1 Axiomatic Systems

Total Page:16

File Type:pdf, Size:1020Kb

1 Axiomatic Systems 1 Axiomatic Systems 1.1 Features of Axiomatic Systems One motivation for developing axiomatic systems is to determine precisely which prop erties of certain ob jects can b e deduced from which other prop erties. The goal is to cho ose a certain fundamental set of prop erties the axioms from which the other prop erties of the ob jects can b e deduced e.g., as theorems . Apart from the prop erties given in the axioms, the ob jects are regarded as unde ned. As a p owerful consequence, once you haveshown that any particular collection of ob jects satis es the axioms however unintuitive or at variance with your preconceived notions these objects may be, without any additional e ort you may immediately conclude that all the theorems must also b e true for these ob jects. Wewanttocho ose our axioms wisely. Wedonotwant them to lead to contradictions; i.e., wewant the axioms to b e consistent. We also strive for economy and wanttoavoid redundancy|not assuming any axiom that can b e proved from the others; i.e., wewantthe axiomatic system to b e independent.Finally,wemaywishtoinsistthatwebeabletoprove or disproveany statement ab out our ob jects from the axioms alone. If this is the case, we say that the axiomatic system is complete. We can verify that an axiomatic system is consistentby nding a model for the axioms|a choice of ob jects that satisfy the axioms. We can verify that a sp eci ed axiom is indep endent of the others by ndingtwo mo dels|one for which all of the axioms hold, and another for which the sp eci ed axiom is false but the other axioms are true. We can verify that an axiomatic system is complete by showing that there is essentially only one mo del for it all mo dels are isomorphic ; i.e., that the system is categorical. Reference: Kay, Section 2.2. 1 1.2 Examples Let's lo ok at three examples of axiomatic systems for a collection of committees selected from a set of p eople. In each case, determine whether the axiomatic system is consistentor inconsistent. If it is consistent, determine whether the system is indep endent or redundant, complete or incomplete. 1. a There is a nite numb er of p eople. b Each committee consists of exactly two p eople. c Exactly one p erson is on an o dd numb er of committees. 2 2. a There is a nite numb er of p eople. b Each committee consists of exactly two p eople. c No p erson serves on more than two committees. d The numb er of p eople who serve on exactly one committeeiseven. 3 3. a Each committee consists exactly two p eople. b There are exactly six committees. c Each p erson serves on exactly three committees. 4 1.3 Kirkman's Scho olgirl Problem Consider the following axiomatic system for p oints and lines, where lines are certain subsets of p oints, but otherwise p oints and lines are unde ned. Given anytwo distinct p oints, there is exactly one line containing b oth of them. Given anytwo distinct lines, they intersect in a single p oint. There exist four p oints, no three of which are contained in a common line. The total numb er of p oints is nite. 1. Try to come up with some mo dels for this axiomatic system. 2. Find a mo del containing exactly 15 p oints. 3. Solve the following famous puzzle prop osed byT.P. Kirkman in 1847: A school-mistress is in the habit of taking her girls for a daily walk. The girls are fteen in number, and are arrangedin verows of threeeach, so that each girl might have two companions. The problem is to dispose them so that for seven consecutive days no girl wil l walk with any of her school-fel lows in any triplet more than once. 5 1.4 Finite Pro jective Planes The axioms describ ed in the previous section de ne structures called nite projective planes.I have a game called Con gurations that is designed to intro duce the players to the existence, construction, and prop erties of nite pro jective planes. When I checked in August 1997 the game was available from WFF 'N PROOF Learning Games Asso ciates, 1490 South Boulevard, Ann Arb or, MI 48104, phone: 313 665-2269, fax: 313 764-9339, for a cost of $12.50. Here are examples of some problems from this game: 1. In eachbox b elowwriteanumb er from 1 to 7, sub ject to the two rules: 1 The three numb ers in each column must b e di erent; 2 the same pair of numb ers must not occurintwo di erent columns. Col 1 Col 2 Col 3 Col 4 Col 5 Col 6 Col 7 Row1 Row2 Row3 6 2. Use the solution to the ab ove problem to lab el the seven p oints of the following diagram with the numb ers 1 through 7 so that the columns of the ab ove problem corresp ond to the triples of p oints in the diagram b elow that lie on a common line or circle. This is called the Fano plane. 7.
