students to understand all the different facets of limits of functions.
References Bartolini Bussi, M. G., & Sierpinska, A. (2000). The relevance of historical studies in designing and analysing classroom activities. In J. Fauvel, & J. van Maanen (Eds.), History in mathematics education. The ICMI Study (pp. 154-161). Dordrecht /Boston/London: Kluwer. Belhoste, B. (1991). Augustin-Louis Cauchy, A biography. Berlin: Springer Verlag. Boyer, C. B. (1949). The history of the calculus and its conceptual development. New York: Dover Publications. Cornu, B. (1991). Limits. In D. Tall (Ed.), Advanced mathematical thinking (pp. 153-166), Dordrecht: Kluwer Academic Publishers. Davis, R., & Vinner, S. (1986). The notion of limit: Some seemingly unavoidable misconception stages. Journal of Mathematical Behavior, 5, 281-303. Edwards, C. H. (1979). The historical development of the calculus. Berlin: Springer Verlag. Furina, G. (1994). Personal reconstruction of concept definitions: limits. Proceedings of the 17:th annual conference of the Mathematical Education Research Group of Australasia, Lismore, Australia: Southern Cross University.
23 Furinghetti, F., & Radford, L. (2002). Historical conceptual developments and the teaching of mathematics: from phylogenesis and ontogenesis theory to classroom practice. In L. English (Ed.), Handbook of international research in mathematics education (pp. 631-654). New Jersey: Lawrence Erlbaum. Gray, E., & Tall, D. (1994). Duality, ambiguity and flexibility: A procedural view of simple arithmetic. The Journal for Research in Mathematics Education, 26(2), 115-141. Grugnetti, L., & Rogers, L. (2000). Philosophical, multicultural and interdiciplinary issues. In J. Fauvel, & J. van Maanen (Eds.), History in mathematics education. The ICMI Study (pp. 39-62). Dordrecht /Boston/London: Kluwer. Guicciardini, N. (2003). Newton’s method and Leibniz’s calculus. In H. N. Jahnke (Ed.), A history of analysis (pp. 1-39). Rhode Island, USA: American Mathematical Society. Juter, K. (2003). Learning limits of function, University students’ development during a basic course in mathematics. (Licentiate thesis) Sweden: Luleå University of Technology, Department of Mathematics. Juter, K. (2004). Limits of functions – How students solve tasks. In C. Bergsten & B. Grevholm (Eds.), Mathematics and language, Proceedings of MADIF 4, the 4:th Swedish Mathematics Education Research Seminar (pp. 146-156). Linköping, Sweden: Svensk Förening för Matematikdidaktiska Skrifter. Juter, K. (2005a). Limits of functions – how do students handle them? Pythagoras, 61, 11-20. Juter, K. (2005b). Students’ attitudes to mathematics and performance in limits of functions, Mathematics Education Research Journal, 17(2), 91-110. Katz, V. (1998). A history of mathematics: An introduction. (2nd ed.). United States of America: Addison-Wesley Education Publishers, Inc. Katz, V., Dorier, J., Bekken, O., & Sierpinska, A. (2000). The role of historical analysis in predicting and interpreting students’ difficulties in mathematics. In J. Fauvel, & J. van Maanen (Eds.), History in mathematics education. The ICMI Study (pp. 149-154). Dordrecht /Boston/London: Kluwer. McGinn, M. K., & Boote, D. N. (2003). A first-person perspective on problem solving in a history of mathematics course. Mathematical Thinking and Learning, 5(1), 71-107.
24 Radford, L. (1997). On psychology, historical epistemology, and the teaching of mathematics: Towards a socio-cultural history of mathematics. For the Learning of Mathematics, 17(1), 26-33. Radford, L. (2000). Historical formation and student understanding of mathematics. In J. Fauvel, & J. van Maanen (Eds.), History in mathematics education. The ICMI Study (pp. 143-170). Dordrecht /Boston/London: Kluwer. School of Mathematics. (2004). Mathematics 11CFN, Notes on Calculus. Melbourne: La Trobe University. Sfard, A. (1991). On the dual nature of mathematical conceptions: Reflections on processes and objects as different sides of the same coin. Educational Studies in Mathematics, 22, 1-36. Sierpinska, A. (1994). Understanding in mathematics. London: The Falmer Press. Stergiou, V., & Patronis, T. (2002). Exploring a modern path to infinitesimals: mathematics students’ understanding of the concept of rate of convergence. International Journal of Mathematical Education in Science and Technology, 33(5), 651-659. Szydlik, J. (2000). Mathematical beliefs and conceptual understanding of the limit of a function. Journal for Research in Mathematics Education, 31(3), 258-276. Tall, D. (1980). Mathematical intuition, with special reference to limiting processes. Proceedings of the Fourth International Congress on Mathematical Education (pp. 170-176), Berkeley. Tall, D. (2001). Natural and formal infinities. Educational Studies in Mathematics, 48, 199-238. Tall, D. (2004). Thinking through three worlds of mathematics. In M. Johnsen Høines, & A. B. Fugelstad (Eds.), Proceedings of the 28:th Conference of the International Group for the Psychology of Mathematics Education (pp. 281-288). Bergen, Norway. Tall, D., & Vinner, S. (1981). Concept image and concept definition in mathematics with particular reference to limits and continuity. Educational Studies in Mathematics, 12, 151-169. Thiele, R. (2003). Antiquity. In H. N. Jahnke (Ed.), A history of analysis (pp. 1-39). Rhode Island, USA: American Mathematical Society. Todorov, T. (2001). Back to classics: Teaching through infinitesimals. International Journal of Mathematical Education in Science and Technology, 32(1), 1-20. van Maanen, J. (2003). Precursors of differentiation and integration. In H. N. Jahnke (Ed.), A history of analysis (pp. 1-39). Rhode Island, USA: American Mathematical Society.
25 Vinner, S. (1991). The role of definitions in the teaching and learning of mathematics. In D. Tall (Ed.), Advanced mathematical thinking (pp 65-81). Dordrecht: Kluwer Academic Publishers. Williams, S. (1991). Models of limit held by college calculus students. Journal for Research in Mathematics Education, 22(3), 219-236.
26 Article V
Limits of functions - Students solving tasks
Kristina Juter Kristianstad University College Sweden
Abstract Students solve problems to learn mathematics. How do they cope with the problems and explain their solutions? This study is about students at a Swedish University, solving tasks about limits of functions. The students are enrolled in their first University course in mathematics. They study algebra and calculus for 20 weeks full time. There are many possible problems for the students to handle, including the notion of function, the formal definition and limits as unattainable values for functions. Many students do not possess enough knowledge about limits to solve and explain solutions to non-standard problems. Some students arrive at correct answers through erroneous reasoning.
Introduction This study was conducted to reveal how students at University level justify their solutions to tasks with various degrees of difficulty. The study is part of a larger study of students’ concept formation of limits. The mathematical area is limits of functions. The study was carried out at a Swedish University at the first level of mathematics. The results are, however, applicable to other countries as well since students meet similar challenges in their learning of limits. I have, in discussions with some Australian mathematics teachers at University level, found out that the topics taught in basic mathematics courses in Australia are similar to Swedish courses. Two groups of students taking the same course in successive semesters have been solving tasks. Their solutions are categorised here and analysed to create a picture of how students reason about limits.
Background and questions When students take a course in analysis, they solve a vast number of tasks. They can check if they got correct answers and then continue to work with their problems. But what can be said if some of the solutions are correct for the wrong reasons? Or wrong by accident despite a thorough attempt? This could lead to serious errors in the students’ mental representations of the concept at hand. For a mathematics teacher, to be able to assist his or her students, it is essential to be aware of the different ways students justify their claims. In this study I address the following questions: x How do students solve problems with limits? x How do they explain their solutions?
1 Theoretical background About problem solving in general Mathematics is often expressed with symbols operated by certain rules. The rules have to be known to an individual engaged in mathematical activity and they can be memorised. This is not enough if he or she wants to understand mathematics though. Instead of only memorising formulas and procedures, the individual needs to have an exploring attitude to problem solving (Schoenfeld, 1992). Pólya (1945) describes a way to go about it in terms of decomposing and recombining. The problem is at first considered as a whole, then details are examined to give more information for the solution process. The details are combined in different ways and this may give a new perspective to the problem as a whole.
