算术的非标准模型

鲁宾逊基于超实数的无穷小微积分就是传统微积分的非标准微积分。

深入学习、理解算术的非标准模型对于正确理解无穷小微积分是十分有益的。

建议读者仔细研读本文附件。，

袁萌陈启清 12月10日

附件：算术的非标准模型

Non-Standard Models of Arithmetic

Asher M. Kach

1 May 2004

Abstract Almost everyone, mathematician or not, is comfortable with the standard model (N : +,·) of arithmetic. Less familiar, even among logicians, are the non-standard models of arithmetic. In this talk we prove their existence, explore their structure, discuss their uniqueness, and examine various model and computability theoretic properties they possess. As much as is reasonably possible, the history of their discovery and study will be integrated into the talk.

Deﬁnitions and Notation

Throughout we will refer to N0 = (N : +,·) as the standard model of arithmetic. Any model of Th(N0) not isomorphic to N0 will be termed a non-standard model of arithmetic, or more brieﬂy a non-standard model. An element x ∈M will be called ﬁnite if x ∈N0; otherwise x will be called inﬁnite. Working in a model M, we will say that x is less than or equal to y, denoted x ≤ y, if there is a z ∈M such that x + z = y. We deﬁne y− · x to be z if such a z exists and 0 if no such z exists. We will use N, Z, Q, and R to denote both the usual set and its order type. If α and β are order types, we will use αβ to denote the order type obtained by replacing each element of β by a copy of α.

Existence of Non-Standard Models

Theorem: (Skolem, 1934 / 1955) There is a countable non-standard model of arithmetic. Proof 1: (Idea) Letting F ={fi : i ∈ ω} be the set of deﬁnable functions in N0, deﬁne a one-to-one increasing function g that allows an ordering to be put on equivalence classes of F. Deﬁne addition and multiplication to be pointwise and verify that (F/≡: +/≡,·/≡)∈Mod(Th(N0)).

Proof 2: (Idea) After augmenting the language with a constant c, use the Compactness Theorem to show the consistency of an inﬁnite number via the set of sentences Φ ={c > n : n ∈ ω}.

Order Types of Non-Standard Models

Theorem: (Henkin, 1950) The order type of any non-standard model of arithmetic is of the form N+Zθ for some dense linear order θ without endpoints. Proof: (Sketch) For denseness, between any two elements a b, either q = (a + b)/2 or q = (a + b +1)/2 exists. In either case, it must be that a q b. Extend q to a Z-chain by adding and subtracting ﬁnite integers. For unboundedness, for any inﬁnite a, the element 2a must satisfy a 2a. Corollary: Any countable non-standard model of arithmetic has order type N+ZQ. Proof: Up to isomorphism, the only countable dense linear order without endpoints is Q.

Continuum Many Countable Non-Standard Models

Theorem: There are exactly 2ℵ0 non-isomorphic countable non-standard models of arithmetic. Proof: (Sketch) Augment the language with an extra constant c. Let P be any set of (ﬁnite) prime numbers and let ΦP be the set of sentences {p | c : p ∈ P}∪{p6| c : p 6∈ P}. Use the Compactness Theorem to show consistency of ΦP and L¨owenheim - Skolem to get a countable model. Finally argue that if there were fewer than continuum many countable models, then not all types would be realised.

The Overspill Principle

Theorem: Let M be a non-standard model of arithmetic and let b be any element of M.

(Weak Overspill) Let ϕ(x,y) have free variables x and y. Then ∀x ∈ ω[M|= ϕ(x,b)] if and only if ∃a inﬁnite [M|= (∀x < a)ϕ(x,b)].

Proof: (Sketch) Show that the set of natural numbers ω ⊂ M is not deﬁnable (using parameters) in M. For the non-trivial direction, if there was no such inﬁnite a, use that ω is not deﬁnable to contradict that M was assumed to be a non-standard model.

The Overspill Principle (Continued)

(Strong Overspill) Let ϕ(x,y,z) have free variables x, y, and z, and suppose ϕ(x,y,b) deﬁnes a function F : M → M. Then ∀x ∈ ω[F(x) is inﬁnite] if and only if ∃a inﬁnite ∀x < a[F(x) is inﬁnite]. Proof: For the non-trivial direction, apply Weak Overspill to the formula ϕ(x) given by Fx > x.

Corollary: For any inﬁnite integer a, there is an inﬁnite integer c with 2c < a. Proof: As 2x is ﬁnite for all ﬁnite integers x, and hence smaller than a, there must be an inﬁnite integer c with 2c < a by Weak Overspill.

Order Type of the Reals Not Realised

Theorem: (Klaus Potthoﬀ) There is no non-standard model of arithmetic with order type N+ZR. Proof: (Sketch) Assume there was a non-standard model of order type N+ZR, and let a be any inﬁnite element of it. Identify real numbers r with the corresponding Z-chain, Zr. Let rn for n ∈ ω be the Zr in which the element na resides. As the rn are increasing and bounded by the copy of Z in which the element a2 resides, the sequence {rn} converges to some real number r. Let b be any element of Zr, choosing b smaller than the multiple of a in Zr if one exists. Deﬁne S ={x : a|x and x < b}. Then ω ={n : na ∈ S}, a contradiction to ω not being deﬁnable in any non-standard model of arithmetic.

