There are two concepts of standard/nonstandard models in simple type theory.

The first concept—we might call it the pragmatical one—interprets type theory as a first order logic with countably many sorts of variables: the variables for the urelements of type 0,…, the n-ary relational variables of type (τ1, …, τn) with arguments of type (τ1,…,τn), respectively. If A ≠ ∅ then 〈Aτ〉 is called a model of type logic, if A0 = A and . 〈Aτ〉 is called full if, for every τ = (τ1,…,τn), . The variables for the urelements range over the elements of A and the variables of type (τ1,…, τn) range over those subsets of which are elements of . The theory Th(〈Aτ〉) is the set of all closed formulas in the language which hold in 〈Aτ〉 under natural interpretation of the constants. If 〈Bτ〉 is a model of Th(〈Aτ〉), then there exists a sequence 〈fτ〉 of functions fτ: Aτ → Bτ such that 〈fτ〉 is an elementary embedding from 〈Aτ〉 into 〈Bτ〉. 〈Bτ〉 is called a nonstandard model of 〈Aτ〉, if f0 is not surjective. Otherwise 〈Bτ〉 is called a standard model of 〈Aτ〉.

This first concept of model theory in type logic seems to be preferable for applications in model theory, for example in nonstandard analysis, since all nice properties of first order model theory (completeness, compactness, and so on) are preserved.

The minimal Gödel class with identity (MGCI) is the class of closed, prenex quantificational formulas whose prefixes have the form ∀x1∀x2∃x3 and whose matrices contain arbitrary predicate letters and the identity sign “=”, but contain no function signs or individual constants. The MGCI was shown undecidable (for satisfiability) in 1983 [Go2]; this both refutes a claim of Gödel's [Gö, p. 443] and settles the decision problem for all prefix-classes of quantification theory with identity.

In this paper, we show the decidability of a natural subclass of the MGCI. The formulas in this subclass can be thought of as exploiting only half of the power of the existential quantifier. That is, since an MGCI formula has prefix ∀x1∀x2∃x3, in general its truth in a model requires for any elements a and b, the existence of both a witness for and a witness for . The formulas we consider demand less: they require, for any elements a and b, a witness for the unordered pair {a, b}, that is, a witness either for or for .

This paper develops in certain directions the work of Meyer in [3], [4], [5] and [6] (see also Routley [10] and Asenjo [11]). In those works, Peano's axioms for arithmetic were formulated with a logical base of the relevant logic R, and it was proved finitistically that the resulting arithmetic, called R#, was absolutely consistent. It was pointed out that such a result escapes incautious formulations of Gödel's second incompleteness theorem, and provides a basis for a revived Hilbert programme. The absolute consistency result used as a model arithmetic modulo two. Modulo arithmetics are not ordinarily thought of as an extension of Peano arithmetic, since some of the propositions of the latter, such as that zero is the successor of no number, fail in the former. Consequently a logical base which, unlike classical logic, tolerates contradictory theories was used for the model. The logical base for the model was the three-valued logic RM3 (see e.g. [1] or [8]), which has the advantage that while it is an extension of R, it is finite valued and so easier to handle.

The resulting model-theoretic structure (called in this paper RM32) is interesting in its own right in that the set of sentences true therein constitutes a negation inconsistent but absolutely consistent arithmetic which is an extension of R#. In fact, in the light of the result of [6], it is an extension of Peano arithmetic with a base of a classical logic, P#. A generalisation of the structure is to modulo arithmetics with the same logical base RM3, but with varying moduli (called RM3i here).

Forcing with [Κ]κ over a model of set theory with a strong partition cardinal, M. Spector produced a generic ultrafilter G on κ such that κκ/G is not well-founded.

Theorem. Let G be Spector-generic over a model M of , for all α < κ.

1) Every cardinal (well-ordered or not) of M is a cardinal of M[G].

2) If A ∈ M[G] is a well-ordered subset of M, then Ae M. Let Φ = Κκ/G.

