The literature on modal logic includes a number of general completeness and decidability results. The work of Schiller Joe Scroggs [5], R.A. Bull [1], Kit Fine [2], and Krister Segerberg [6] provide examples.

Scroggs showed that the proper extensions of S5 have the finite model property and are axiomatizable. (Harrop [3] then argued that logics having these properties are decidable.) Bull extended Scroggs' result by showing that the normal extensions of S4.3 have the finite model property. Fine subsequently provided a model-theoretic proof of Bull's result and also proved the axiomatizability of these logics. In a different direction Segerberg proved that every normal logic containing the characteristic axioms of Lewis' systems S4 and S5 is decidable.

The present paper is in this tradition. We extend the results of Scroggs and Segerberg by showing that every normal modal logic containing the S5 axiom has the finite model property, is axiomatizable, and thus is decidable.

A Boolean algebra (henceforth abbreviated B.A.) is said to be recursive if B is a recursive subset of the natural numbers N and the operations ∧ (meet), ∨ (join), and ¬ (complement) are partial recursive. Let denote the set of atoms of and denote the ideal generated by the atoms of . Given recursive B.A.s and , we write ≈ if is isomorphic to and ≈r if is recursively isomorphic to , i.e., if there is a partial recursive isomorphism from onto .

Recursive B.A.s have been studied by several authors including Ershov [2], Fiener [3], [4], Goncharov [5], [6], [7], LaRoche [8], Nurtazin [7], and the author [10], [11]. This paper continues a study of the recursion theoretic relationships among , , and the recursive isomorphism type of a recursive B.A. we started in [11]. We refer the reader to [11] for any unexplained notation and definitions. In [11], we were mainly concerned with the possible recursion theoretic properties of the set of atoms in recursive B.A.s. We found that even if we insist that be recursive, there is considerable freedom for the properties of . For example, we showed that if is a recursive B.A. such that is recursive and is infinite, then (i) there exists a recursive B.A. such that and both and are recursive and (ii) for any nonzero r.e. degree δ, there exist recursive B.A.s , , … such that for each i, is of degree δ, is recursive, is immune if i is even and is not immune if i is odd, and no two B.A.s in the sequence are recursively isomorphic.

A generalized version of Martin's axiom, called BACH, is shown to be equivalent to one of its combinatorial consequences, a generalization of P(c).

In Gabbay [1] it is stated as an open problem whether or not Craig's Theorem holds for the logic of constant domains CD, i.e. for the extension of the intuitionistic predicate calculus, IPC, obtained by adding the schema; . Then in the later article, [2], Gabbay gives a proof of it. The proof given in [2] is via Robinson's (weak) consistency theorem and depends on relatively complicated (Kripke-) model-theoretical constructions developed in [1] (see p. 392 of [1] for a brief sketch of the method). The aim of this note is to show that the interpolation theorem for CD can also be obtained, by simple proof-theoretic methods, from §80 of Kleene's Introduction to Metamathematics [3].

GI is the classical formal system whose postulates are given on p. 442 of [3]. Let GD be the system obtained from GI by the following modifications: (1) the sequents of GD are to have at most two formulas in their succedents and (2) the intuitionistic restriction that Θ be empty is required for the succedent rules (→ ¬) and (→ ⊃). It is a simple matter to show that: , x not free in . It then follows that, using Theorem 46 of [3], if then .

Since the discovery of forcing in the early sixties, it has been clear that many natural and interesting mathematical questions are not decidable from the classical axioms of set theory, ZFC. Therefore some mathematicians have been studying the consequences of stronger set theoretic assumptions. Two new types of axioms that have been the subject of much research are large cardinal axioms and axioms asserting the determinacy of definable games. The two appear at first glance to be unrelated; one of the most surprising discoveries of recent research is that this is not the case.

In this paper we will be assuming the axiom of determinacy (AD) plus the axiom of dependent choice (DC). AD is false, since it contradicts the axiom of choice. However every set in L[R] is ordinal definable from a real. Our axiom that definable games are determined implies that every game in L[R] is determined (in V), and since a strategy is a real, it is determined in L[R]. That is, L[R] ⊨ AD. The axiom of choice implies L[R] ⊨ DC. So by embedding ourselves in L[R], we can assume AD + DC and begin proving theorems. These theorems true in L[R] imply corresponding theorems in V, by e.g. changing “every set” to “every set in L[R]”. For more information on AD as an axiom, and on some of the points touched on here, the reader should consult [14], particularly §§7D and 8I. In this paper L[R] will no longer even be mentioned. We just assume AD for the rest of the paper.