Students learn new and improved methods for problem solving as they take courses in mathematics. This means that they eventually can have quite a few methods to choose from, both new and old ones. When an individual encounters a problem he or she might not fetch the optimum solution method from the mind (Davis & Vinner, 1986; Pólya, 1945). This is not the same as saying that the student cannot solve the problem in a better way. We cannot know what strategies are available in an individual’s mind, but we can see the chosen method. The students’ actions are shaped by their abilities (Star, 2000). Whatever effective and numerous methods a person has in his or her mind, if they are unreachable at the time they are needed, they are of no use.
A concept image is the total cognitive representation of a notion that an individual has in his or her mind (Tall & Vinner, 1981). It might be partially evoked and different parts can be active in different situations leading to possible inconsistencies. An individual’s concept image might differ from the formal concept definition or the concept image in itself can be confusing or incoherent. It may be that an individual is convinced that limits are upper or lower bounds, impossible for the function to pass or even to reach (Cornu, 1991). Then his or her concept image is incompatible with the formal concept definition. Such confusion on a critical issue may provoke the individual to try to understand how it should actually be, but there is also the danger of giving up because of the obstacle.
Lithner (2000; 2003) describes, influenced by Pólya, different types of reasoning in problem solving. He states plausible reasoning (PR) to be if the argumentation in the reasoning structure: (i) is founded on intrinsic mathematical properties of the components involved in the reasoning, and (ii) is meant to guide towards what probably is the truth, without necessarily having to be complete or correct. (Lithner, 2003, page 33)
2 Established experience (EE) is at hand when the argumentation: (i) is founded on notions and procedures established on the basis of the individual’s previous experiences from the learning environment, and (ii) is meant to guide towards what probably is the truth, without necessarily having to be complete or correct. (Lithner, 2003, page 34). Identification of similarities (IS) occurs when the reasoning satisfies the two conditions: (i) The strategy choice is founded on identifying similar surface properties in an example, theorem, rule, or some other situation described earlier in the text. (ii) The strategy implementation is carried through by mimicking the procedure from the identified situation. (Lithner, 2003, page 35).
Lithner’s study showed that almost all the time the students in his study spent on mathematics at home was devoted to exercises and IS was the preferred way to reason. The students compared the problems with solutions in the textbook to the problems they should solve and used the strategy from the textbook. One problem for the students was to identify the essential surface properties in order to select the correct procedure for the solution. This way of reasoning has a weak foundation since it is based on memory and not understanding. It is easy to make mistakes. The students did not display much reflection on intrinsic properties or awareness of relations in the study. The author suggests that one reason for this can be that the students are not worried about their inadequate insight into the critical features and therefore do not regard them as useful. This kind of judgement of their own capability has occurred among the students in the study presented in this paper as well (Juter, 2003). Many of the students were unable to solve non-routine tasks or explain given solutions about limits of functions but they thought they had control over the notion of limits anyway. This sense of control can come from successful problem solving (Cornu, 1991) and the difficulties with a few non-routine problems do not change that feeling.
Hiebert and Lefevre (1986) present a different perspective. They speak of conceptual and procedural knowledge. Conceptual knowledge has an emphasis on relations. The items of a notion are connected through relations and together they form a mental web. A part of conceptual knowledge cannot be thought of as a disjointed piece of information. Conceptual knowledge develops via construction of relations between items. The items can be other relations or concepts where the connection can be between two (or more) items that are existing already in the mind or between a new and an existing item. When this connection is created, the result often becomes more than its parts jointly (Dreyfus, 1991). Parts with no prior relations become connected and suddenly more things fit together. Hiebert and Lefevre (1986) present two levels at which relationships between items of mathematical knowledge can be created. At the
3 primary level, the connections are of the same or lower degree of abstractness than the items that are connected. At the reflective level, the relations are at a higher level than the connected items. At this level, the relationships are often based on identification of similar crucial features in seemingly different items. A relation can be at more than one level. Procedural knowledge is divided in two parts (Hiebert & Lefevre, 1986). One is knowledge of the formal language of mathematics, i.e. the symbols and words we use to represent the mathematics. Knowing this is not the same as knowing what the mathematics described actually means. It is just the accepted forms that are known. The other part consists of the algorithms used for solving tasks. Characteristics of these algorithms are their step-by-step descriptions which are similar to an assembly description for a shelf. There are no relational needs except that one instruction follows the former one.
Mathematical knowledge needs links between conceptual and procedural knowledge and both of them are essential for students to be able to perform mathematics satisfactorily as I see it. Otherwise, the result might be that they are able to produce an answer to a task but not to understand what they have done. On the other hand students can have an accurate intuition for the mathematics, but be unable to make the necessary calculations. Such links can make it easier to remember algorithms and when to use each of them. If there is a connection between an algorithm and the explanation to it, it is a big help for the students when they are solving problems. Links to the underlying meaning of the symbols are also important for an individual to be able to understand what is actually going on. Then it might be possible to rephrase the task in an easier way and to use a preferred algorithm rather than a perhaps harder one tied to the initial formulation of the task. Connections from conceptual to procedural knowledge in the use of symbols make the thinking smoother, according to Hiebert and Lefevre (1986). With a compact symbolic representation it is easier to manipulate objects and processes and detect their features. In this way, new knowledge can be formed. Powerful procedures create more space for other thoughts necessary for completion of the task.
Mathematical thinking in a problem solving situation is a dynamic process and the relations between items of conceptual and procedural knowledge are therefore vital (Silver, 1986). It is very rare that a person only shows evidence of one sort of knowledge. Problem solving is an action involving different kinds of knowledge. Non-standard tasks, but also easier tasks, can reveal relations between the different knowledge types. The former types generally require understanding or interpretation of some kind and thereby the interplay becomes more visible.
School mathematics has a tradition of focusing on manipulation of symbols rather than on understanding what the symbols represent (Davis, 1986). Davis suggests that experience of the mathematical area should come before the
4 symbols for it are introduced. Then the meaning of the symbols will be familiar from the start and this will perhaps make problem solving less demanding.
I consider the students’ reasoning and problem solving to be expressions of their concept images (Tall & Vinner, 1981).
About problems related to limits Infinity is a notion that can cause trouble. It is something for which an individual has one or several intuitive representations (Tall, 1980). If there are multiple, different representations evoked simultaneously, the result might be erroneous. When dealing with limits of functions one has no specific method or algorithm as one has for diofantic equations for example. The limit process appears potentially infinite and students can get the impression that there is no end to it (Tall, 2001). It can be hard for them to work with items that are confusing in identity. Is it an object or a process?
A common error in students’ concept interpretations of limits of functions is that functions do not attain their limit values (Cornu, 1991; Tall, 1993; Tall, 2001; Szydlik, 2000). There is also a possible mix-up of f(a) and lim f (x) (Davis & xoa Vinner, 1986). These two flaws combined can totally block students in their struggle with tasks that could easily be solved with an equation for instance.
When students meet the concept of limits at Universities for the first time they have already been working with functions and their graphs. The goals in the curriculum for upper secondary school in Sweden do not mention limits explicitly, but the students are expected to learn about derivatives and integrals (Skolverket, 2003). This implies that limits of functions are discussed in some form. The students at universities therefore have an existing concept image of limits of functions that has been satisfactory in the contexts they have worked in so far. Hence there is no need to learn the formal limit concept to be able to analyse functions (Williams, 1991). The students have to experience the need for further sophistication in their mathematical development to adjust their perhaps blunt existing mental representation of limits. Szydlik’s (2000) data implies that there is a connection between the understanding of limits and functions and infinitesimals. Students who used infinitesimal language in their descriptions of limits did not show evidence of increasing alternate conceptions of limits. Their ability to solve problems about limits was not diminished either. Infinitesimals can on the other hand have the opposite effect too. Milani and Baldino (2002) found cases where students had trouble with their concept images of infinitesimals and limits. Images and definitions of limits were different but this was not the case for infinitesimals. The authors ask the question whether students perceive definitions as useful for their mathematical activities.
5 The definition of the limit concept often causes difficulties for students (Cornu, 1991; Juter, 2003; Vinner, 1991). The students’ concept definitions are not always compatible with the formal concept definition and that can cause an incoherent concept image with different rules for different situations. If a problem is stated in a manner that is not specifically represented in the students’ concept images, there can be more than one representation evoked in the effort to solve the problem. This can make the students confused and unable to proceed.
There are many things that can disturb the solving process. The goal of this study is to find out more about the students’ solution strategies when solving problems involving limits of functions and the justifications of their choices.