3) There is an ultrafilter U on Φ such that every member of U has a subset of type Φ, and the intersection of any well-ordered subset of U is in U.

4) Φ satisfies for all α <ℵ1 and all ordinals β.

5) There is a linear order Φ′ with property 3) above which is not “weakly compact”, i.e., Φ′ ↛ (Φ′)2.

In a series of papers [K2], [K3], [K4], E. M. Kleinberg established the extensive properties of what are now called “strong partition cardinals”, cardinals satisfying for all λ < κ. The purpose of this note is to show that all these consequences and the results in [H] and [W] can be obtained from the weaker relation and many from .

We assume the reader is generally familiar with Kleinberg's machinery and with the definition of . We recall that a cardinal κ satisfies iff for every partition F: [κ]κ → A there is a p ∈ [κ]κ such that F″ [p]κ ≠ A. We take the liberty of regarding a p ∈ [κ]κ both as a subset of κ and as a function from κ to κ. We assume DC throughout.

§1. From. Our results stem from the observation that the proofs in the papers cited above only require homogeneous sets for certain classes of partitions.

Definition. A partition F: [κ]κ → λ, λ < λ, is called clopen if for all p ∈ [κ]κ there is an α < κ such that whenever -clopen is the assertion that all clopen partitions have homogeneous sets (Spector-Watro).

We denote by and the classes of torsion-free nilpotent groups of nilpotency class at most two and three, respectively. In this paper we show that most of the known results about existentially closed (e.c.) groups in remain true in : Up to isomorphism, there exist only countably many countable e.c. groups and they are distinguished by the ranks of their centers. An e.c. group is finitely (infinitely) generic if and only if the center has dimension one (≥ω). Apart from trivial exceptions, e.c, algebraically closed, and “closed with respect to systems of equations in one unknown” are equivalent.

Let K be a class of groups. A group G ϵ K is called existentially closed in K if G contains a solution of any finite system Σ of equations and inequations with constants in G and one or more unknowns, provided that Σ has a solution in some in K. If Σ may contain equations (in at most n unknowns) only, G is called algebraically closed (n-unknown closed [1]). We use the abbreviations e.c, a.c and n-u.c. throughout. For more background on these terms the reader is referred to [3] and [4]. We also assume a basic knowledge of generic structures. The main group theoretic notions needed here are explained in §2; [2] and [11] are the general references for this field.

We prove that if every uncountable model of a first-order theory T is ω-saturated and T is superstable then T is categorical in some infinite power.

Let L be a first order predicate calculus with equality which has a fixed binary predicate symbol <. In this paper, we shall deal with quantifiers Cx, ∀x ≦ y, ∃x ≦ y defined as follows: CxA(x) is ∀y∃x(y ≦ x ∧ A(x)), ∀x ≦ yA{x) is ∀x(x ≦ y ⊃ A(x)), and ∃x ≦ yA(x) is ∃x(x ≦ y ∧ 4(x)). The expressions x̄, ȳ, … will be used to denote sequences of variables. In particular, x̄ stands for 〈x1, …, xn〉 and ȳ stands for 〈y1,…, ym〉 for some n, m. Also ∃x̄, ∀x̄ ≦ ȳ, … will be used to denote ∃x1 ∃x2 … ∃xn, ∀x1 ≦ y1 ∀x2 ≦ y2 … ∀xn ≦ yn, …, respectively. Let X be a set of formulas in L such that X contains every atomic formula and is closed under substitution of free variables and applications of propositional connectives ¬(not), ∧(and), ∨(or). Then, ∑(X) is the set of formulas of the form ∃x̄B(x̄), where B ∈ X, and Φ(X) is the set of formulas of the form

.

Since X is closed under ∧, ∨, the two sets Σ(X) and Φ(X) are closed under ∧, ∨ in the following sense: for any formulas A and B in Σ(X) [Φ(X)], there are formulas in Σ(X)[ Φ(X)] which are obtained from A ∧ B and A ∨ B by bringing some quantifiers forward in the usual manner.