Beginning in 1960, and continuing for about a decade and a half, Shepherdson's self-referential formulae dominated the applications of diagonalization in metamathematics. In 1976, it was doubly toppled from its position of supremacy by two demonstrably more powerful such sentences introduced by D. Guaspari. The goal of the present note is to understand and explain the first (and, for the time being, more important) of Guaspari's self-referential sentences, which we dub “Guaspari sentences of the first kind”, or, less poetically, “Guaspari fixed points”.

Both Shepherdson's and Guaspari's fixed points are generalizations of the Rosser sentence. But, where Shepherdson merely tacks on side-formulae, Guaspari takes a more revolutionary step: He views the basic components, PrT(⌈¬φ⌉) and PrT(⌈φ⌉), of the Rosser sentence as attempts to refute something and replaces them by refutations of something else. Ignoring their Shepherdsonesque side-formulae (tacked on for the sake of more esoteric applications), our analysis of Guaspari fixed points can be viewed as merely the isolation of those properties whose refutations can be used in this context.

In §1 we offer the outcome of this analysis—i.e. a delineation of properties whose refutations can so be used. Some simple examples are cited and the Guaspari fixed points are defined. The main theorem—a master Fixed Point Calculation—is proven in §2.

The study of effectiveness in classical mathematics is rapidly expanding, through recent research in algebra, topology, model theory, and functional analysis. Well-known contributors are Barwise (Wisconsin), Crossley (Monash), Dekker (Rutgers), Ershoff (Novosibirsk), Feferman (Stanford), Harrington (Berkeley), Mal′cev (Novosibirsk), Morley (Cornell), Nerode (Cornell), Rabin (Hebrew University), Shore (Cornell). Further interesting work is due to Kalantari (University of California, Santa Barbara), Metakides (Rochester), Millar (Wisconsin), Remmel (University of California, San Diego), Nurtazin (Novosibirsk). Areas investigated include enumerated algebras, models of complete theories, vector spaces, fields, orderings, Hilbert spaces, and boolean algebras.

We investigate the effective content of the structure theory of p-groups. Recall that a p-group is a torsion abelian group in which the (finite) order of each element is some power of a fixed prime p. (In the sequel, “group” = “additively written abelian group”.)

The structure theory of p-groups is based on the two elementary notions of order and height. Recall that the order of x is the least integer n such that nx = 0. The height of x is the number of times p divides x, that is, the least n such that x = pny for some y in the group but x ≠ pn+1y for any y. If for each n ∈ ω there is a “pnth-root” yn, so that x = pnyn, then we say that x has infinite height. In 1923, Prüfer related the two notions as criteria for direct sum decomposition, proving

Theorem. Every group of bounded order is a direct sum of cyclic groups, and

Theorem. Every countable primary group with no (nonzero) elements of infinite height is a direct sum of cyclic groups.

Ever since Cohen invented forcing in 1963, people have studied the properties that cardinals can have in generic extensions of the ground model. A very early result of Lévy shows that if κ is a regular cardinal and λ > κ is strongly inaccessible, then there is a notion of forcing which collapses every cardinal strictly between κ and λ yet preserves every other cardinal. This, of course, answers one question of the genre “What properties can a cardinal have in a generic extension?”

Another question of the same genre that can be asked is the following: Is it possible to have a generic extension of the ground model in which all cardinals are preserved and yet the cofinalities of some cardinals are different? This question was first answered in the affirmative by Prikry, who proved the following theorem.

Theorem 1.1 (Prikry [5]). Assume that V ⊨ “ZFC + κ is measurable”. Then there is a notion of forcing, P, such that for G V-generic over P:

(1) V and V[G] have the same cardinals.

(2) V and V[G] have the same bounded subsets of κ.

(3) V[G], i.e, V[G] ⊨ “κ is Rowbottom”.

(4) V[G] ⊨ “cof(κ) = ω”.

Prikry's result naturally raises the following question: Is it possible to get a generic extension in which cardinals are preserved and yet the cofinalities of certain cardinals are different from the ground model's but are uncountable? This question was first answered in the affirmative by Magidor, who proved the following theorem.

