Categorical characterizations of the natural numbers require primitive recursion

Simpson and the second author asked whether there exists a characterization of the natural numbers by a second-order sentence which is provably categorical in the theory RCA$^*_0$. We answer in the negative, showing that for any characterization of the natural numbers which is provably true in WKL$^*_0$, the categoricity theorem implies $\Sigma^0_1$ induction. On the other hand, we show that RCA$^*_0$ does make it possible to characterize the natural numbers categorically by means of a set of second-order sentences. We also show that a certain $\Pi^1_2$-conservative extension of RCA$^*_0$ admits a provably categorical single-sentence characterization of the naturals, but each such characterization has to be inconsistent with WKL$^*_0$+superexp.Comment: 17 page

The computational content of Nonstandard Analysis

Kohlenbach's proof mining program deals with the extraction of effective information from typically ineffective proofs. Proof mining has its roots in Kreisel's pioneering work on the so-called unwinding of proofs. The proof mining of classical mathematics is rather restricted in scope due to the existence of sentences without computational content which are provable from the law of excluded middle and which involve only two quantifier alternations. By contrast, we show that the proof mining of classical Nonstandard Analysis has a very large scope. In particular, we will observe that this scope includes any theorem of pure Nonstandard Analysis, where `pure' means that only nonstandard definitions (and not the epsilon-delta kind) are used. In this note, we survey results in analysis, computability theory, and Reverse Mathematics.Comment: In Proceedings CL&C 2016, arXiv:1606.0582

Reverse mathematics & nonstandard analysis: towards a dispensability argument

Reverse Mathematics is a program in the foundations of mathematics initiated by Harvey Friedman and developed extensively by Stephen Simpson. Its aim is to determine which minimal axioms prove theorems of ordinary mathematics. Nonstandard Analysis plays an important role in this program. We consider Reverse Mathematics where equality is replaced by the predicate 'approx', i.e. equality up to infinitesimals from Nonstandard Analysis. This context allows us to model mathematical practice in Physics particularly well. In this way, our mathematical results have implications for Ontology and the Philosophy of Science. In particular, we prove the dispensability argument, which states that the very nature of Mathematics in Physics implies that real numbers are not essential (i.e. dispensable) for Physics (cf. the Quine-Putnam indispensability argument)

Reverse Mathematics and parameter-free Transfer

Recently, conservative extensions of Peano and Heyting arithmetic in the spirit of Nelson's axiomatic approach to Nonstandard Analysis, have been proposed. In this paper, we study the Transfer axiom of Nonstandard Analysis restricted to formulas without parameters. Based on this axiom, we formulate a base theory for the Reverse Mathematics of Nonstandard Analysis and prove some natural reversals, and show that most of these equivalences do not hold in the absence of parameter-free Transfer.Comment: 22 pages; to appear in Annals of Pure and Applied Logi

The computational content of Nonstandard Analysis

Kohlenbach's proof mining program deals with the extraction of effective information from typically ineffective proofs. Proof mining has its roots in Kreisel's pioneering work on the so-called unwinding of proofs. The proof mining of classical mathematics is rather restricted in scope due to the existence of sentences without computational content which are provable from the law of excluded middle and which involve only two quantifier alternations. By contrast, we show that the proof mining of classical Nonstandard Analysis has a very large scope. In particular, we will observe that this scope includes any theorem of pure Nonstandard Analysis, where 'pure' means that only nonstandard definitions ( and not the epsilon-delta kind) are used. In this note, we survey results in analysis, computability theory, and Reverse Mathematics

Automorphisms of models of bounded arithmetic

We establish the following model theoretic characterization of the fragment I∆0 +Exp+BΣ1 of Peano arithmetic in terms of fixed points of automorphisms of models of bounded arithmetic (the fragment I∆0 of Peano arithmetic with induction limited to ∆0-formulae). Theorem A. The following two conditions are equivalent for a countable model M of the language of arithmetic: (a) M satisfies I∆0 + BΣ1 + Exp. (b) M = Ifix(j) for some nontrivial automorphism j of an end extension N of M that satisfies I∆0. Here Ifix(j) is the largest initial segment of the domain of j that is pointwise fixed by j, Exp is the axiom asserting the totality of the exponential function, and BΣ1 is the Σ1-collection scheme consisting of the universal closure of formulae of the form [∀x < a ∃y ϕ(x, y)] → [∃z ∀x < a ∃y < z ϕ(x, y)], where ϕ is a ∆0-formula. Theorem A was inspired by a theorem of Smoryński, but the method of proof of Theorem A is quite different and yields the following strengthening of Smoryński’s result: Theorem B. Suppose M is a countable recursively saturated model of P A and I is a proper initial segment of M that is closed under exponentiation. There is a group embedding j ↦− → ̂j from Aut(Q) int

The reverse mathematics of elementary recursive nonstandard analysis: a robust contribution to the foundations of mathematics

Reverse Mathematics (RM) is a program in the Foundations of Mathematics founded by Harvey Friedman in the Seventies ([17, 18]). The aim of RM is to determine the minimal axioms required to prove a certain theorem of ‘ordinary’ mathematics. In many cases one observes that these minimal axioms are also equivalent to this theorem. This phenomenon is called the ‘Main Theme’ of RM and theorem 1.2 is a good example thereof. In practice, most theorems of everyday mathematics are equivalent to one of the four systems WKL0, ACA0, ATR0 and Π1-CA0 or provable in the base theory RCA0. An excellent introduction to RM is Stephen Simpson’s monograph [46]. Nonstandard Analysis has always played an important role in RM. ([32,52,53]). One of the open problems in the literature is the RM of theories of first-order strength I∆0 + exp ([46, p. 406]). In Chapter I, we formulate a solution to this problem in theorem 1.3. This theorem shows that many of the equivalences from theorem 1.2 remain correct when we replace equality by infinitesimal proximity ‘≈’ from Nonstandard Analysis. The base theory now is ERNA, a nonstandard extension of I∆0 + exp. The principle that corresponds to ‘Weak K ̈onig’s lemma’ is the Universal Transfer Principle (see axiom schema 1.57). In particular, one can say that the RM of ERNA+Π1-TRANS is a ‘copy up to infinitesimals’ of the RM of WKL0. This implies that RM is ‘robust’ in the sense this term is used in Statistics and Computer Science ([25,35]). Furthermore, we obtain applications of our results in Theoretical Physics in the form of the ‘Isomorphism Theorem’ (see theorem 1.106). This philosophical excursion is the first application of RM outside of Mathematics and implies that ‘whether reality is continuous or discrete is undecidable because of the way mathematics is used in Physics’ (see paragraph 3.2.4, p. 53). We briefly explore a connection with the program ‘Constructive Reverse Mathematics’ ([30,31]) and in the rest of Chapter I, we consider the RM of ACA0 and related systems. In particular, we prove theorem 1.161, which is a first step towards a ‘copy up to infinitesimals’ of the RM of ACA0. However, one major aesthetic problem with these results is the introduction of extra quantifiers in many of the theorems listed in theorem 1.3 (see e.g. theorem 1.94). To overcome this hurdle, we explore Relative Nonstandard Analysis in Chapters II and III. This new framework involves many degrees of infinity instead of the classical ‘binary’ picture where only two degrees ‘finite’ and ‘infinite’ are available. We extend ERNA to a theory of Relative Nonstandard Analysis called ERNAA and show how this theory and its extensions allow for a completely quantifier- free development of analysis. We also study the metamathematics of ERNAA, motivated by RM. Several ERNA-theorems would not have been discovered without considering ERNAA first