The main result of this paper will be that various notions of majorizability and compactness coincide in the full typestructure over the natural numbers. Moreover we shall show that the extensional typestructure of strongly majorizable functionals can be obtained by applying Zucker's construction ( )E to any of these coinciding intensional typestructures. A different result is proved in the typestructure of effective operations, where not every majorizable functional is compact. Finally we shall introduce the concept of relative compactness in the full typestructure and prove that there are just two degrees of compactness.

It is a theorem of Prikry [7] that if κ carries a uniform η-descendingly complete ultrafilter then the stationary reflection property fails. In this paper we will derive similar results, but here from properties of filters (or ideals) rather than ultrafilters.

Throughout κ and η will denote regular cardinals with η < κ (in particular κ will be uncountable), and I will denote an ideal on κ, by which we mean a set I ⊆ P(κ) such that (i) I is closed under taking subsets and finite unions and (ii) αЄ I for each α < κ, but κ ∉ I. I is said to be μ-complete if it is closed under taking unions of size < μ, I* = {X ⊆ κ ∣ κ − X Є I} is the filter dual to I and if A Є I+ (= P(κ) − I), then I∣A is the ideal on κ given by I ∣ A = {X ⊆ κ ∣ X ∩ A Є I}. If h: A → κ then h is said to be (i) unbounded mod I if for each α < κ, h−1(α) = {ξ Є A ∣ h(ξ) < α} Є I and (ii) a least function for I if h is unbounded mod I and whenever g: A → κ is a function, unbounded mod I, then {ξ Є A ∣ g{ξ) < h{ξ)} Є I.

This article was inspired by the question: is there a definable equivalence relation on the field of complex numbers, each of whose equivalence classes has exactly two elements? The answer turned out to be no, as we now explain in greater detail.

Let Κ be an algebraically closed field and let E be a definable equivalence relation on Κ. [Note: By “definable” we will always mean “definable with parameters”.] Either E has one cofinite class, or all classes are finite and there is a number d such that all but a finite set of classes have cardinality d. In the latter case let B be the finite set of elements of Κ which are not in a class of size d. We prove the following result.

Theorem 1. a) If char(Κ) = 0 or char(Κ) = p > d, then ∣B∣ ≡ 1 (mod d).

b) If char(Κ) = 2 and d = 2, then ∣B∣ ≡ 0 (mod 2).

c) If char(Κ) = p > 2 and d = p + s, where 1 ≤ s ≤ p/2, then ∣B∣ ≡ p + 1 (mod d).

Furthermore, a)−c) are the only restrictions on ≡B≡.

If one is in the right mood, one can view this theorem as saying that the “algebraic cardinality” of the complex numbers is congruent to 1 (mod n) for every n.

§1 contains a reduction of the problem to the special case where E is induced by a rational function in one variable. §2 contains the main calculations and the proofs of a)−c). §3 contains eight families of examples showing that all else is possible. In §4 we prove an analogous result for real closed fields.

In a model of Peano arithmetic, the isomorphism type of the multiplicative semigroup uniquely determines the isomorphism type of the additive semigroup. In fact, for any prime p of , the function x ↦ px is an isomorphism of the additive semigroup with the multiplicative subsemigroup of powers of p. It was observed by Jensen and Ehrenfeucht [3] that for countable models of PA, the isomorphism type of the additive semigroup (or even the additive group) determines the isomorphism type of the multiplicative semigroup. (See Theorem 3 below.)

In this note we will show that the countability restriction cannot be dropped. First, we show (as Theorem 2) that for uncountable models of PA the isomorphism type of the additive group never determines the isomorphism type of the multiplicative semigroup. Our main result is Theorem 5 in which we show that the isomorphism type of the additive semigroup need not determine the isomorphism type of the multiplicative semigroup, thereby improving upon Harnik [2], where Theorem 5 is proved under the assumption of ♢. For completeness, a sketch of the proof of the Jensen-Ehrenfeucht result is included.

The history of this paper begins with Nadel's question, asked in 1981, whether the countability assumption can be eliminated in the Jensen-Ehrenfeucht theorem. Soon afterwards, Nadel obtained the strong counterexample of Theorem 2, which applied to the additive group rather than the additive semigroup. A result of Pabion [8] shows that such a strong result is not possible for the additive semigroup.