Recommended publications
  • Finding an Interpretation Under Which Axiom 2 of Aristotelian Syllogistic Is
    Finding an interpretation under which Axiom 2 of Aristotelian syllogistic is False and the other axioms True (ignoring Axiom 3, which is derivable from the other axioms). This will require a domain of at least two objects. In the domain {0,1} there are four ordered pairs: <0,0>, <0,1>, <1,0>, and <1,1>. Note first that Axiom 6 demands that any member of the domain not in the set assigned to E must be in the set assigned to I. Thus if the set {<0,0>} is assigned to E, then the set {<0,1>, <1,0>, <1,1>} must be assigned to I. Similarly for Axiom 7. Note secondly that Axiom 3 demands that <m,n> be a member of the set assigned to E if <n,m> is. Thus if <1,0> is a member of the set assigned to E, than <0,1> must also be a member. Similarly Axiom 5 demands that <m,n> be a member of the set assigned to I if <n,m> is a member of the set assigned to A (but not conversely). Thus if <1,0> is a member of the set assigned to A, than <0,1> must be a member of the set assigned to I. The problem now is to make Axiom 2 False without falsifying Axiom 1 as well. The solution requires some experimentation. Here is one interpretation (call it I) under which all the Axioms except Axiom 2 are True: Domain: {0,1} A: {<1,0>} E: {<0,0>, <1,1>} I: {<0,1>, <1,0>} O: {<0,0>, <0,1>, <1,1>} It’s easy to see that Axioms 4 through 7 are True under I.
    [Show full text]
  • The Axiom of Choice and Its Implications
    THE AXIOM OF CHOICE AND ITS IMPLICATIONS KEVIN BARNUM Abstract. In this paper we will look at the Axiom of Choice and some of the various implications it has. These implications include a number of equivalent statements, and also some less accepted ideas. The proofs discussed will give us an idea of why the Axiom of Choice is so powerful, but also so controversial. Contents 1. Introduction 1 2. The Axiom of Choice and Its Equivalents 1 2.1. The Axiom of Choice and its Well-known Equivalents 1 2.2. Some Other Less Well-known Equivalents of the Axiom of Choice 3 3. Applications of the Axiom of Choice 5 3.1. Equivalence Between The Axiom of Choice and the Claim that Every Vector Space has a Basis 5 3.2. Some More Applications of the Axiom of Choice 6 4. Controversial Results 10 Acknowledgments 11 References 11 1. Introduction The Axiom of Choice states that for any family of nonempty disjoint sets, there exists a set that consists of exactly one element from each element of the family. It seems strange at first that such an innocuous sounding idea can be so powerful and controversial, but it certainly is both. To understand why, we will start by looking at some statements that are equivalent to the axiom of choice. Many of these equivalences are very useful, and we devote much time to one, namely, that every vector space has a basis. We go on from there to see a few more applications of the Axiom of Choice and its equivalents, and finish by looking at some of the reasons why the Axiom of Choice is so controversial.
    [Show full text]
  • Equivalents to the Axiom of Choice and Their Uses A
    EQUIVALENTS TO THE AXIOM OF CHOICE AND THEIR USES A Thesis Presented to The Faculty of the Department of Mathematics California State University, Los Angeles In Partial Fulfillment of the Requirements for the Degree Master of Science in Mathematics By James Szufu Yang c 2015 James Szufu Yang ALL RIGHTS RESERVED ii The thesis of James Szufu Yang is approved. Mike Krebs, Ph.D. Kristin Webster, Ph.D. Michael Hoffman, Ph.D., Committee Chair Grant Fraser, Ph.D., Department Chair California State University, Los Angeles June 2015 iii ABSTRACT Equivalents to the Axiom of Choice and Their Uses By James Szufu Yang In set theory, the Axiom of Choice (AC) was formulated in 1904 by Ernst Zermelo. It is an addition to the older Zermelo-Fraenkel (ZF) set theory. We call it Zermelo-Fraenkel set theory with the Axiom of Choice and abbreviate it as ZFC. This paper starts with an introduction to the foundations of ZFC set the- ory, which includes the Zermelo-Fraenkel axioms, partially ordered sets (posets), the Cartesian product, the Axiom of Choice, and their related proofs. It then intro- duces several equivalent forms of the Axiom of Choice and proves that they are all equivalent. In the end, equivalents to the Axiom of Choice are used to prove a few fundamental theorems in set theory, linear analysis, and abstract algebra. This paper is concluded by a brief review of the work in it, followed by a few points of interest for further study in mathematics and/or set theory. iv ACKNOWLEDGMENTS Between the two department requirements to complete a master's degree in mathematics − the comprehensive exams and a thesis, I really wanted to experience doing a research and writing a serious academic paper.
