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Numbers and Models, Standard and Nonstandard Numbers and Models, Standard and Nonstandard. Dedicated to the memory of Abraham Robinson. 4 April 2010 The following is a somewhat extended manuscript for a talk at the \Algebra Days", May 2008, in Antalya. I talked about my personal recollections of Abraham Robinson. Contents 1 How I met Abraham Robinson and his infinitesimals 2 2 What is Nonstandard Analysis? 4 2.1 A preliminary Axiom . 4 2.2 The Axiom in its final form . 7 2.3 Some excercises . 9 3 Robinson's visits 12 3.1 T¨ubingen . 12 3.2 Heidelberg . 13 4 Nonstandard Algebra 15 5 Nonstandard Arithmetic 17 References 20 1 1 How I met Abraham Robinson and his in- finitesimals It was in the early months of 1963. I was visiting the California Institute of Technology on my sabbatical. Somehow during this visit I learned that one year ago Wim Luxemburg had given a lecture on A. Robinson's theory of infinitesimals and infinitely large numbers. Luxemburg was on leave but I got hold of his Lecture Notes [Lux62]. Although the topic was somewhat distant from my own work I got interested and, after thorough reading I wished to meet the person who had been able to put Leibniz' infinitesimals on a solid base and build the modern analysis upon it. At that time Abraham Robinson was at the nearby University of Cali- fornia at Los Angeles, and I managed to meet him there. I remember an instructive discussion about his theory which opened my eyes for the wide range of possible applications; he also showed me his motivations and main ideas about it. Abraham Robinson 1918-74 Perhaps I am allowed to insert some personal words explaining why I had been so excited about the new theory of infintesimals. This goes back to my school days in K¨onigsberg, when I was 16. At that time the school syllabus required that we were to be instructed in Calculus or, as it was called in German, in Diffentialrechnung. Our Math teacher at that time was an elderly lady who had been retired already but was reactivated again for 2 school work in order to fill the vacancy of our regular teacher; the latter had been drafted to the army. (It was war time: 1944.) I still remember the sight of her standing in front of the blackboard where she had drawn a ∆y wonderfully smooth parabola, inserting a secant and telling us that ∆x is its dy slope, until finally she convinced us that the slope of the tangent is dx where dx is infinitesimally small and dy accordingly. My school in K¨onigsberg1 This, I admit, impressed me deeply. Until then our school Math had con- sisted largely of Euclidean geometry, with so many problems of constructing triangles from some given data. This was o.k. but in the long run that stuff did not strike me as to be more than boring exercises. But now, with those infinitesimals, Math seemed to have more interesting things in stock than I had met so far. And I decided that I would study Mathematics if I survived the dangers of war which we knew we would be exposed to very soon. After all, I wanted to find out more about these wonderfully strange infinitesimals. Well, I survived. And I managed to enter University and start with Mathematics. The first lecture I attended to was Calculus, with Professor Otto Haupt in Erlangen. There we were told to my disappointment that my Math teacher had not been up to date after all. We were warned to 1Wilhelmsgymasium. This is the same school where Hilbert in the year 1880 obtained his Abitur. 3 beware of infinitesimals since they do not exist, and in any case they lead dy to contradictions. Instead, although one writes dx then this does not really mean a quotient of two entities, but it should be interpreted as a symbolic ∆y notation only, namely the limit of the quotients ∆x . I survived this disappointment too. Later I learned that dy and dx can be interpreted, not as infinitesimals but as some entities of an abstract con- struction called differential module, and if that module is one-dimensional dy then the quotient dx would make sense and yield what we had learned any- how. Certainly, this sounded nice but in fact it was only an abstract frame ignoring the natural idea of infinitesimally small numbers. So when I learned about Robinson's infinitesimals, my early school day experiences came to my mind again and I wondered whether that lady teacher had not been so wrong after all. The discussion with Abraham Robinson kindled my interest and I wished to know more about it. Some time later there arose the opportunity to invite him to visit us in Germany where he gave lectures on his ideas, first in T¨ubingenand later in Heidelberg, after I had moved there. Before continuing with this let me briefly explain what I am talking about, i.e., Robinson's theory of nonstandard analysis. 