The study and the students The students in this study were enrolled in a 20 week, full time course in mathematics. They were learning analysis and algebra at basic University level. In the Spring Semester of 2002, 111 students solved Task 1 to Task 3 described below. They had treated limits of functions in the course and it was nothing new for them. Eleven days later they were given Task 4 and Task 5. There were 87 students who participated in that session. The last set of tasks provided solutions that could be wrong or incomplete. This was stated on the sheet with the tasks and the students were to give a complete and correct solution to each task. A new group of 78 students tackled the same tasks the following Semester (Autumn of 2002). They were all given the five tasks at the same time, after the notion of limits of functions had been dealt with. The group was smaller than the previous Semester and one reason is that the students who had biology or chemistry as a main topic were offered another course more suitable for them but this was not an option for the students in the Spring Study. In both cases, the tasks formed part of three questionnaires along with questions about limits and attitudes towards mathematics in general. Two interviews with each of 15 of the students were conducted in the Autumn Study (Juter, 2003). The first three tasks in this presentation were slightly altered in the Second Study since many of the students misinterpreted or did not understand what the tasks were about. The change was from “Can the function f(x) = 2x 3 attain the limit value?” to “Can the function f(x) = 2x 3 attain the limit value in 1a?” with respective functions in Task 1 to Task 3 below.
Method The tasks were constructed to focus on different aspects of the limit concept. The degree of difficulty varied to identify the level the students could handle. I explained what I wanted the students to do at each session they responded to the questionnaires to make sure that it was clear to them.
The collected data has been rewritten and categorised with the aid of the computer program NUD*IST (N6, 2003). The categories were decided from the
6 raw material. I did not create them in advance, other than that the right and wrong answers made different categories. There were subcategories in each of them based, on the students’ justifications of their responses. They were chosen from the different reasons the students used for their solutions. This process led to a number of categories. Categories with similar types of reasoning were merged together to make the presentation more accessible. Some solutions are in more than one category since some students gave more than one solution or solutions that fitted more than one category for other reasons. This way to work with the data gave different types of category systems for the different tasks.
Empirical data Examples of typical student answers are provided in a table after each task. The tables also include the number of students from each semester in each category. The numbers within brackets are percentages of participating students in each class. (R) indicates that the answer is right and (W) denotes a wrong answer.
Task 1: a) Decide the limit: lim(2x 3) . xo3 b) Explanation. c) Can the function f(x) = 2x 3 attain the limit value in 1a? d) Why?
Table 1(i). Typical student answers in the categories for Task 1a – b. Number of students (%). Category 1a 1b Spring Autumn 2002 2002 Tends to (R) 9 x tends to 3 2x+3 becomes close to 9 39 (35) 33 (42) Replace x by 3 (R) 9 2*3+3=9 37 (33) 19 (24) Mixed (R) 9 x tends to 3 and 2*3+3=9 9 (8.1) 9 (12) Theory (R) 9 Continuous function that attains the 7 (6.3) 3 (3.8) function value in the point 3 No explanation (R) 9 - 9 (8.1) 8 (10) Wrong or empty The answer is either wrong or missing 12 (11) 16 (21)
Table 1 (ii). Typical student answers in the categories for Task 1c – d. Number of students (%). Category 1c 1d Spring Autumn 2002 2002 Theory (R) Yes The function is continuous in the 22 (20) 23 (29) point Replace x by 3 (R) Yes 2x+3=9 for x=3 22 (20) 21 (27) No explanation (R) Yes - 15 (14) 8 (10) Limits not attainable No A function does not attain the limit 9 (8.1) 10 (13) (W) value, it only comes very close, it is in the definition No reason (W) No - 3 (2.7) 3 (3.8) Empty or The answer has no connection to the 40 (36) 16 (21) misinterpretation question or is missing 7 x 3 2 Task 2: a) Decide the limit: lim . xof x 3 1 b) Explanation. x 3 2 c) Can the function f(x) = attain the limit value in 2a? x 3 1 d) Why?
Table 2 (i). Typical student answers in the categories for Task 2a – b. Number of students (%). Category 2a 2b Spring Autumn 2002 2002 Exclude –2 & 1 (R) 1 Thex 3 terms dominate, -2 and 53 (48) 32 (41) 1 insignificant as x o f Algebra (R) 1 Divide with x 3 , 17 (15) 28 (36) x 3 2 2 3 3 1 3 x x x o 1 x 3 1 1 1 x 3 x 3 x 3 No explanation (R) 1 - 6 (5.4) 4 (5.1) Algebra (W) -2 x 3 cancel out since it is the 17 (15) 12 (15) same number Infinity reason (W) Does divided by infinity 11 (10) 0 (0) not exist Empty or The answer has no connection 10 (9) 9 (12) misinterpretation to the question or is missing
Table 2 (ii). Typical student answers in the categories for Task 2c – d. Number of students (%). Category 2c 2d Spring Autumn 2002 2002 x 3 2 z x 3 1 (R) No x 3 2 can never be equal to 7 (6.3) 17 (22) x 3 1 for the same value on x -2 & 1 (R) No terms –2 and 1 will always 7 (6.3) 5 (6.4) remain No explanation (R) No - 18 (16) 8 (10) Infinity reason (W) No Since x never attains the value f 16 (14) 21 (27) Theory (W) No The function tends to the limit 7 (6.3) 6 (7.7) value, it does not attain it No reason (W) Yes - 15 (14) 7 (9.0) Empty or The answer has no connection to 42 (38) 17 (22) misinterpretation the question or is missing
8 x 5 Task 3: a) Decide the limit: lim . xof 2 x b) Explanation. x 5 c) Can the function f(x) = attain the limit value in 3a? 2 x d) Why?
Table 3 (i). Typical student answers in the categories for Task 3a – b. Number of students (%). Category 3a 3b Spring Autumn 2002 2002 Exp. dominant (R) 0 2x grows faster than x 5 80 (72) 64 (82) No explanation (R) 0 - 6 (5.4) 3 (3.8) Infinity or no limit (W) f x 5 is larger than 2 x when x is 11 (10) 7 (9.0) large Empty or The answer has no connection to 17 (15) 4 (5.1) misinterpretation the question or is missing
Table 3 (ii). Typical student answers in the categories for Task 3c – d. Number of students (%). Category 3c 3d Spring Autumn 2002 2002 x=0 (R) Yes 0 15 (14) 16 (21) For x=0o f(0) = = 0 1 No explanation (R) Yes - 5 (5) 6 (7.7) Does not reach limit (W) No We can only get infinitely close 14 (13) 16 (21) 0 No Then the numerator has to be 22 (20) 12 (15) x 5 z 0 or (W) 0 zero and it never is Right for wrong reason Yes Because the denominator 5 (5) 16 (21) (W) attains a much larger number for large x Empty or The answer has no connection 47 (42) 12 (15) misinterpretation to the question or is missing
x2 x Task 4: Problem: Decide the following limit value: lim . xo1 x2 1 The students were given the following: x 2 x x(x 1) x Solution: o f when x o1 . x 2 1 (x 1)(x 1) x 1 The task was for the students to decide the proper adjustments to make the solution correct:
Adjustments (What changes or complements are needed and why):
9 Table 4. Typical student answers in the categories for Task 4. Number of students (%). Category 4 Spring Autumn 2002 2002 Both sides (R) x o1 from minus or plus 18 (21) 28 (36) One side (R) Incomplete Asx o1 the denominator becomes 10 (11) 5 (6.4) negative
Dominant factor (W) 2 § 1 · 15 (17) 12 (15) x ¨1 ¸ x 11 © ¹ o o f 2 § 1 · 11 x ¨1 ¸ © x 2 ¹ Reasoning (W) x tends to 1 hence it can not be infinite 14 (16) 18 (23) No change (W) It is entirely correct!! 8 (9.2) 4 (5.1) Empty or unclear The answer is missing or does not make 24 (28) 17 (22) any sense
§ 2 · Task 5: Problem: Decide the following limit value: lim xsin¨ ¸ . xof © x ¹ The students were given the following:
§ 2 · sin¨ ¸ § 2 · © x ¹ 2 Solution: xsin¨ ¸ . We know that o 0 when x o f and © x ¹ 1 x x 1 o 0 when x o f . x § 2 · sin¨ ¸ sin x © x ¹ The limit value lim 1 implies that o1 when x o f . xo0 x 1 x The task was for the students to decide the proper adjustments to make the solution correct:
Adjustments (What changes or complements are needed and why):
10 Table 5. Typical student answers in the categories for Task 5. Number of students (%). Category 5 Spring Autumn 2002 2002 All right (R) § 2 · § 2 · 10 (11) 17 (22) sin¨ ¸ sin¨ ¸ x x © ¹ © ¹ 2 o 2 1 2 x x Part right (R) 2 1 9 (10) 11 (14) is not the same thing as therefore you Incomplete x x can not use limit values on limit values Reasoning (W) 1 31 (36) 18 (23) o 0 when x o f so the denominator has x to go o 0 and the expressiono f No change (W) Nothing 4 (4.6) 1 (1.3) Empty or unclear The answer is missing or does not make any 33 (38) 32 (41) sense
Analysis The students’ solutions to Task 1a-b are mainly correct. The correct solutions were divided into categories, with most responses falling into two categories. The second category explicitly suggests that the limit value is attainable and a large proportion of the students have chosen this way to solve the task. An even larger number of the students solved the task in words of approaching the limit value. The idea that limits are not attainable comes again in Task 1c-d where students use this as an argument for the function not to attain the value 9. It is a documented fact that some students have this misinterpretation of the limit value definition (Cornu, 1991; Tall, 1991). Some students do not separate the part with the limit value from the part with the function, that is they mix up f(a) and lim f (x) as Davis and Vinner (1986) describe. If the students have the conviction xoa that limits are unattainable, there might be problems analysing the function. This is something that follows through Task 1 to Task 3. A large part of the students did not answer or answered Task 1c-d in a way that did not make any sense. Some wrote that it depends on what x tends to and this is the reason for the additional words in the formulation of the task in the Autumn of 2002. The result of the change is that this category contained a smaller number of students the Second Semester. This applies for all the c-d questions.
