Our main goal in this paper is to establish a Glimm-Effros type dichotomy for arbitrary analytic equivalence relations.

The original Glimm-Effros dichotomy, established by Effros [Ef], [Ef1], who generalized work of Glimm [G1], asserts that if an Fσ equivalence relation on a Polish space X is induced by the continuous action of a Polish group G on X, then exactly one of the following alternatives holds:

(I) Elements of X can be classified up to E-equivalence by “concrete invariants” computable in a reasonably definable way, i.e., there is a Borel function f: X → Y, Y a Polish space, such that xEy ⇔ f(x) = f(y), or else

(II) E contains a copy of a canonical equivalence relation which fails to have such a classification, namely the relation xE0y ⇔ ∃n∀m ≥ n(x(n) = y(n)) on the Cantor space 2ω (ω = {0,1,2, …}), i.e., there is a continuous embedding g: 2ω → X such that xE0y ⇔ g(x)Eg(y).

Moreover, alternative (II) is equivalent to:

(II)′ There exists an E-ergodic, nonatomic probability Borel measure on X, where E-ergodic means that every E-invariant Borel set has measure 0 or 1 and E-nonatomic means that every E-equivalence class has measure 0.

We give a self-contained geometric proof of the completeness theorem for the infinite-valued sentential calculus of Łukasiewicz.

We investigate in an algebraic setting the question of which logical languages can express the properties integral, permutational, and rigid for algebras of relations.

We consider fine hierarchies in recursion theory, descriptive set theory, logic and complexity theory. The main results state that the sets of values of different Boolean terms coincide with the levels of suitable fine hierarchies. This gives new short descriptions of these hierarchies and shows that collections of sets of values of Boolean terms are almost well ordered by inclusion. For the sake of completeness we mention also some earlier results demonstrating the usefulness of fine hierarchies.

The purpose of this paper is to show that semantics for relevance logic, based on the Routley-Meyer semantics, can be given without using the Routley star operator to treat negation. In the resulting semantics, negation is treated implicationally. It is shown that, by the use of restrictions on the ternary accessibility relation, simplified by the use of some definitions, a semantics can be stipulated over which R is complete.

We prove that in IST, Nelson's internal set theory, the Uniqueness and Collection principles, hold for all (including external) formulas. A corollary of the Collection theorem shows that in IST there are no definable mappings of a set X onto a set Y of greater (not equal) cardinality unless both sets are finite and #(Y) ≤ n #(X) for some standard n. Proofs are based on a rather general technique which may be applied to other nonstandard structures. In particular we prove that in a nonstandard model of PA, Peano arithmetic, every hyperinteger uniquely definable by a formula of the PA language extended by the predicate of standardness, can be defined also by a pure PA formula.

We show that it is consistent with ZFC (relative to large cardinals) that every infinite Boolean algebra B has an irredundant subset A such that 2∣A∣ = 2∣B∣. This implies in particular that B has 2∣B∣ subalgebras. We also discuss some more general problems about subalgebras and free subsets of an algebra.

The result on the number of subalgebras in a Boolean algebra solves a question of Monk from [6]. The paper is intended to be accessible as far as possible to a general audience, in particular we have confined the more technical material to a “black box” at the end. The proof involves a variation on Foreman and Woodin's model in which GCH fails everywhere.

Let p be a fixed prime integer, other than 2, Qp the field of p-adic numbers, and Ωp the completion of the algebraic closure of Qp. Let Rp be the ring of entire functions in one variable t over Ωp; that is, Rp is the ring of functions f: Ωp → Qp such that f(t) is given by a power series around 0, of infinite radius of convergence:

and where ∣a∣p is the p-adic norm of a in Ωp. We prove:

Theorem A. The positive existential theory of Rp in the language L = {0, 1, t, +, ·} is undecidable.

Theorem A gives a negative answer to the analogue of Hilbert's tenth problem for Rp in the language L. Related results include those of [2] where it is shown that the first-order theory of entire functions on the complex plane is undecidable and the similar result for analytic functions on the open unit disk (this is due to Denef and Gromov, communicated to us by Cherlin and is as of now unpublished).