Let α be any countable admissible ordinal greater than ω. There is a transitive set A such that A is admissible, locally countable, OnA = α, and A satisfies Σ1-separation. In fact, if B is any nonstandard model of KP + ∀x ⊆ ω (the hyperjump of x exists), the ordinal standard part of B is greater than ω, and every standard ordinal in B is countable in B, then HCB ∩ (standard part of B) satisfies Σ-separation.

We improve the canonization theorems generalizing the Erdös-Rado theorem, and as a result complete the answer to “When does a Hausdorff space of cardinality χ necessarily have a discrete subspace of cardinality k” We also improve the results on existence of free subsets.

Let T be an inductive, first-order theory in a language L, let E(L) denote the set of existential L-formulas, and let E(T) denote the distributive lattice of equivalence-classes φT of formulas φ ∈ E(L) with respect to equivalence in T. We consider three types of ‘complements’ in E(T): Let φT, ψT ∈ E(T) and suppose φT ∏ ψT = 0. Then ψT is a complement of φT, if φT ∐ ψT = 1; ψT is a pseudo-complement of φT, if for all μT ∈ E(T), (φT ∐ ψT) = 0 implies μT ≤ ψT; ψT is a “weak complement of φT, if for all μT ∈ E(T), (φT ⋰ ψT) ∐ μT = 0 implies μT = 0. The following facts are obvious: A complement of φT is also a pseudo-complement of φT and a pseudo-complement of φT is also a weak complement of φT. Any φT has at most one pseudo-complement; it is denoted by φT*. The relations ‘ψT is the complement of φT’ and ‘ψT is a weak complement of φT’ are symmetrical. We call φT (weakly, pseudo-) complemented if φT has a (weak, pseudo-) complement, and we call E(T) (weakly, pseudo-) complemented if every φT is (weakly, pseudo-) complemented.

The object of this note is to characterize (weakly, pseudo-) complemented existential formulas in model-theoretic terms, and conversely to characterize some classical notions of Robinson style model theory in terms of these formulas. The following theorems illustrate the second approach.

We define a first-order theory FIN which has a recursive axiomatization and has the following two properties. Each finite part of FIN has finite models. FIN is strong enough to develop that part of mathematics which is used or has potential applications in natural science. This work can also be regarded as a consistency proof of this hitherto informal part of mathematics. In FIN one can count every set; this permits one to prove some new probabilistic theorems.

In this paper we are concerned with the formal provability of certain true implications of Σ1-sentences. Old completeness and incompleteness results already give some information about this. For example by Σ1-completeness of PRA (primitive recursive arithmetic) every true implication of the form D → E, where D is a Σ0-sentence and E a Σ1-sentence, or D a Σ1-sentence and E a true Σ1-sentence, is provable in PRA. On the other hand, by Gödel's incompleteness theorems one can define for every suitable theory S a false Σ1-sentence Ds such that for every false Σ0-sentence E the true implication Ds → E is not provable in S, but is provable in PRA + Cons. So one sees that for a suitable fixed conclusion the provability of true implications of Σ1-sentences depends on the content of the premise.

Now we ask, if and how for a suitable fixed premise the provability of true implications of Σ1-sentences depends on the conclusion. As remarked above, by Σ1-completeness of PRA this question is settled, if the premise is true. For a false premise it is answered in § 1 as follows: Let D be a false Σ1-sentence, S an extension of PRA, and S+ ≔ PRA + IAΣ1 + RFNΣ1S). Depending on D and S one can define a Σ1-sentence Es such that S+ ⊢ D → Es, but S ⊬ D → Es provided that S+ is not strong enough to refute D. (IAΣ1 denotes the scheme of the induction axiom for Σ1-formulas, and RFNΣ1(S) the uniform Σ1-reflection principle for S.)

Profinite groups are Galois groups. The effective study of infinite Galois groups was initiated by Metakides and Nerode [8] and further developed by LaRoche [5]. In this paper we study profinite groups without considering Galois extensions of fields. The Artin method of representing a finite group as a Galois group has been generalized (effectively!) by Waterhouse [14] to profinite groups. Thus, there is no loss of relevance in our approach.