In 1949 Nelson [5] proposed a constructive logic in which falsity is conceived in a fashion analogous to that for intuitionistic truth. The predicate calculus N (for strong negation) was characterized by the usual axioms and rules for positive intuitionistic connectives (see Kleene [4, p. 82, la–7 and 9–12]), with the additional axiom schemata for strong negation: A ⊃ (¬A ⊃ B),

Nelson's paper showed that N may be interpreted with concepts for constructive truth (P-realizability) and falsity (N-realizability). The paper also gave mappings between the intuitionistic and strong negation systems of arithmetic.

These mappings are easily adapted to pure predicate calculus. One shows that intuitionistic predicate calculus I is a subsystem of N by reading the intuitionistic negation of A as A ⊃ B & ¬B in N. It is possible to map N into a subsystem of I using the definition of A′ in [5, p. 19] changing only the definition of (¬A)′ for atomic A to A ⊃ B & ¬B.

We deal here with two modal logics, GL and Grz, that are known to have interesting arithmetical interpretations connected with the notion of provability. GL is the extensiom of K (or K4) by the schema □(□ A → A) → □ A, and Grz is the extension of S4 by □(□(A → □A) →A) → □A. GL is also known to be sound and complete with respect to the class of all Kripke models that are transitive, irreflexive and well founded. Grz bears the same relation to the corresponding reflexive models. We refer the reader to [1] for a full exposition of the subject. (See also [4], [2], [6].)

In §I we develop a sequential calculus for both GL and Grz and give a semantical proof that both systems admit cut-elimination. (Incidentally, this provides an easy proof of the semantical completeness of the two systems.) With respect to GL this yields a correction of an error in [2].

In §II we show that cut-elimination fails for QGL (the extension of GL to a language with quantifiers). We further show that, despite this failure, QGL still has some of GL's interesting properties (e.g., the disjunction property). We also show, using fixed-point techniques, that similar properties obtain if we take as semantics for QGL the arithmetical interpretation extended in the obvious way.

We want to thank Professor H. Gaifman for his help while working on the subject.

A strong partition cardinal is an uncountable well-ordered cardinal κ such that every partition of [κ]κ (the size κ subsets of κ) into less than κ many pieces has a homogeneous set of size κ. The existence of such cardinals is inconsistent with the axiom of choice, and our work concerning them is carried out in ZF set theory with just dependent choice (DC). The consistency of strong partition cardinals with this weaker theory remains an open question. The axiom of determinacy (AD) implies that a large number of cardinals including ℵ1 have the strong partition property. The hypothesis that AD holds in the inner model of constructible sets built over the real numbers as urelements has important consequences for descriptive set theory, and results concerning strong partition cardinals are often applied in this context. Kechris [4] and Kechris et al. [5] contain further information concerning the relationship between AD and strong partition cardinals.

We assume familiarity with the basic results on strong partition cardinals as developed in Kleinberg [6], [7], [8] and Henle [2]. Recall that a strong partition cardinal κ is measurable; in fact every stationary subset of κ is measure one under some normal measure on κ. If μ is a countably additive ultrafilter extending the closed unbounded filter on κ, then the length of the ultrapower [κ]κ under the less than almost everywhere μ ordering is again a measurable cardinal. In §1 below we establish a polarized partition property on these measurable cardinals.

Definition. For ordinals α ≤ κ, = [X]α = {p ⊆ X ∣ ot (p) = α}. For α ≤, κ, κ a cardinal, holds iff for every partition F: [Κ]κ → A, there is an X ∈ [Κ]κ with F constant on [X]α. X is called homogeneous for F. When A = 2 the subscript is omitted.

It has been known since the sixties that for finite exponents all such properties are equivalent, i.e., κ→ (κ)2 iff for all n < ω, β < κ. The infinite case is much more difficult, though it is not actually known to be different. The usual methods for dealing with partitions fail, and as the weakest of these properties violates the Axiom of Choice, many techniques are unavailable.