Task 2 is harder for the students to handle and one serious problem is algebra. There are several students who think that x 3 in the numerator and the denominator are cancelling out in a way that erases the terms and leaves only the constants or that x 3 can be replaced by 1 with a similar reasoning. There are more students in the Autumn Study using algebra in a correct manner to solve the task than in the Spring Study. The majority of the students are unable to solve Task 2c-d. One problem is attainability as discussed above. Only some
11 students regard this problem as an equation to solve and this is obvious in Task 3c-d as well. 22% of the students in the Autumn Study solved Task 2c-d with an equation in a correct way whereas the corresponding figure in the Spring Study is 6.3. Eight students from each Semester abused the equal sign and wrote x3 2 x3 in their explanations in Task 2b. Their results are in the column x3 1 x3 “Exclude –2 & 1 (R)”.
Task 3a-b is a standard limit value that is well known to the students apart from about 10% who got it backwards (“Infinity or no limit (W)”). There are fewer categories for this task since over 70% of all the students used the standard limit value reasoning. The problems come in Task 3c-d where the mix up of limits and functions is clear. Students claim that x is never zero since x tends to infinity, but this has nothing to do with the functions ability to attain the value 0. Algebra is a problem for some students here too. Some believe that 20 0 , for example.
Task 4 offers a challenge for many students. Forty-six of the 165 students were able to solve the task completely. One mistake many made was to use the dominant factor and divide, but that is not solving the problem with left and right limit value. This method is often used when x tends to infinity and the students seem to just go through the motions without considering the characteristics of the task they are involved with.
Task 5 is apparently the most demanding one since only 27 of all the students managed to solve it properly. The 20 students in the “Part right (R) Incomplete” category were also correct but they did not give the limit value, they only pointed at the inaccuracy so they might or might not be able to carry out the calculations. The majority of the students either left the task unsolved or reasoned incorrectly.
Discussion The tables show that the students’ explanations of choices of solutions vary. They are good at finding limits in the first three tasks. The problems seem to come when the tasks are a bit different from what the students are used to. A task can be a problem for one student but just routine for another (Björkqvist, 2001; Grevholm, 1991) and this obviously leads to different outcomes for the students. Part c and d are not mathematically more demanding than the other parts of Task 1 to Task 3 but there is something that troubles the students in them. The outcome might have been different if the parts were not presented together. The students were working in the context of limits when they were asked to examine the functions for attainability. The effect was that the functions were only considered locally in some cases and the misconception that limits are unattainable (Cornu, 1991; Tall, 1993; Szydlik, 2000) made some students claim that the function could not attain the value even if it obviously
12 could (Task 1 and Task 3). There were also other types of confusion. One student from the Study in the Autumn of 2002 answered Task 1 like this:
1a: o 9 1b: If x o ( 3) the result becomes 9, the function will never attain that. 1c: Yes. 1d: If you let x o 3 .
The confusion of functions with limits of functions is a problem that indicates a lack of relations between the concepts. If the students were more confident about the roles and possibilities of the notions they would have a better chance to solve problems correctly. An insufficient mathematical base to work from can cause constraints on the individual, in that he or she is not sure what operations are allowed and how to carry them out. This uncertainty can be the reason for the many empty answers.
Infinity is obviously an element that can cause confusion (Tall, 1980). All tasks revealed problems with infinity in different degrees. One thing that is connected with infinity is the notion of local limits in a wider context. Table 5 indicates this by the categories “Reasoning (W)” and “No change (W)”. Many students are reasoning about the local limits for the functions 2 and 1 separately or x x dissect the given function in other ways and locally consider limits. The students follow part of Pólya’s (1945) model with decomposing and recombining, but the recombining to check at the whole again is overlooked. The students’ reasoning hints an attempt for plausible reasoning or established experience (Lithner, 2003), but there appears to be a lack of parts in the mental web that represents this fraction of the concept image (Tall & Vinner, 1981) since they do not have access to the essential information about the properties of the limit process and functions. The development of conceptual knowledge (Hiebert & Lefevre, 1986) has not had a satisfactorily progress.
Table 4 shows an example of identification of similarities (Lithner, 2003) as the students in the category “Dominant factor (W)” use a solving technique that is usually effective on rational functions as x tends to infinity. Here the students recognise the rational function but they do not consider what x tends to. The resemblances at first sight are not the same on all crucial points and the chosen method is not working any better than the given solution. This task requires reasoning rather than an algorithm.
The choices of methods seem to be triggered by first sight resemblances in other cases too. There are comparisons with standard limit values. Sometimes the method is working, as it did for most students’ solutions to Task 3a-b. Other times it does not work, as for some suggested solutions to Task 5. The students do not appear to have a global view of the important characteristics of the
13 mathematics at hand. The effect of this can be that critical features are overlooked and the solution is beyond the possibility to reach for the students.
Table 2(ii) and Table 3(ii) show examples of solutions with correct answer and wrong explanation. The students from the Autumn Study do this to a higher extent than the other students which can be due to various reasons since the two groups learn mathematics under different circumstances. This is something that students must be confronted with to be able to repair. The textbook only gives the answer and not a full solution to the tasks so the first confrontation is in the worst scenario at the exam. If the students have used the wrong arguments for a long time, an adjustment can be hard to make. The students represented in Table 2(ii) who have answered correctly with no explanation can also belong to the category of students with correct answer for the wrong reasons since we do not know why they answered the way they did.
Algebra is the reason for a number of mistakes. Many of the algebraic errors are serious. Table 2 (i) shows the existence of some of them, but there are similar errors in other places too. Some examples are:
2 x 2 0 x 3 3 2 x 2 3 1 2 x x 2 x 3 1 x 3 11 1 x 3 x3 2 2 x 2 x 3 1 1 x3 2 x 3 1 2 1 x x 3 1 x 3 11 2
These examples expose a lack of knowledge in basic calculation rules that should not exist at University level. Investigations at upper secondary school and among student teachers in mathematics show that students find it hard to work with algebra and similar mistakes as those in this study are common (Grevholm, 2003; Olteanu, Grevholm & Ottosson, 2003). If the solution process is hindered by such matters, there is much for the students to work with to improve their concept images. The students have to be aware of the problems before they feel a need to alter anything and if the errors are not discovered nothing will happen.
Conclusion I have shown students’ different types of solutions to tasks about limits of functions and how they are explained. A variety of solutions of different accuracy appeared in the study. I did not expect to see so much confusion about functions and limits of functions or the problems caused by algebra. These are
14 examples of problems related to what is allowed and what is not. The large number of correct solutions based on wrong facts is also a serious problem that shows that, for many students, connections between concepts are wrong or not there at all. Perhaps students need to experience a larger variety of problems to understand the rules and properties of mathematics better. Problems that demand thought and provoke the students’ concept images give opportunities for the students to make appropriate adjustments. Then they would probably get to know vital intrinsic properties for notions at a deeper level as well. Large groups of students and time shortage make this kind of extra effort to help students very hard to carry out, but students should be able to see the core of the mathematics they work with and to recognise its characteristic features. These abilities can be developed if teachers can provide an environment that inspires the students to discuss mathematical issues.