    [Show full text]
  • Axiomatic Systems for Geometry∗
    Axiomatic Systems for Geometry∗ George Francisy composed 6jan10, adapted 27jan15 1 Basic Concepts An axiomatic system contains a set of primitives and axioms. The primitives are Adaptation to the object names, but the objects they name are left undefined. This is why the primi- current course is in tives are also called undefined terms. The axioms are sentences that make assertions the margins. about the primitives. Such assertions are not provided with any justification, they are neither true nor false. However, every subsequent assertion about the primitives, called a theorem, must be a rigorously logical consequence of the axioms and previ- ously proved theorems. There are also formal definitions in an axiomatic system, but these serve only to simplify things. They establish new object names for complex combinations of primitives and previously defined terms. This kind of definition does not impart any `meaning', not yet, anyway. See Lesson A6 for an elaboration. If, however, a definite meaning is assigned to the primitives of the axiomatic system, called an interpretation, then the theorems become meaningful assertions which might be true or false. If for a given interpretation all the axioms are true, then everything asserted by the theorems also becomes true. Such an interpretation is called a model for the axiomatic system. In common speech, `model' is often used to mean an example of a class of things. In geometry, a model of an axiomatic system is an interpretation of its primitives for ∗From textitPost-Euclidean Geometry: Class Notes and Workbook, UpClose Printing & Copies, Champaign, IL 1995, 2004 yProf. George K.
    [Show full text]
  • Axioms and Theorems for Plane Geometry (Short Version)
    Axioms and theorems for plane geometry (Short Version) Basic axioms and theorems Axiom 1. If A; B are distinct points, then there is exactly one line containing both A and B. Axiom 2. AB = BA. Axiom 3. AB = 0 iff A = B. Axiom 4. If point C is between points A and B, then AC + BC = AB. Axiom 5. (The triangle inequality) If C is not between A and B, then AC + BC > AB. ◦ Axiom 6. Part (a): m(\BAC) = 0 iff B; A; C are collinear and A is not between B and C. Part (b): ◦ m(\BAC) = 180 iff B; A; C are collinear and A is between B and C. Axiom 7. Whenever two lines meet to make four angles, the measures of those four angles add up to 360◦. Axiom 8. Suppose that A; B; C are collinear points, with B between A and C, and that X is not collinear with A, B and C. Then m(\AXB) + m(\BXC) = m(\AXC). Moreover, m(\ABX) + m(\XBC) = m(\ABC). Axiom 9. Equals can be substituted for equals. Axiom 10. Given a point P and a line `, there is exactly one line through P parallel to `. Axiom 11. If ` and `0 are parallel lines and m is a line that meets them both, then alternate interior angles have equal measure, as do corresponding angles. Axiom 12. For any positive whole number n, and distinct points A; B, there is some C between A; B such that n · AC = AB. Axiom 13. For any positive whole number n and angle \ABC, there is a point D between A and C such that n · m(\ABD) = m(\ABC).
    [Show full text]
  • Axioms of Set Theory and Equivalents of Axiom of Choice Farighon Abdul Rahim Boise State University, [email protected]
    Boise State University ScholarWorks Mathematics Undergraduate Theses Department of Mathematics 5-2014 Axioms of Set Theory and Equivalents of Axiom of Choice Farighon Abdul Rahim Boise State University, [email protected] Follow this and additional works at: http://scholarworks.boisestate.edu/ math_undergraduate_theses Part of the Set Theory Commons Recommended Citation Rahim, Farighon Abdul, "Axioms of Set Theory and Equivalents of Axiom of Choice" (2014). Mathematics Undergraduate Theses. Paper 1. Axioms of Set Theory and Equivalents of Axiom of Choice Farighon Abdul Rahim Advisor: Samuel Coskey Boise State University May 2014 1 Introduction Sets are all around us. A bag of potato chips, for instance, is a set containing certain number of individual chip’s that are its elements. University is another example of a set with students as its elements. By elements, we mean members. But sets should not be confused as to what they really are. A daughter of a blacksmith is an element of a set that contains her mother, father, and her siblings. Then this set is an element of a set that contains all the other families that live in the nearby town. So a set itself can be an element of a bigger set. In mathematics, axiom is defined to be a rule or a statement that is accepted to be true regardless of having to prove it. In a sense, axioms are self evident. In set theory, we deal with sets. Each time we state an axiom, we will do so by considering sets. Example of the set containing the blacksmith family might make it seem as if sets are finite.