It would be desirable to have a similar result in the language which, instead of the variable t, has a predicate for the transcendental (that is, nonconstant) elements of Rp. A related problem is the similar problem for meromorphic functions on the real or p-adic plane or on the unit open or closed disk. These problems seem for the moment rather hard in view of the fact that the analogue of Hilbert's Tenth Problem for the field of rational functions over the complex numbers (or any algebraically closed field of characteristic zero) is an open problem.

Treating the existential quantification ∃νi as a diamond ♢i and the identity νi = νj as a constant δij, we study restricted versions of first order logic as if they were modal formalisms. This approach is closely related to algebraic logic, as the Kripke frames of our system have the type of the atom structures of cylindric algebras; the full cylindric set algebras are the complex algebras of the intended multidimensional frames called cubes.

The main contribution of the paper is a characterization of these cube frames for the finite-dimensional case and, as a consequence of the special form of this characterization, a completeness theorem for this class. These results lead to finite, though unorthodox, derivation systems for several related formalisms, e.g. for the valid n-variable first order formulas, for type-free valid formulas and for the equational theory of representable cylindric algebras. The result for type-free valid formulas indicates a positive solution to Problem 4.16 of Henkin, Monk and Tarski [16].

If it is difficult to give the exact significance of consistency proofs from a classical point of view, in particular the proofs of Gentzen [2, 6], and Novikoff [14], the motivations of these proofs are quite clear intuitionistically. Their significance is then less to give a mere consistency proof than to present an intuitionistic explanation of the notion of classical truth. Gentzen for instance summarizes his proof as follows [6]: “Thus propositions of actualist mathematics seem to have a certain utility, but no sense. The major part of my consistency proof, however, consists precisely in ascribing a finitist sense to actualist propositions.” From this point of view, the main part of both Gentzen's and Novikoff's arguments can be stated as establishing that modus ponens is valid w.r.t. this interpretation ascribing a “finitist sense” to classical propositions.

In this paper, we reformulate Gentzen's and Novikoff's “finitist sense” of an arithmetic proposition as a winning strategy for a game associated to it. (To see a proof as a winning strategy has been considered by Lorenzen [10] for intuitionistic logic.) In the light of concurrency theory [7], it is tempting to consider a strategy as an interactive program (which represents thus the “finitist sense” of an arithmetic proposition). We shall show that the validity of modus ponens then gets a quite natural formulation, showing that “internal chatters” between two programs end eventually.

We first present Novikoff's notion of regular formulae, that can be seen as an intuitionistic truth definition for classical infinitary propositional calculus. We use this in order to motivate the second part, which presents a game-theoretic interpretation of the notion of regular formulae, and a proof of the admissibility of modus ponens which is based on this interpretation.

We study the I-ultrafilters on ω, where I is a a collection of subsets of a set X, usually ℝ or ω1. The I-ultrafilters usually contain the P-points, often as a small proper subset. We study relations between I-ultrafilters for various I, and closure of I-ultrafilters under ultrafilter sums. We consider, but do not settle, the question whether I-ultrafilters always exist.

Let us define the intuitionistic part of a classical theory T as the intuitionistic theory whose proper axioms are identical with the proper axioms of T. For example, Heyting arithmetic HA is the intuitionistic part of classical Peano arithmetic PA.

It's a well-known fact, proved by Heyting and Myhill, that ZF is identical with its intuitionistic part.

In this paper, we mainly prove that TT, Russell's Simple Theory of Types, and NF, Quine's “New Foundations,” are not equal to their intuitionistic part. So, an intuitionistic version of TT or NF seems more naturally definable than an intuitionistic version of ZF.

In the first section, we present a simple technique to build Kripke models of the intuitionistic part of TT (with short examples showing bad properties of finite sets if they are defined in the usual classical way).

In the remaining sections, we show how models of intuitionistic NF2 and NF can be obtained from well-chosen classical ones. In these models, the excluded middle will not be satisfied for some non-stratified sentences.