Despite all errors and misunderstandings that have been documented here, there are skilful problem solvers among the students and that is something we must not forget. One goal of our teaching is to make able problem solvers of as many students as possible. We need not only to know how the mathematically weak students reason but also how the mathematically developed students do it.
References Björkqvist, O. (2001). Matematisk problemlösning. In B. Grevholm (Ed), Matematikdidaktik – ett nordiskt perspektiv (pp. 115-132), Lund: Studentlitteratur. Cornu, B. (1991). Limits. In D. Tall (Ed), Advanced mathematical thinking (pp. 153-166), Dordrecht: Kluwer Academic Publishers. Davis, R. (1986). Conceptual and procedural knowledge in mathematics: A summary analysis. In J. Hiebert (Ed), Conceptual and procedural knowledge: The Case of Mathematics (pp. 265-300), London: Lawrence Erlbaum Associates Publishers. Davis, R. & Vinner, S. (1986). The notion of limit: Some seemingly unavoidable misconception stages. Journal of Mathematical Behavior, 5, 281-303. Dreyfus, T. (1991). Advanced mathematical thinking processes. In D. Tall (Ed), Advanced mathematical thinking (pp. 25-41), Dordrecht: Kluwer Academic Publishers. Grevholm, B. (1991). Problem för lärare (Problems for teachers). In G. Emanuelsson, B. Johansson & R. Ryding (Eds) Problemlösning, Lund: Studentlitteratur. Grevholm, B. (2003) Student teachers’ conceptions of equations and inequalities. Paper presented at the Pre ICME10 conference in Växjö. Available at http://www.msi.vxu.se/picme10/SEMINAR.html (2003-11- 26).
15 Hiebert, J. & Lefevre, P. (1986). Conceptual and procedural knowledge in mathematics: An introductory analysis. In J. Hiebert (Ed), Conceptual and procedural knowledge: The case of mathematics (pp. 1-28), London: Lawrence Erlbaum Associates Publishers. Juter, K. (2003). Learning limits of function, University students’ development during a basic course in mathematics. (Licentiate thesis) Luleå: Luleå University of Technology, Department of Mathematics. Lithner, J. (2000). Mathematical reasoning in task solving, Educational Studies in Mathematics, 41, 165-190. Lithner, J. (2003). Students’ mathematical reasoning in university textbook exercises. Educational Studies in Mathematics, 52, 29-55. Milani, R. & Baldino, R. (2002). The theory of limits as an obstacle to infinitesimal analysis. PME26 2002, 345-352. N6, (NUD*IST computer program) (2003). www.qsrinternational.com Olteanu, C., Grevholm, B. & Ottosson, T. (2003) Algebra in upper secondary school: A study of teachers’ teaching and student learning. Paper presented at CERME3, Italy. Available at http://www.dm.unipi.it/~didattica/CERME3/WG6/TG6-list.html (2003-11-21). Pólya, G. (1945). How to solve it. London: Penguin Books Ltd. Schoenfeld, A. (1992). Learning to think mathematically: problem solving, metacognition and sence-making in mathematics. In D. Grouws (Ed), Handbook for research on mathematics teaching and learning (pp. 334- 370), New York: Macmillan. Silver, E. (1986). Using conceptual and procedural knowledge: A focus on relationships. In J. Hiebert (Ed), Conceptual and procedural knowledge: The case of mathematics (pp. 181-198), London: Lawrence Erlbaum Associates Publishers. Skolverket, (2003). Kursinfo 2005/2006. http://www3.skolverket.se Star, J. (2000). On the relationship between knowing and doing in procedural learning. In B. Fishman & S. O’Connor-Divelbiss (Eds.), Fourth International Conference of the Learning Sciences (pp. 80-86), Mahwah, NJ: Erlbaum. Szydlik, J. (2000). Mathematical beliefs and conceptual understanding of the limit of a function, JRME online, vol 31, issue 3, 258-276. Tall, D. (1980). Mathematical intuition, with special reference to limiting processes. In R. Karplus (Ed), Proceedings of the Fourth International Conference for the Psychology of Mathematics Education (pp. 170-176). Berkeley, CA: PME. Tall, D. (1991). The psychology of advanced mathematical thinking. In D. Tall (Ed), Advanced mathematical thinking (pp. 3-24), Dordrecht: Kluwer Academic Publishers. Tall. D. (1993). Students’ difficulties in calculus. Proceedings of working group 3 on students’ difficulties in calculus, ICME-7, Québec, Canada, 13-28.
16 Tall, D. (2001). Natural and formal infinities. Educational Studies in Mathematics, 48, 199-238. Tall, D. & Vinner, S. (1981). Concept image and concept definition in mathematics with particular reference to limits and continuity. Educational Studies in Mathematics, 12, 151-169. Vinner, S. (1991). The role of definitions in the teaching and learning of mathematics. In D. Tall (Ed), Advanced mathematical thinking (pp. 65-81), Dordrecht: Kluwer Academic Publishers. Williams, S. (1991). Models of limit held by college calculus students. Journal for Research in Mathematics Education, 22 (3), 219-236.
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Article VII
LIMITS AND INFINITY - A STUDY OF UNIVERSITY STUDENTS’ PERFORMANCE Kristina Juter Barbro Grevholm Kristianstad University College, Sweden Agder University College, Norway
Students’ work with limits of functions includes the concept of infinity. These concepts are complex but necessary for mathematics studies. A study of student solutions to limit tasks was conducted to reveal how students handled limits of functions and infinity at a Swedish university. As a background to the students’ and teachers’ situations we have also analysed textbooks and curricula. One result is that many students were unable to handle limits correctly. Some students gave correct answers with incorrect explanations to tasks. Textbooks used at upper secondary schools do not provide much theory or many tasks about limits and infinity so most students new to university do not have a developed image of these concepts.
BACKGROUND AND QUESTIONS Students at universities taking courses in analysis are supposed to solve a considerable number of tasks. The students presented in this article were suggested about 30 to 40 tasks per week to solve. Students can check in their textbook if they get correct answers and then continue to work with their problems. An important question is: What if some of the solutions are correct for the wrong reasons? In this article we discuss some of the results from an investigation into students learning limits of functions at university level (Juter, 2003). The main aim of the whole study was to learn more about the development of the students’ conceptions of limits of functions. No such study had been done in Sweden before, but there are researchers in other countries (e.g. Cornu, 1991; Davis, & Vinner, 1986; Milani, & Baldino, 2002; Szydlik, 2000; Tall, 1980; Tall, & Vinner, 1981; Williams, 1991) who have investigated students’ learning of limits. It appears as if the concept of limit is so complicated for students to handle that some of them develop an incoherent interpretation of it. Infinity is also a concept that causes difficulties for many students when they solve problems about limits of functions (Tall, 2001; Tirosh, 1991). The research question posed in this article is the following: How do students explain their solutions to limit tasks in relation to their use of infinity? Limits and infinity in curricula and textbooks are discussed in connection to the students’ achievements.