    [Show full text]
  • Math 310 Class Notes 1: Axioms of Set Theory
    MATH 310 CLASS NOTES 1: AXIOMS OF SET THEORY Intuitively, we think of a set as an organization and an element be- longing to a set as a member of the organization. Accordingly, it also seems intuitively clear that (1) when we collect all objects of a certain kind the collection forms an organization, i.e., forms a set, and, moreover, (2) it is natural that when we collect members of a certain kind of an organization, the collection is a sub-organization, i.e., a subset. Item (1) above was challenged by Bertrand Russell in 1901 if we accept the natural item (2), i.e., we must be careful when we use the word \all": (Russell Paradox) The collection of all sets is not a set! Proof. Suppose the collection of all sets is a set S. Consider the subset U of S that consists of all sets x with the property that each x does not belong to x itself. Now since U is a set, U belongs to S. Let us ask whether U belongs to U itself. Since U is the collection of all sets each of which does not belong to itself, thus if U belongs to U, then as an element of the set U we must have that U does not belong to U itself, a contradiction. So, it must be that U does not belong to U. But then U belongs to U itself by the very definition of U, another contradiction. The absurdity results because we assume S is a set.
    [Show full text]
  • Mathematics and Mathematical Axioms
    Richard B. Wells ©2006 Chapter 23 Mathematics and Mathematical Axioms In every other science men prove their conclusions by their principles, and not their principles by the conclusions. Berkeley § 1. Mathematics and Its Axioms Kant once remarked that a doctrine was a science proper only insofar as it contained mathematics. This shocked no one in his day because at that time mathematics was still the last bastion of rationalism, proud of its history, confident in its traditions, and certain that the truths it pronounced were real truths applying to the real world. Newton was not a ‘physicist’; he was a ‘mathematician.’ It sometimes seemed as if God Himself was a mathematician. The next century and a quarter after Kant’s death was a tough time for mathematics. The ‘self evidence’ of the truth of some of its basic axioms burned away in the fire of new mathematical discoveries. Euclid’s geometry was not the only one possible; analysis was confronted with such mathematical ‘monsters’ as everywhere-continuous/nowhere-differentiable curves; set theory was confronted with the Russell paradox; and Hilbert’s program of formalism was defeated by Gödel’s ‘incompleteness’ theorems. The years at the close of the nineteenth and opening of the twentieth centuries were the era of the ‘crisis in the foundations’ in mathematics. Davis and Hersch write: The textbook picture of the philosophy of mathematics is a strangely fragmented one. The reader gets the impression that the whole subject appeared for the first time in the late nineteenth century, in response to contradictions in Cantor’s set theory.
    [Show full text]
  • The Entscheidungsproblem and Alan Turing
    The Entscheidungsproblem and Alan Turing Author: Laurel Brodkorb Advisor: Dr. Rachel Epstein Georgia College and State University December 18, 2019 1 Abstract Computability Theory is a branch of mathematics that was developed by Alonzo Church, Kurt G¨odel,and Alan Turing during the 1930s. This paper explores their work to formally define what it means for something to be computable. Most importantly, this paper gives an in-depth look at Turing's 1936 paper, \On Computable Numbers, with an Application to the Entscheidungsproblem." It further explores Turing's life and impact. 2 Historic Background Before 1930, not much was defined about computability theory because it was an unexplored field (Soare 4). From the late 1930s to the early 1940s, mathematicians worked to develop formal definitions of computable functions and sets, and applying those definitions to solve logic problems, but these problems were not new. It was not until the 1800s that mathematicians began to set an axiomatic system of math. This created problems like how to define computability. In the 1800s, Cantor defined what we call today \naive set theory" which was inconsistent. During the height of his mathematical period, Hilbert defended Cantor but suggested an axiomatic system be established, and characterized the Entscheidungsproblem as the \fundamental problem of mathemat- ical logic" (Soare 227). The Entscheidungsproblem was proposed by David Hilbert and Wilhelm Ackerman in 1928. The Entscheidungsproblem, or Decision Problem, states that given all the axioms of math, there is an algorithm that can tell if a proposition is provable. During the 1930s, Alonzo Church, Stephen Kleene, Kurt G¨odel,and Alan Turing worked to formalize how we compute anything from real numbers to sets of functions to solve the Entscheidungsproblem.