2 What is Nonstandard Analysis? 2.1 A preliminary Axiom Consider the hierarchy of numbers which we present to our students in their first year: N ⊂ Z ⊂ Q ⊂ R Everything starts with the natural numbers N which, as Kronecker allegedly has maintained, are \created by God" (or whatever is considered to be equiv- alent to Him). The rest is constructed by mankind, i.e., by the minds of mathematicians. In each step, the structure in question is enlarged such as to admit greater flexibility with respect to some operations defined in the structure. In Z the operation of subtraction is defined such that Z becomes an additive group; in fact Z is a commutative ring without zero divisors. In Q the operation of division is defined such that Q becomes a field. Finally, in R every Cauchy sequence is convergent, such that R becomes a complete or- dered field. In each step we tell our students that the respective enlargement exists and we explain how to construct it. 4 In order to develop what nowadays is called \analysis" the construction usually stops with the real field R; this is considered to be adequate and quite sufficient as a basis for (real) analysis. But it had not always been the case that way. Since Leibniz had used the natural idea of infinitesimals to build a systematic theory with it, many generations of mathematicians (including my lady teacher) had been taught in the Leibniz way. Prominent people like Euler, the Bernoullis, Lagrange and even Cauchy (to name only a few) did not hesitate to use them. Gottfried Wilhelm Leibniz The Leibniz idea for analysis, as interpreted by Robinson, is to work in a further enlargement: ∗ R ⊂ R such that the following Axiom is satisfied. In order to explain the main idea I will first state the Axiom in a preliminary form which, however, will not yet be sufficient. Later I will give the final, more general form. Axiom (preliminary form). (1.) ∗R is an ordered field extension of R. (2.) ∗R contains infinitely large elements. An element ! 2 ∗R is called “infinitely large" if j!j > n for all n 2 N. Part (2.) says that the ordering of ∗R does not satisfy the axiom of Archimedes. 5 Fields with the properties (1.) and (2.) were known for some time2 but the attempts to build analysis on this basis were not quite satisfactory. Among all such fields one has to select those which in addition have more sophisticated properties. But for the moment let us stay with the Axiom in this preliminary form and see what we can do with it. The elements of R are called standard real numbers, while the elements of ∗R not in R are nonstandard. This terminology is taken from model theory but I find it not very suggestive in the present context. Sometimes the elements of ∗R are called hyperreal numbers. Perhaps someone some time will find a more intuitive terminology. The finite elements α in ∗R are those which are not infinitely large, i.e. which satisfy Archimedes' axiom: jαj < n for some n 2 N (depending on α). These finite elements form a subring E ⊂ ∗R. It contains all infinitesimal 1 elements " which are defined by the property that j"j < n for all n 2 N. It follows from the definition that the set of infinitesimals is an ideal I ⊂ E. We have: ! infinitely large () !−1 infinitesimal 6= 0: It is well known that this property characterizes E as a valuation ring in the sense of Krull [Kru32]. Theorem: The finite elements E form a valuation ring of ∗R with the infinitesimals I as its maximal ideal. The residue class field E=I = R. Two finite elements α; β are said to be infinitely close to each other if α − β is infinitesimal, i.e., if they belong to the same residue class modulo the ideal I of infinitesimals. This is written as α ≈ β : The residue class of α 2 E is called the \monad" of α; this terminology has been introduced by Robinson in reference to Leibniz' theory of monads. Every monad contains exactly one standard number a 2 R; this is called the standard part of α, and denoted by st(α). There results the standard part map st : E ! R which in fact is nothing else than the residue class map of E modulo its maximal ideal I. 2See, e.g., [AS27]. 6 In this situation let us consider the example of a parabola y = x2 which, as I have narrated above, had been used by my school teacher to introduce us to analysis.
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