THEORETICAL FRAMEWORK A concept image is the total cognitive representation of a notion that an individual has in his or her mind (Tall, & Vinner, 1981). A concept image is similar to a schema, which is a commonly used word for a mental representation (e.g. Cottrill, Dubinsky, Nichols, Schwingendorf, & Vidakovic, 1996; Juter, 2005a). The concept
1 image might be partially evoked and different parts can be active in different situations leading to possible inconsistencies. An individual’s concept image might differ from the formal concept definition or the concept image in itself can be confusing or incoherent. The students in the study presented here are placed in problem solving situations designed to reveal how their concept images of limits develop during a semester of mathematics studies. The students’ solutions and explanations of their solutions are considered to be expressions of their concept images. The concept of infinity can cause difficulties. It is a notion about which an individual may have one or several intuitive representations (Tall, 1980). If there are multiple different representations evoked simultaneously the individual might be confused. When dealing with limits of functions one has no specific method or algorithm as one has for diophantic equations for example. The limit process appears potentially infinite and students can get the impression that there is no end to it (Tall, 2001). It can be difficult for them to work with items that are confusing in identity. Is it an object or a process? Infinity is not dealt with in a systematic manner in school (Skolverket, 2005) so the students have their own constructions of the concept. If defective or limited constructions are not challenged until the students need them, they may have a long time to become established in the students’ minds. Tirosh (1991) discussed different types of infinities. Conflicting properties can result in a variety of intuitive meanings and previous experience can cause conflict with formal theory. Limits are frequently connected to measuring infinity, which comes from situations where, for example, a line segment of length L contains half as many infinitesimally small points as a line segment of length 2L, since limits often are in a context where something becomes larger or smaller (Tall, 1992). This stands in contrast to cardinal infinity in Cantor’s sense, which, for example, results in the fact that the number of rational numbers and the number of even numbers are the same. Intuitions of cardinal infinity are resistant to development and regular mathematical training, and sensitive to the context in which a problem is posed (Tirosh, 1991). These statements point to the importance of an explicit discussion of infinity at universities and schools and the careful choices of tasks. A common obstacle in students’ understanding of limits of functions is that they think that functions can not attain their limits (Cornu, 1991; Juter, 2005a; Tall, 1993; Tall, 2001; Szydlik, 2000). There is also a possible mix-up of f (a) and lim f (x) xoa (Davis, & Vinner, 1986). These two flaws combined can block students in their struggle with tasks that could easily be solved with an equation for instance. The theoretical framework is presented in greater detail in an exposition of the whole study (Juter, 2003).
2 EDUCATIONAL CONTEXTS OF STUDENTS AND TEACHERS Students’ and teachers’ understanding are products of curricula from earlier years. Teachers are fostered by these documents and they pass on traditions to textbooks and students. It is therefore important to consider students’ achievements in the context of curricula and textbooks. Even if there is a new curriculum, it can take some time to implement it in the classrooms. The intended curriculum, expressed in formal documents, might differ from the implemented curriculum, expressed in, for example, textbooks. There is also a third factor, the achieved curriculum, which is what the students actually manage to accomplish (Valverde, Bianchi, Wolfe, Schmidt, & Houang, 2002). In this article we place our study about students’ accomplishments in a broader educational context of textbooks and curricula. Limits in the curricula The curriculum of today for upper secondary school mathematics in Sweden states that the concept of limit shall be introduced in an informal way and with the purpose of enabling the pupil to handle derivatives. Limits are not mentioned per se, but required for the concepts that are mentioned, as illustrated by the following passage from the curriculum (Skolverket, 2005, http://www3.skolverket.se/ki03/front.aspx? sprak=SV&ar=0506&infotyp=8&skolform=21&id=MA&extraId=) Matematik C builds on Matematik B within arithmetic, algebra and functions. It also comprises differential calculus. In the course, problems concerning optimisation, changes and extreme values are treated. (Authors’ translation) The history behind this treatment is interesting. In the curriculum from 1965 (Läroplan för gymnasiet, 1969, p. 266) it is stated: Limits: Limits, when x tends to infinity, minus infinity and to a, shall be given a correct definition. As an illustration functions will be drawn in a coordinate system or in set diagrams, whereby the concept neigbourhood can be used. Further, examples shall be given of functions without limit and functions with no finite limit. In some case a calculation of an omega to a given epsilon shall be done such that for x !Z it follows that fx b . Rules for limits shall be presented for the sum, product and quotient H. of limits…(Authors’ translation) This plan was a consequence of the new math movement and changes followed quickly. The psychological reactions from students and teachers convinced curriculum developers that the plan was not realistic. A revision came already five years later in Lgy70 (Skolöverstyrelsen, 1971). Then there is a division between central topics and less central topics in the curriculum. Under “Central Topics” it says (Skolöverstyrelsen, p. 105, authors’ translation) “Limits and continuity intuitively with support from drawings.” As “Less Central Topics” is taken “Formal definitions of limit and continuity.” But there was a time when students at the age of 17 were supposed to learn about limits in a formal way based on a definition involving epsilon and omega and formal proofs. Students in the
3 present study show examples of how problematic the learning of the limit concept can be at university level. The study illustrates that there is a need to consider carefully how to treat the concept of limit in the teaching of mathematics at different levels of schooling. In 1981 a new supplement for the curriculum in mathematics for a natural science programme was agreed on (Skolöverstyrelsen, 1981). In this text the concept of infinity was not mentioned, and the concept of limit was also not mentioned per se. It was implicitly presented through examples both in the definition of the derivative of a function and of convergence of series. “Stringent treatment of convergence is not required” (Skolöverstyrelsen, p. 38, authors’ translation). The example given was 1 cos x lim . Thus students are expected to work with the limit concept despite the fact xo0 3x 2 that no information or instruction is given in the curriculum that the concept is supposed to be introduced to them. A consequence of this lack of transparency of the curriculum constructors’ intentions could be that the teachers go on teaching as they did before the new curriculum came. Or it could be that the teachers are not enough aware of how to work with limits and do not pay enough attention to the concept. Limits and infinity in textbooks Limits of functions are introduced at upper secondary school before the students work with derivatives. The two textbooks (Björup, Körner, Oscarsson, Sandhall, & Selander, 1995; Björk, & Brolin, 2000) investigated are commonly used at upper secondary schools in Sweden. They both present the theory of limits of functions very briefly as an introduction to derivatives. One has a definition stated informally, but the other one does not even have a description of limits. The tasks in the textbooks were designed to develop the students’ ability to calculate derivatives and all examples and tasks with limits are intended to prepare students to meet the definition of the derivative with x or h tending to zero. Both textbooks present the infinity symbol in examples, but none of the books have a discussion or tasks dealing with different properties of infinity. In an earlier textbook for upper secondary school (Nyman, Emanuelsson, Bergman, & Bergström, 1980) the concept of limit is introduced as early as page five in connection with the definition of the derivative of a function. The differential quotient for a tangent is constructed, f (x) f (a) , and the text says (Nyman et al., p. x a 5): We now let B tend to A, that is x tend to a. If the quotient above has a limit, that is tends to a specific value, it is the slope of the tangent. The limit of the quotient is called the derivative of the function f(x) in the point a. (Authors’ translation) The same textbook starts to explain limits of functions about 65 pages later by looking at a sequence of examples that are increasingly complicated. The concept of infinity is introduced without any explanation at all. Additionally, so are non-finite
4 limits. Thus there is a long tradition in school mathematics of introducing complicated concepts without really discussing them or even mentioning or illustrating their meaning or definition. Limits are reintroduced at university level. The textbook used in the course studied (Hellström, Morander, & Tengstrand, 1991) introduces the concept of infinity briefly with some warnings about treating infinity without caution, and one of Zenon’s paradoxes at the beginning of the chapter about limits. The texts about limits are rigorously stated with definitions and theorems. The examples show the reader how to use the definition of limits to solve problems and how to carry through different methods of solution. There are a few explorative examples. The exercises are of the same character as the examples.
RATIONALE AND DESIGN OF THE STUDY The empirical study reported here was divided into two parts. The first part was conducted in a spring semester and the second part the following semester, that is, the autumn semester of the following academic year. The students in both parts were learning analysis and algebra at basic university level. Different methods were used to collect data to address the question stated in the introduction: How do students explain their solutions to limit tasks in relation to their use of infinity? Apart from analyzing curricula and textbooks, as aforementioned, questionnaires were used to document students’ competencies and explanations. The use of questionnaires gives an opportunity to gather more data than, for example, interviews, which were conducted in other parts of the study. One drawback of using questionnaires is that students can not be asked to further elaborate their answers, but in this part of the study, the students are regarded as a group and there is hence a value in many student responses that may reveal patterns of strategies and explanations. In the spring semester 111 students solved five tasks about limits. Each of the first three tasks had four parts for the students to respond to, for example: Task 1: a) Determine the limit: lim(2x 3) . b) Explanation. xo3 c) Can the function f (x) 2x 3 attain the limit in 1a? d) Why? The students had met limits of functions in their course and hence this was nothing new for them. Eleven days later they were given two additional tasks. 87 students participated in this second session. The last two tasks provided solutions that could be wrong or incomplete. This was stated on the sheet with the tasks and the students were asked to give a complete and correct solution for each task. The tasks were selected to show students’ responses at different levels of difficulty. The three first tasks were very similar to some tasks in the students’ textbook, while the following two tasks were presented in an unfamiliar manner, but with content that was not too dissimilar from some tasks in their textbook. These types of tasks were not common in the textbooks for upper secondary school, which were studied.