It is extremely unlikely that an infinite exponent can be increased, but infinite subscripts are generally possible.

Theorem. If ∥ α ∥ · 2 ≤ α, then κ → (κ)α implies for all λ < κ.

In §1, we show the subscript can be any λ < κ. In §2 we extend this to 2λ. In §3 we discuss the possibilities with limited amounts of well-ordered choice, and apply the theorem to the problem of obliging ordinals. Except in §3, we assume no choice. For the story of finite exponents, see [1]. For the relationship between infinite-exponent partition relations and choice, see [4].

The concepts “classical valuation” and “supervaluation” were introduced by van Fraassen around 1966, to provide a semantic analysis of the then extant axiomatic systems of free logic. Consider an atomic sentence

and a “partial” model which fails to interpret c. Then (1) has no truth value in , nor does

While the valuelessness of (1) was found intuitively acceptable, that of (2) was not. Indeed, (2) and all other tautologies are theorems of free logic.

Van Fraassen found a way to accommodate both intuitions. He interprets the unproblematic atomic sentences as usual, while “interpreting” those like (1) by simply assigning them a truth-value in arbitrary fashion. Then a truth-value for every sentence can be defined in the usual way; the result van Fraassen calls a “classical valuation” of the language. The arbitrary element in any given classical valuation is then eliminated by passage to the “supervaluation” over , which agrees with the classical valuations where they agree among themselves, and otherwise is undefined. In the supervaluation over , (1) is valueless but (2) true (since true on all classical valuations), as was required.

There is a slight, but crucial oversimplification in the preceding account. Evaluation of the sentence

requires prior evaluation of the open formula

But here mere assignment of truth-value is not enough; a whole set must be arbitrarily assigned as extension. The quantification over classical valuations involved in passage to the supervaluation thus involves an implicit quantification over subsets of the domain of : supervaluations are second order.

It is shown that if ZF is consistent, then so is ZFC + GCH + “There is a graph with cardinality ℵ2 and chromatic number ℵ2 such that every subgraph of cardinality ≤ ℵ1, has chromatic number ≤ ℵ0”. This partially answers a question of Erdös and Hajnal.

In Baire space we define a sequence of equivalence relations ‹Ev ∣ v < , each Ev being with classes in + v + 1 and such that (i) Ev does not have perfectly many classes, and (ii) is countable iff < ω1. This construction can be extended cofinally in . A new proof is given of a theorem of Hausdorff on partitions of R into ω1 many sets.

An effective translation of the fact that any infinite ordered set contains an infinite ascending or descending sequence is that any infinite recursive set A ⊆ Q has a recursive subset with order type ω or ω*. Tennenbaum's theorem states that this translation is false, and there is a counterexample A with order type ω + ω*. Tennenbaum suggested that this counterexample is an infinite recursive linearly ordered set which is effectively finite, and that the collection of all such counterexamples could provide a concrete model of nonstandard arithmetic. The purpose of this paper is to determine the collection of order types for which there is a counterexample.

It is readily seen that any counterexample to the effective translation must have order type ω + Z · α + ω* for some α [2], [3]. Let be the collection of order types α for which there is a counterexample. As a test case, we have previously shown that contains the constructive scattered orderings [3], [4]. In this paper we determine exactly which order types are in . We easily show that if ω+ Z · α + ω* is recursive, then α is . The main result is that . Consequently, .