Extension Types

Let M⊆N be models of arithmetic, not necessarily non-standard. Deﬁnition: An element a ∈ N −M is said to be M-inﬁnite if a > b for all b ∈ M; otherwise it is said to be M-ﬁnite. Deﬁnition: If every element of N −M is M-inﬁnite, then N is said to be an end extension of M. We write M⊆e N in this case. If every element of N −M is M-ﬁnite, then N is said to be a coﬁnal extension of M. We write M⊆c N in this case. If N−M contains both M-ﬁnite and M-inﬁnite integers, then N is said to be a mixed extension of M. We write M⊆m N in this case.

Extension Existence

Theorem: Every non-standard model M of PA has a proper elementary mixed extension (trivial), a proper elementary end extension (MacDowell and Specker, 1961), and a proper elementary coﬁnal extension (Rabin, 1962).

Theorem: (Gaifman, 1971) Let M⊆N be models of PA. Then there is a unique model ˆ M of PA such that M⊆c ˆM⊆e N. Moreover, the coﬁnal extension M⊆c ˆ M is elementary.

Corollary: Coﬁnal extensions are necessarily elementary. Proof: Let M⊆N be any coﬁnal extension. Then ˆ M=N by uniqueness and so the extension is elementary.

Recovering a Structure From End Segments

Theorem: (Smoryn´ski, 1977) Let M be a model of arithmetic with M = I ∪E, an initial segment and end segment. Then M can be completely recovered from the structure E = (E : +,·). Proof: (Idea) Similar to the construction of a ﬁeld of quotients from an integral domain. In E, deﬁne x < y, S(x) = y (i.e. x +1 = y), and x|y. With M0 ={(a,b)∈ E2 : E |= b|a}, argue that M∼ = (M0/≡: +/≡,·/≡), where ≡ is the equivalence relation given by (a,b)≡(c,d) if and only if ad = bc.

Corollaries

Corollary: If M1 and M2 are models of arithmetic with isomorphic end segments, then M1 and M2 are isomorphic.

Corollary: The theory of non-standard parts of non-standard models of PA is Π1 1 complete.

Corollary: The theory of end segments of non-standard models of PA is recursively axiomatizable.

A Number Theoretic Result Theorem: (Rabin, 1962) For every non-standard model of arithmetic M, there are parameters ai1...in for 0≤ ij ≤ k in M such that the diophantine equation X0 ≤ij≤k ai1...inti1 1 ...tin n = 0 is not solvable in M but is solvable in some model of arithmetic M0 with M⊂M0.

Remark: Note that a diophantine equation with parameters from the standard model is solvable in the standard model if and only if it is solvable in some non-standard model.

Standard Systems

Deﬁnition: Let M be a model of arithmetic. A set X ⊆ ω is standard in M if there is a formula ϕ(x, ¯ y) and ¯ b ∈M such that X ={x ∈ ω :M|= ϕ(x,¯ b)}. The standard system of M is the collection of standard sets in M. Proposition: If X is a standard set in M, then X has arbitrarily small inﬁnite codes. Proof: Let X be deﬁned by ϕ(x, ¯ y) and ¯ b. For any inﬁnite integer a, let c be an inﬁnite integer with 2c < a. Deﬁne F(x) =   2x if ϕ(x,¯ b) 0 otherwise. Then d =Pc−2 i=0 F(x)+2c−1 < 2c < a is an inﬁnite code for X.

Embeddability Results

Theorem: Let M and N be countable non-standard models of arithmetic. • (Friedman) Then M is embeddable in N if and only if SSym(M)⊆SSym(N) and Th∃(M)⊆Th∃(N). • (Friedman) Then M is isomorphic to an initial segment of N if and only if SSym(M) = SSym(N) and ThΣ1(M)⊆ThΣ1(N). • (Wilkie, 1977) Then M is isomorphic to arbitrarily large initial segments of N if and only if SSym(M) = SSym(N) and ThΠ2(M)⊆ThΠ2(N).

Corollary: Any countable non-standard model of arithmetic is isomorphic to a proper initial segment of itself.

Random Theorems

Theorem: (Rabin, 1962) There is an ascending chain M1 ⊂M2 ⊂ ... of models of arithmetic such that the union M=∪iMi is not a model of arithmetic. Corollary: There is no AE set S ⊂Th(N0) such that S |= Th(N0). Proof: The union of a ascending chain of models of an AE set S is a model of S.

Theorem: (Rabin, 1962) There is a non-standard model of arithmetic M with elementary submodels M1 and M2 such that M1 ∩M2 is not a model of arithmetic. Theorem: (Knight, 1973, 1975) Let Σ be a complete type with respect to Th(N0) omitted in N0. Then for every cardinal κ, there is a model of Th(N0) with cardinality κ omitting the type Σ.

Bibliography

Leon Henkin, Completeness in the theory of types, The Journal of Symbolic Logic, vol 15 (1950), pp. 81-91.

J. F. Knight, Complete types and the natural numbers, The Journal of Symbolic Logic, vol. 38 (1973), pp. 413-415.

J. F. Knight, Types Omitted in Uncountable Models of Arithmetic, The Journal of Symbolic Logic, vol. 40 (1975), pp. 317-320.

Kenneth McAloon, On the Complexity of Models of Arithmetic, The Journal of Symbolic Logic, vol. 47 (1982), pp. 403-415.

M. Rabin, Diophantine equations and non-standard models of arithmetic, in Logic, Methodology & Philosophy of Science, Stanford University Press, 1962, pp. 151-158.

Thoralf Skolem, Peano’s axioms and models of arithmetic, in Mathematical Interpretations of Formal Systems, North-Holland, Amsterdam, 1955, p. 1-14.

C. Smoryn´ski, Lectures on nonstandard models of arithmetic: Commemorating Guise

算术的非标准模型

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