5 A new group of students were given the same tasks the following autumn semester. They were given all the five tasks at the same time, after the notion of limits of functions had been dealt with in their course. The difference in treatment was for practical reasons. 78 students took part in that study. In both cases the tasks were part of three questionnaires with questions about limits and attitudes towards mathematics. Two interviews with each of 15 of the students were conducted in the autumn study (Juter, 2003; Juter, 2005a). The tasks were constructed to focus on different aspects of the limit concept. The difficulty varied in order to identify the level the students could handle. The students were instructed both orally and in writing at each session when they responded to the questionnaires to make sure that it was clear to them. The data collected have been rewritten and then categorised with the aid of a computer program, NUD*IST. The categories were decided from the raw interview and questionnaire material. They were not created in detail in advance other than that the right and wrong answers made different categories. There were sub-categories in each of them based on the students’ justifications of their responses. The sub- categories were roughly determined before the data was collected. Then they were finally determined from the different reasons the students used in their solutions. This process led to a number of categories. Categories with similar types of reasoning were merged together to make the presentation more accessible (Juter, 2003; Juter 2004). This way to work with the data gave different types of category systems for the different tasks. The category systems were flexible so that new categories emerging from the data could be implemented if they were relevant to the study. The work with the categories is similar to the methods of grounded theory (Glaser, & Strauss, 1967) where categories evolve from data. It does not stand in conflict with the theories of concept formations presented in the theory section since the aim is to get as much information about how the students learn as possible.
SAMPLE OF DATA AND RESULTS Examples and student responses are presented in this section to show some results from the study. The selection of tasks was particularly done to bring forward cases where students reasoned about infinity. The results of the first two parts of Tasks 2 and 3 are briefly presented and the last two parts of the tasks are presented in greater detail. The results of Task 5 are also presented in detail. x 3 2 Task 2: a) Determine the limit: lim . b) Explanation. xof x 3 1 x 3 2 c) Can the function f (x) attain the limit in 2a? d) Why? x 3 1 Most students were able to correctly solve Task 2a-b. A majority of the students recognized the dominant term and disregarded the constants. Algebra was also used to explicitly calculate the limit. 15 % of the students were unable to handle the
6 algebra required for such calculations, i.e. divide numerator and denominator with the dominant term. Grevholm (2003) confirmed that university students often have inadequate algebraic knowledge to be able to work with mathematics at this level. The results of Task 2c-d are displayed in Table 1. (R) and (W) denote right and wrong answers respectively.
Table 1. Typical student answers to tasks 2c - d. Number of students (% of the group) Category 2c 2d Spring Autumn x 3 2 z x 3 1 (R) No x 3 2 can never be equal to 7 (6.3) 17 (22) x 3 1 for the same value on x -2 & 1 (R) No terms –2 and 1 will always 7 (6.3) 5 (6.4) remain No explanation (R) No - 18 (16) 8 (10) Infinity reason (W) No Since x never attains the value 16 (14) 21 (27) f Theory (W) No The function tends to the limit, 7 (6.3) 6 (7.7) it does not attain it No reason (W) Yes - 15 (14) 7 (9.0) Empty or The answer has no connection 42 (38) 17 (22) misinterpretation to the question or is missing
There are five categories for the answer “No”, which is the correct answer, but the categories labelled “Infinity reason” and “Theory” are marked “W” for wrong. The explanations in these categories do not correctly justify why the function can not attain the limit in the task. Some explanations in the “Infinity reason” category are difficult to label as right or wrong, for instance the example given in the table. The student’s explanation is that x never can be equal to infinity and there is a philosophical dimension to the reasoning. The explanation relies on the fact that infinity is unreachable and a termination of the limit process is therefore impossible, but then there would be no functions able to attain their limits in similar situations. Table 1 shows that about one quarter of the students gave a correct answer based on incorrect reasoning. The limit definition and infinity caused many of the errors here. None of the students who answered “Yes” provided an explanation to their answer. The third task was the following: x 5 Task 3: a) Determine the limit: lim . b) Explanation. xof 2 x x 5 c) Can the function f (x) attain the limit in 3a? d) Why? 2 x
7 Almost all students managed to calculate the limit in Task 3a and to explain their answer with a standard limit in Task 3b. There was a greater variety in the students’ answers to Tasks 3c and 3d as shown in Table 2 below. (R) and (W) denote right and wrong answers respectively.
Table 2. Typical student answers to tasks 3c - d. Number of students (% of the group) Category 3c 3d Spring Autumn 0 x = 0 (R) Yes For x = 0 o f(0) = = 0 15 (14) 16 (21) 1 No explanation (R) Yes - 5 (5) 6 (7.7) Does not reach No We can only get infinitely 14 (13) 16 (21) limit (W) close.
5 0 No Then the numerator has to be 22 (20) 12 (15) x z 0 or (W) 0 zero and it never is. Right for wrong Yes Because the denominator 5 (5) 16 (21) reason (W) attains a much larger number for large x. Empty or The answer has no connection 47 (42) 12 (15) misinterpretation to the question or is missing.
Less than twenty percent of the students delivered a correct answer with a correct explanation even though the task is of standard type and the function is well known to the students. A large proportion of the students was unable to correctly decide if the function could attain the limit. The third category includes responses that reveal the confusion of limits of functions with functions as described by Davis and Vinner (1986). The function’s ability to attain the value zero is influenced by the context of limits and the properties of limits as the students perceive them. This confusion is apparent in other parts of the results from the study as well (Juter, 2003; Juter, 2005a) where students often think that the limit definition states that limits are unattainable for functions. The fifth category includes students who demonstrate problems in dealing with infinity. The students in this category had the correct answer but did not offer a correct justification for it. The student who gave the example in Table 2 (“Yes”, “Because the denominator attains a much larger number for large x”) drew his or her conclusions too far in the reasoning about infinity and stated that the function eventually reaches the value zero as x tends to infinity. It is again a philosophical matter of what happens at infinity. Such an interpretation of infinity is the opposite of the example in the fourth category in Table 1 where the student claimed that x could not reach infinity.
8 The fifth task was stated as a problem with a given solution: § 2 · Task 5: Problem: Determine the following limit: lim xsin¨ ¸ . xof © x ¹ The students were given the following solution: § 2 · sin¨ ¸ § 2 · © x ¹ 2 1 Solution: xsin¨ ¸ . We know that o 0 when x o f and o 0 © x ¹ 1 x x x § 2 · sin¨ ¸ sin x © x ¹ when x o f . The limit lim 1 implies that o1 xo0 x 1 x when x o f . Then the students were asked to make the solution correct and complete. Table 3 shows the categories for the students’ explanations. (R) and (W) denote right and wrong answers respectively.
Table 3. Typical student answers to task 5. Number of students (% of the group) Category 5 Spring Autumn All right (R) § 2 · § 2 · 10 (11) 17 (22) sin¨ ¸ sin¨ ¸ x x © ¹ © ¹ 2 o 2 1 2 x x
Part right (R) 2 is not the same thing as 1 therefore 9 (10) 11 (14) Incomplete x x you can not use limits on limits Reasoning (W) 1 31 (36) 18 (23) o 0 when x o f so the denominator x has to go o 0 and the expression o f No change (W) Nothing 4 (4.6) 1 (1.3) Empty or unclear The answer is missing or does not 33 (38) 32 (41) make any sense
Not many students gave a complete and correct solution to this task. Ten from the spring study and seventeen from the autumn study did. Nine respectively eleven students pointed to the error in the given solution but did not give an alternative one. Several students were reasoning about infinity in an incorrect manner as exemplified
9 by the student answer given in the third category in Table 3. Such answers illustrate how some of the students calculated limits locally and did not consider the function as an entity. Student answers in the category labelled “No change” also showed evidence of this local view on limits. The difference is that in the latter case, the solution was given and in the former the students made the solutions themselves.