Logics LF(M) are considered, in which M (“most”) is a new first-order quantifier whose interpretation depends on a given filter F of subsets of ω. It is proved that countable compactness and axiomatizability are each equivalent to the assertion that F is not of the form {(⋂F) ∪ X: ∣ω − X∣ < ω} with ∣ω − ⋂F∣ = ω. Moreover the set of validities of LF (M) and even of depends only on a few basic properties of F. Similar characterizations are given of the class of filters F for which LF (M) has the interpolation or Robinson properties. An omitting types theorem is also proved. These results sharpen the corresponding known theorems on weak models (, where the collection q is allowed to vary. In addition they provide extensions of first-order logic which possess some nice properties, thus escaping from contradicting Lindström's Theorem [1969] only because satisfaction is not isomorphism-invariant (as it is tied to the filter F). However, Lindström's argument is applied to characterize the invariant sentences as just those of first-order logic.

The recursively unsolvable halting problem for Turing machines is reduced to the problem of the existence or not of an algorithm for deciding whether a field is finite. The latter problem is further reduced to the decision problem of each of properties

for recursive sets Σ of equations of strong algebraic languages with infinitely many operation symbols.

Decision problems concerning properties of sets of equations were first raised by Tarski [9] and subsequently examined by Perkins [6], McKenzie [4], McNulty [5] and Pigozzi [7]. Perkins is the only one who studied recursive sets; the others investigated finite sets. Since the undecidability of properties Pi for recursive sets of equations does not imply any answer to the corresponding decision problems for finite sets, the latter problems remain open.

The work presented here is part of my Ph.D. thesis [2]. I thank Wilfrid Hodges, who supervised it.

An algebraic language is a first-order language with equality but without relation symbols. It is here denoted by , where Qi is an operation symbol and cj, is a constant symbol.

One of the main ideas of calculi of natural deduction, as introduced by Jaśkowski and Gentzen, is that assumptions may be discharged in the course of a derivation. As regards sentential logic, this conception will be extended in so far as not only formulas but also rules may serve as assumptions which can be discharged. The resulting calculi and derivations with rules of any finite level are informally introduced in §1, while §§2 and 3 state formal definitions of the concepts involved and basic lemmata. Within this framework, a standard form for introduction and elimination rules for arbitrary n-ary sentential operators is motivated in §4, understood as a contribution to the theory of meaning for logical signs. §5 proves that the set {&, ∨, ⊃, ⋏} of standard intuitionistic connectives is complete, i.e. &, ∨, ⊃, and ⋏ suffice to express each n-ary sentential operator having rules of the standard form given in §4. §6 makes some remarks on related approaches. For an extension of the conception presented here to quantifier logic, see [11].

Classical set theory is considered as the framework of contemporary mathematics. But there are aspects of our understanding of the real world about what it is not clear that they are described in Cantor's set theory in the best possible way. For example the formalization of some intuitive concepts such as the description of vague properties (which led to the theory of fuzzy sets) and the interpretation of the notion of infinitely small quantities (modelled by nonstandard models) is not quite simple in the classical set theory. Therefore it is natural to look for an alternative to Cantor's set theory which would enable us to formalize these considerations more naturally and which would be a sufficiently strong framework for mathematics at the same time.

The alternative set theory which was created by P. Vopěnka (cf. [V]) is an attempt to construct a theory that could serve as an alternative to Cantor's set theory. The ideas of the alternative set theory make it possible to constitute a new approach to mathematics. An effort to formalize mathematically our intuitive concepts once more is made, and moreover it seems that one can obtain in the alternative set theory mathematical formalizations of concepts which up to now were not adequately formalized.

In the alternative set theory we can build up all essential parts of mathematics. Development of the classical disciplines is described in [V]. Let us mention a few further results obtained in the alternative set theory: a connection between the discrete and the continuous on the basis of which it is possible to define topology (cf. [V]), investigation of motion (cf. [V]), construction of different types of σ-additive ultrafilters (cf. [S-V4]), classification of classes (measurement of vagueness of properties; cf. [Č-V]), possibility of definition of notions of nonstandard methods (cf. [S-V 1]) and valuations of ideals (cf. [M]). A series of articles has been written developing mathematics in the alternative set theory (for the full list of papers cf. [S]). For some remarks concerning the connection between the alternative set theory and nonstandard methods, the reader is refered to [S 4].