1. Introduction. Since this paper is a written version of one presented as a survey lecture at a meeting of the Association for Symbolic Logic, its form and content have been essentially determined by the form and content of that lecture, and these latter were themselves considerably influenced by a deliberate attempt to avoid the lecture consisting essentially either of a catalogue of results in some fairly wide field or of the presentation of detailed proofs of a few particular results in a very limited field, with, in either case, a consequential serious limitation of the time available for discussion of the motivation for the results and of the connections between them. There is thus no attempt to cover, even in outline, the full range of topics which could reasonably fall within the scope of its title.

The paper consists mainly of a general discussion of some results concerned with finite models of axiomatically defined calculi of propositional form, with particular reference to the occurrence of such models in proofs that certain propositional calculi are decidable. In addition, there is an account of some results concerned with the existence of undecidable propositional calculi and of others connected with the decidability or undecidability of certain structure problems which relate either to sets of propositional calculi or to different presentations of the same calculus.

In this paper we prove:

Theorem 1. Any finite model of the axiom of power-set also satisfies the axioms of extensionality, sum-set and choice.

Clearly, it will follow from (2) below that in a finite model the axiom of power-set is satisfied if and only if every set is a power-set. Thus, Theorem 1 follows immediately from Theorem 2 below, where by a theory of sets we mean a first-order theory without identity and with only one binary predicate symbol ∈.

Theorem 2. If in a theory of sets every set is a power-set and if the axiom of power-set is valid, then the axioms of extensionality, sum-set and choice are valid.

The proof of Theorem 2 will follow from the lemmas which we establish below.

We mean by x = y that x and y have the same elements. We denote a power-set of x by P(x) when it exists; similarly, we denote a sum-set of x by Ux.

Clearly, in every theory of sets we have:

(1) (x ⊂ y) ↔ (P(x) ⊂ P(y)),

(2) (x = y) ↔ (P(x) = P(y)),

(3) (x = y) → ((x ∈ P(z)) → (y ∈ P(z))),

(4) ⋃P(x) = x.

In view of (2), (3) and the definition of equality, we have:

Lemma 1. If in a theory of sets every set is a power-set, then equal sets are elements of the same sets.

We have also, in view of (4):

Lemma 2. If in a theory of sets every set is a power-set, then every set has a sum-set.

Although Peano's arithmetic can be developed in set theories, it can also be developed independently. This is also true for the theory of ordinal numbers. The author formalized the theory of ordinal numbers in logical systems GLC (in [2]) and FLC (in [3]). These logical systems which contain the concept of ‘arbitrary predicates’ or ‘arbitrary functions’ are of higher order than the first order predicate calculus with equality. In this paper we shall develop the theory of ordinal numbers in the first order predicate calculus with equality as an extension of Peano's arithmetic. This theory will prove to be easy to manage and fairly powerful in the following sense: If A is a sentence of the theory of ordinal numbers, then A is a theorem of our system if and only if the natural translation of A in set theory is a theorem of Zermelo-Fraenkel set theory. It will be treated as a natural extension of Peano's arithmetic. The latter consists of axiom schemata of primitive recursive functions and mathematical induction, while the theory of ordinal numbers consists of axiom schemata of primitive recursive functions of ordinal numbers (cf. [5]), of transfinite induction, of replacement and of cardinals. The latter three axiom schemata can be considered as extensions of mathematical induction.

In the theory of ordinal numbers thus developed, we shall construct a model of Zermelo-Fraenkel's set theory by following Gödel's construction in [1]. Our intention is as follows: We shall define a relation α ∈ β as a primitive recursive predicate, which corresponds to F′ α ε F′ β in [1]; Gödel defined the constructible model Δ using the primitive notion ε in the universe or, in other words, using the whole set theory.

Our ultimate purpose is to give an axiomatic treatment of recursion theory sufficient to develop the priority method. The direct or abstract approach is to keep in mind as clearly as possible the methods actually used in recursion theory, and then to formulate them explicitly. The indirect or experimental approach is to look first for other mathematical theories which seem similar to recursion theory, to formulate the analogies precisely, and then to search for an axiomatic treatment which covers not only recursion theory but also the analogous theories as particular cases.

The first approach is more general because it does not depend on the existence of (familiar) analogues. A concrete mathematical theory, it seems, need have no such analogues and still be important, as e.g. classical number theory. In such a case, an axiomatic treatment may still be useful for exhibiting the mathematical structure of the theory considered and the assumptions on which it rests. However, it will lack one of the most heavily advertised advantages of the axiomatic method, namely, the “economy of thought” which results from an uniform theory for several different and interesting cases: we cannot hope for this if, by hypothesis, we know of only one particular case. In contrast, the second approach, if successful at all, is bound to achieve such “economy” because we start out with several interesting particular cases. Another possible virtue of the second approach is that of field work over insight: the abstract pattern that we are looking for and hoping to formalize in axioms, may not be evident in any one mathematical theory, but may spring to the eye if one happens to look simultaneously at several theories which happen to realize the pattern.

Introduction. This paper is a sequel to our paper [3]. In that paper we introduced the notion of a finite approximation to an infinitely long formula, in a language L with infinitely long expressions of the type considered by Henkin in [2]. The results of the paper [3] show relationships between the models of an infinitely long sentence and the models of its finite approximations. In the present paper we shall apply the main result of [3] to prove a number of theorems about ordinary finitely long sentences; these theorems are of a type which one might call “preservation theorems”. The following two known results are typical preservation theorems: A sentence φ is preserved under substructures if and only if φ is (logically) equivalent to some universal sentence (Łoś [7] and Tarski [11]); φ is preserved under homomorphic images if and only if φ is equivalent to some positive sentence (Lyndon [8]). An expository account of preservation theorems may be found in Lyndon [9].

We shall use freely all of the notation introduced in [3], and shall not attempt to make this paper selfcontained. However, the reader should have no difficulty following this paper after he has read [3]. In § 1 we cover some preliminary topics. In § 2 and § 3 we shall illustrate our method in familiar situations by proving anew the two preservation theorems mentioned above. We shall then obtain four new preservation theorems in §§ 4, 5, and 6, involving respectively direct powers and roots, strong homomorphisms and retracts, and direct factors. The result in § 6 was stated in the abstract [4], while the results in § 5 were stated in the abstract [5].

In [S, pp. 77–88], Smullyan introduced the class of rudimentary relations, and showed that they form a basis for the recursively enumerable sets. He also asked [S, p. 81] if the addition and multiplication relations were rudimentary. In this note we answer one of these questions by showing that the addition relation is rudimentary. This result was communicated to Smullyan orally in 1960 and is announced in [S, p. 81, footnote 1]. However, the proof has not yet appeared in print. (Shortly after the publication of [S], James H. Bennett, using much more subtle methods than those of this note, showed that the multiplication relation is also rudimentary. That result appears in his doctoral dissertation [B], and is being prepared for publication.)

Let us begin by reviewing Smullyan's definition [S, p. 10] of dyadic notation for the positive integers. Each positive integer a is identified with the unique string anan−1…a1a0 of 1's and 2's such that a = Σin=0ai2i Because of this identification, we are able to speak of typographical properties of numbers.