DISCUSSION A large number of the students solved the tasks in words of approaching the limit. The idea that limits are not attainable is revealed in quite a few parts of the data. Some students used this as an argument for functions not to attain limits in parts c and d of the tasks. There is well-documented evidence that some students have this misinterpretation of the limit definition (Cornu, 1991; Tall, 1991). Some students did not separate the part with the limit from the part with the function, that is, they mixed upf (a) and lim f (x) as Davis and Vinner (1986) described. If students believe that xoa limits are unattainable, there might be problems for them to analyse functions. Infinity, as treated in the fifth category in Table 2, does not seem to be explicitly handled at upper secondary school or at universities, judged from the textbooks studied. Students are influence by their personal conceptions of infinity when they solve problems which can lead to conflicts (Tirosh, 1991). If the students do not have any prior experience of discussing or even thinking of different features of infinity they will not have the ability to correctly analyse limits. All tables show problems with infinity. Table 1 (category “No reason”) and Table 2 (category “Right for wrong reason”) have categories where the solutions suggest that limits are attainable for very large values of x or if x is equal to infinity. In the first case, this is not explicitly stated and the students could be guessing. When the constants are disregarded in the first part of the task there is a possibility that the students keep the constants away in their reasoning and therefore claim that the function can attain the limit. It can also be that the constants are regarded to be embedded into infinity and hence not considered anymore. The absence of explanations makes it impossible to know exactly why the students answered “Yes”. More than one quarter of the solutions were correct with an inaccurate explanation in Task 2, and more than every tenth of the solutions to Task 3 were correct with an inaccurate explanation. Many of the explanations are wrong based on an inadequate conception of infinity, or perhaps an inability to express a correct conception. Experiences of success with incorrect rationale give a false sense of security about the accuracy of the concept image, and our hypothesis is that it can be avoided with an explicit treatment instead of, as suggested in curricula and textbooks, an intuitive approach. The fact that textbooks do not deal with infinity in a thorough and explicit manner can give students and teachers the impression that it is sufficient for students to work with their possibly vague conceptions from childhood and school. The results
10 presented in this article do not support such impressions. There is a need for students to explicitly learn limits of functions and infinity to prevent erroneous reasoning, as has been exemplified in the present study. This is not the first occurrence of such reports on students’ learning difficulties at universities. Högskoleverket (2005) in Sweden recently issued a warning about students’ lack of interest in mathematics and about their insufficient pre-knowledge in mathematics. The suggested solution is to have a better cooperation between upper secondary schools and universities. The transitions to the formality of the subject treatment at universities can then be less dramatic. In their conclusion they point to the fact that the problems discussed are not new and that long-term and systematic interventions are missing. It is necessary now to make sustainable initiatives and changes (Högskoleverket, 2005, p. 65). We claim that if this is a serious commitment it would be valuable to take into consideration results from studies such as this on learning limits.
CONCLUSIONS Students expose a variety of ways of explaining their solutions to limit problems. Students with adequate reasoning in their explanations are not nearly as many as those who make mistakes, often caused by students’ conceptions of infinity, solving these fairly easy tasks. A result such as this can not be accepted as sufficient. One conclusion is that complex concepts such as infinity and limits should be introduced and handled more carefully by teachers and textbook authors at both school and university level. A way to continue the work could be to implement and evaluate some intervention programme in order to construct alternative learning trajectories for students. Teachers at universities must be aware of how students have created their early conceptions of limits and infinity, and try to offer opportunities to develop these, often limited and insufficiently matured, conceptions to more meaningful concepts. References Björk, L., & Brolin, H. (2000). Matematik tretusen, kurs C och D, (Mathematics three thousand, course C and D). Falköping, Sweden: Bokförlaget Natur och Kultur. Björup, K., Körner, S., Oscarsson, E., Sandhall, Å., & Selander, T. (1995). Delta, matematik för gymnasiet kurs C + D, (Delta, mathematics for upper secondary school course C+D). Malmö, Sweden: Gleerups förlag. Cornu, B. (1991). Limits. In D. Tall (Ed.), Advanced mathematical thinking (pp. 153-166). Dordrecht: Kluwer Academic Publishers. Cottrill, J., Dubinsky, E., Nichols, D., Schwingendorf, K., & Vidakovic, D. (1996). Understanding the limit concept: Beginning with a coordinated process scheme. Journal of Mathematical Behaviour 15, 167-192. Davis, R., & Vinner, S. (1986). The notion of limit: Some seemingly unavoidable misconception stages. Journal of Mathematical Behavior 5, 281-303.
11 Glaser, B., & Strauss, A. (1967). The discovery of grounded theory. London: Weidenfelt & Nicholson. Grevholm, B. (2003). Student teachers’ conceptions of equations and inequalities. Paper presented at the Pre ICME10 conference in Växjö. Retrieved February 6, 2006 from http://www.vxu.se/msi/picme10/L4GB.pdf. Hellström, L., Morander, S., & Tengstrand, A. (1991) Envariabelanalys, (One variable analysis). Lund: Studentlitteratur. Högskoleverket (2005). Stora brister i mattekunskaper hos nybörjarstudenter. Retreived February 4, 2006 from http://web2.hsv.se/publikationer/pressmeddelanden/2005/050818_ny.shtml Juter, K. (2003). Learning limits of functions, university students' development during a basic course in mathematics. (Licentiate thesis) Luleå: Luleå University of Technology. Juter, K. (2004). Limits of functions – How students solve tasks. In C. Bergsten, & B. Grevholm (Eds.), Mathematics and language, Proceedings of MADIF 4, the 4th Swedish Mathematics Education Research Seminar (pp. 146-156). Linköping, Sweden: Svensk Förening för MatematikDidaktisk Forskning. Juter, K. (2005a). Limits of functions – how do students handle them? Pythagoras 61, 11- 20. Juter, K. (2005b). Students’ attitudes to mathematics and performance in limits of functions. Mathematics Education Research Journal 17(2), 91-110. Läroplan för gymnasiet (1969). Skolöverstyrelsens skriftserie 80 (5:e tryckningen). Stockholm: Utbildningsförlaget. Milani, R., & Baldino, R. (2002). The theory of limits as an obstacle to infinitesimal analysis. In A. D. Cockburn, & E. Nardi (Eds.), Proceedings of the 26th Conference of the International Group for the Psychology of Mathematics Education (pp. 345-352), Norwich, UK: University of East Anglia. Nyman, B., Emanuelsson, G., Bergman, M., & Bergström, L. (1980). Matematik för gymnasieskolan, NT2, Huvudbok, (Mathematics for upper secondary school, NT2, Main book). Stockholm: Esselte studium. Skolverket, (2005). Retrieved February 6, 2006 from http://www3.skolverket.se/ki03/front.aspx?sprak=SV&ar=0506&infotyp=8&skolform=2 1&id=MA&extraId= Skolöverstyrelsen (1971). Lgy70. Planeringssupplement III. Stockholm: Utbildningsförlaget. Skolöverstyrelsen (1981). Läroplan för gymnasieskolan. Supplement 75. Matematik för treårig naturvetenskaplig och fyraårig teknisk linje. Stockholm: Utbildningsförlaget. Szydlik, J. (2000). Mathematical beliefs and conceptual understanding of the limit of a function, Journal for Research in Mathematics Education 31(3), 258-276.
12 Tall, D. (1980). Mathematical intuition, with special reference to limiting processes. In R. Karplus (Ed.), Proceedings of the Fourth International Conference for the Psychology of Mathematics Education (pp. 170-176). Berkeley, CA: PME. Tall, D. (1991). The psychology of advanced mathematical thinking. In D. Tall (Ed.), Advanced mathematical thinking (pp. 3-24). Dordrecht: Kluwer Academic Publishers. Tall, D. (1992). The transition to advanced mathematical thinking: Functions, limits, infinity and proof. In D. A. Grouws (Ed.), Handbook of research on mathematics teaching and learning (pp. 495-511). New York: National Council of Teachers of Mathematics, Macmillan. Tall. D. (1993). Students’ difficulties in calculus. Proceedings of Working Group 3 on Students’ Difficulties in Calculus, ICME-7, Québec, Canada, 13-28. Tall, D. (2001). Natural and formal infinities. Educational Studies in Mathematics 48, 199- 238. Tall, D., & Vinner, S. (1981). Concept image and concept definition in mathematics with particular reference to limits and continuity. Educational Studies in Mathematics 12, 151- 169. Tirosh, D. (1991). The role of students’ intuitions of infinity in teaching the cantorian theory. In D. Tall (Ed.), Advanced mathematical thinking (pp. 199-214). Dordrecht: Kluwer Academic Publishers. Valverde, G. A., Bianchi, L. J. Wolfe, R. G., Schmidt, W. H., & Houang, R. T. (2002). According to the book. Using TIMSS to investigate the translation of policy into practice through the world of textbooks. Dordrecht: Kluwer Academic Publishers. Williams, S. (1991). Models of limit held by college calculus students. Journal for Research in Mathematics Education 22(3), 219-236.
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