The fundamental notions of a co-r.e. profinite group, recursively profinite group, and the degree of a co-r.e. profinite group are defined in §1. In this section we prove that every co-r.e. profinite group can be effectively represented as an inverse limit of finite groups. The degree invariant is shown to behave very well with respect to open subgroups and quotients. The work done in this section is basic to the rest of the paper.

The commutator subgroup, the Frattini subgroup, the p-Sylow subgroups, and the center of a profinite group are essential in the study of profinite groups. It is only natural to ask if these subgroups are effective. The following question exemplifies our approach to this problem: Is the center a co-r.e. profinite group? Theorem 2 provides a general method for answering this type of question negatively. Examples 3,4 and 5 are all applications of this theorem.

The Gödel Class is the class of prenex formulas of pure quantification theory whose prefixes have the form ∀y1∀y2∃x1 … ∃xn. The Gödel Class with Identity, or GCI, is the corresponding class of formulas of quantification theory extended by inclusion of the identity-sign “ = ”. Although the Gödel Class has long been kndwn to be solvable, the decision problem for the Gödel Class with Identity is open. In this paper we prove that there is no primitive recursive decision procedure for the GCI, or, indeed, for the subclass of the GCI containing just those formulas with prefixes ∀y1∀y2∃x.

Throughout this paper we take quantification theory to include, aside from logical signs, infinitely many k-place predicate letters for each k > 0, but no function signs or constants. Moreover, by “prenex formula” we include only those without free variables. A decision procedure for a class of formulas is a recursive function that carries a formula in the class to 0 if the formula is satisfiable and to 1 if not. A class is solvable iff there exists a decision procedure for it. A class is finitely controllable iff every satisfiable formula in the class has a finite model. Since we speak only of effectively specified classes, finite controllability implies solvability (but not conversely).

The GCI has a curious history. Gödel showed the Gödel Class (without identity) solvable in 1932 [4] and finitely controllable in 1933 [5].

Every ℵ0-categorical partially ordered set of finite width has a finitely axiomatizable theory. Every ℵ0-categorical partially ordered set of finite weak width has a decidable theory. This last statement constitutes a major portion of the complete (with three exceptions) characterization of those finite partially ordered sets for which any ℵ0-categorical partially ordered set not embedding one of them has a decidable theory.

Although Brouwer became famous for his vehement attacks upon classical logic and set theory, his work did not develop in a vacuum and strongly depended on that of Cantor.

His mind bent on shifting aside nonconstructive arguments, he tried to rebuild Cantor's edifice along new, intuitionistic lines. The continuum hypothesis, lying at the core of set theory, also confronted Brouwer, and he had to face the farthest conclusion Cantor had been able to reach in trying to solve it: every nondenumerable closed subset of the real line has the power of the continuum.

Brouwer's thinking about it seems to have been subject to some development. In 1914 we hear him saying: “Wir sahen oben dass das Cantorsche Haupttheorem für den Intuitionisten keines Beweises bedarf” (“As we saw above, for us, being intuitionists, Cantor's Main Theorem does not need a proof”) [3]. Nevertheless, five years later, he publishes an essay: Theorie der Punktmengen, which might be described as an attempt to reconstruct Cantor's reasonings in detail [4].

This attempt was not entirely successful, as Brouwer comes to admit in 1952, probably having lost, now, some of his youthful rashness [10]. So the question of what the constructive content of Cantor's Main Theorem is, still awaits an answer.

We do not think the answer we will give can be considered a conclusive one, but, in any case, it is a beginning.

It is known (see [1, 3.1.5]) that the order type of the nonstandard natural number system *N has the form ω + (ω* + ω) θ, where θ is a dense order type without first or last element and ω is the order type of N. Concerning this, Zakon [2] examined *N more closely and investigated the nonstandard real number system *R, as an ordered set, as an additive group and as a uniform space. He raised five questions which remained unsolved. These questions are concerned with the cofinality and coinitiality of θ (which depend on the underlying nonstandard universe *U). In this paper, we shall treat nonstandard models where the cofinality and coinitiality of θ coincide with some appropriated cardinals. Using these nonstandard models, we shall give answers to three of these questions and partial answers to the other to questions in [2].

In this paper we prove, under the assumption of a strong partition property for an uncountable cardinal κ, the existence of more than κ-many measurable cardinals greater than κ. Our proof involves so-called seminormal measures, and, along the way, we establish several key facts about such measures.