It is shown that the low basis theorem is meaningful and provable in IΣ1 and that the priority-free solution to Post's problem formalizes in this theory.

In this paper we shall consider some decision problems for ordinary differential equations. All differential equations will be algebraic differential equations, i.e. equations of the form P(x, y, y′, …, y(n)) = 0 (or P(x, y1, …, ym, y′1 …, y′m, …) = 0 in the case of several dependent variables), where P is a polynomial in all its variables with rational coefficients. (We call P a differential polynomial.)

Jáskowski [6] showed that there is no algorithm to determine if a system of algebraic differential equations (in several dependent variables) has a solution on [0, 1]. An easier proof of this using results on Hilbert's tenth problem is given in [2]. It is natural to restrict the problem in the hope of finding something which is decidable. Two ways to do this are the following: (a) One can ask only for the existence of solutions locally, say around x = 0. Here solution may mean (i) analytic functions, (ii) germs of C∞ functions, or (iii) formal power series, (b) One can consider only one equation in one dependent variable, but ask for the existence of a solution on an interval. In [4] we gave an algorithm for deciding (a)(ii) and (a)(iii) and showed that (a)(i) is undecidable. In §4 (Theorem 4.1) we shall show that (b) (for real analytic solutions) is undecidable. This partially answers Problem 9 of Rubel [13], which stimulated this investigation. We shall also prove (Theorem 4.2) that, given an analytic function f(x) which is the solution of an algebraic differential equation, determined by some initial conditions at x = 0, one cannot in general determine if the radius of convergence of f is < 1 or ≥ 1.

Because the main difference between combinatory weak equality and λβ-equality is that the rule

is valid for the latter but not the former, it is easy to assume that another way of defining combinatory β-equality is to add rule (ξ) to the postulates for weak equality. However, to make this true, one must choose the definition of combinatory abstraction in (ξ) very carefully. If one tries to use one of the more common abstraction algorithms, the result will be an equality, =ξ, that is either equivalent to βη-equality (and so strictly stronger than β-equality) or else strictly weaker than β-equality. This paper will study the relations =ξ for several commonly used abstraction-algorithms, distinguish between them, and axiomatize them.

I falsely claimed, as an aside remark in [8] and also implicitly in the abstract [9], that the slow-growing hierarchy

“catches up” with the fast-growing hierarchy

at level Γ0, i.e. that, for all x > 0,

where x′ is some simple (even linear) function of x.

Girard [4] gave the first correct analysis of the deep relationship which exists between G and F, based on his extensive category-theoretic framework for -logic. This analysis indicates that the first point at which G catches up with F is the ordinal of the theory ID<ω(0 of arbitrary finite iterations of an inductive definition. This is very far beyond Γ0! In particular, in order to capture F at level ∣IDn∣ the slow-growing hierarchy must be generated up to ∣IDn+1∣, i.e. one extra iteration of an inductive definition is needed in order to generate sufficient new ordinals.

The set of equations which use only one variable and hold in all representable relation algebras cannot be derived from any finite set of equations true in all representable relation algebras. Similar results hold for cylindric algebras and for logic with finitely many variables. The main tools are a construction of nonrepresentable one-generated relation algebras, a method for obtaining cylindric algebras from relation algebras, and the use of relation algebras in defining algebraic semantics for first-order logic.

This paper continues joint work of the authors with P. Clote, R. Soare and S. Wainer (Annals of Pure and Applied Logic, vol. 31 (1986), pp. 145–163). An element x of the Cantor space 2ω is said have rank α in the closed set P if x is in Dα(P)/Dα + 1(P), where Dα is the iterated Cantor-Bendixson derivative. The rank of x is defined to be the least α such that x has rank a in some set. The main result of the five-author paper is that for any recursive ordinal λ + n (where λ is a limit and n is finite), there is a point with rank λ + n which is Turing equivalent to O(λ + 2n) All ranked points constructed in that paper are singletons. We now construct a ranked point which is not a singleton. In the previous paper the points of high rank were also of high hyperarithmetic degree. We now construct points with arbitrarily high rank. We also show that every nonrecursive RE point is Turing equivalent to an RE point of rank one and that every nonrecursive point is Turing equivalent to a hyperimmune point of rank one. We relate Clote's notion of the height of a singleton in the Baire space with the notion of rank. Finally, we show that every hyperimmune point x is Turing equivalent to a point which is not ranked.