    [Show full text]
  • What Are the Recursion Theoretic Properties of a Set of Axioms? Understanding a Paper by William Craig Armando B
    What are the recursion theoretic properties of a set of axioms? Understanding a paper by William Craig Armando B. Matos [email protected] February 5, 2014 Abstract This note is for personal use. It was written with the goal of understand- ing the paper \On Axiomatizability Within a System" by William Craig. Some of its consequences are also explored. Parts of two other papers on this subject are also transcribed and studied. These three papers are (or were) freely available at the internet. The recursion theoretic properties of a set of axioms are studied. 1 Introduction This note is about the application of a recursion theoretical result discovered by Craig to logical formal systems. We transcribe and study parts of three papers related that result, known as Craig's Theorem or Craig's observation. We hope that some readers may enjoy this note, which is essentially a collection of views on the same subject. General comments on formal deduction systems Due to G¨odelincompleteness theorem, a sufficiently strong consistent theory (view as the set of its theorems) cannot be decidable. Such a theory will be a 0 recursively enumerable (or Σ1) set. Similarly, the set of axioms is also usually infinite, not decidable but recursively enumerable. We quote a Wikipedia entry on G¨odel'sincompleteness theorems. Many theories of interest include an infinite set of axioms [. ] To verify a formal proof when the set of axioms is infinite, it must be possible to determine whether a statement that is claimed to be an axiom is actually an axiom. This issue arises in first order theories of arithmetic, such as Peano arithmetic, because the principle of mathematical induction is expressed as an infinite set of axioms (an axiom schema).
    [Show full text]
  • Axiomatic Systems
    Eucliean and Non-Euclidean Geometry – Fall 2007 Dr. Hamblin Axiomatic Systems An axiomatic system is a list of undefined terms together with a list of statements (called “axioms”) that are presupposed to be “true.” A theorem is any statement that can be proven using logical deduction from the axioms. Examples Here are some examples of axiomatic systems. Committees Undefined terms: committee, member Axiom 1: Each committee is a set of three members. Axiom 2: Each member is on exactly two committees. Axiom 3: No two members may be together on more than one committee. Axiom 4: There is at least one committee. Monoid Undefined terms: element, product of two elements Axiom 1: Given two elements x and y, the product of and , denoted , is a uniquely defined element. Axiom 2: Given elements , , and , the equation is always true. Axiom 3: There is an element , called the identity, such that and for all elements . Silliness Undefined terms: silly, dilly. Axiom 1: Each silly is a set of exactly three dillies. Axiom 2: There are exactly four dillies. Axiom 3: Each dilly is contained in a silly. Axiom 4: No dilly is contained in more than one silly. Notice that in the second example, the axioms defined a new term (“identity”). This isn’t an undefined term because the axiom includes a definition. Also, these axioms refer to basic set theory that you learned in Discrete Math. For our purposes, we will assume all of those basic set theory terms are known. It is possible to view set theory itself as another axiomatic system, but that is beyond the scope of this course.
    [Show full text]
  • Logic in Philosophy of Mathematics
    Logic in Philosophy of Mathematics Hannes Leitgeb 1 What is Philosophy of Mathematics? Philosophers have been fascinated by mathematics right from the beginning of philosophy, and it is easy to see why: The subject matter of mathematics| numbers, geometrical figures, calculation procedures, functions, sets, and so on|seems to be abstract, that is, not in space or time and not anything to which we could get access by any causal means. Still mathematicians seem to be able to justify theorems about numbers, geometrical figures, calcula- tion procedures, functions, and sets in the strictest possible sense, by giving mathematical proofs for these theorems. How is this possible? Can we actu- ally justify mathematics in this way? What exactly is a proof? What do we even mean when we say things like `The less-than relation for natural num- bers (non-negative integers) is transitive' or `there exists a function on the real numbers which is continuous but nowhere differentiable'? Under what conditions are such statements true or false? Are all statements that are formulated in some mathematical language true or false, and is every true mathematical statement necessarily true? Are there mathematical truths which mathematicians could not prove even if they had unlimited time and economical resources? What do mathematical entities have in common with everyday objects such as chairs or the moon, and how do they differ from them? Or do they exist at all? Do we need to commit ourselves to the existence of mathematical entities when we do mathematics? Which role does mathematics play in our modern scientific theories, and does the em- pirical support of scientific theories translate into empirical support of the mathematical parts of these theories? Questions like these are among the many fascinating questions that get asked by philosophers of mathematics, and as it turned out, much of the progress on these questions in the last century is due to the development of modern mathematical and philosophical logic.
    [Show full text]