Intuitively, a topological space is a set together with some notion of “nearness”. Classically, this notion of nearness is usually described by a collection of open sets. With the development of Non-standard analysis by Abraham Robinson [11], we have an alternative way to describe this notion. If X is any set and *X is a nonstandard extension of X then we can describe a topology on X by means of a relationship of “infinitely close” on some points of X. In many ways this latter approach is more intuitive and leads to more straightforward proofs [11] than the classical approach. The first section of this paper explores the connections between these two approaches. In the main section of the paper we extend the notion of “infinitely close” to infinite points of *X. In the final section we give a very intuitive characterization of the compact-open topology (when the domain is locally compact) and use it to give quick, straight-forward proofs of the usual facts about this topology [1], [2]. This last section can be read independently of the second section.

In this paper we describe generalizations of several approaches to the hyperarithmetic hierarchy, show how they are related to the Kripke-Platek theory of admissible ordinals and sets, and study conditions under which the various approaches remain equivalent.

To put matters in some perspective, let us first review various approaches to the theory of hyperarithmetic sets. For most purposes, it is convenient to first define the semi-hyperarithmetic (semi-HA) subsets of N. A set is then said to be hyperarithmetic (HA) if both it and its complement are semi-HA. A total number-theoretic function is HA if its graph is HA.

There seem to be at least two approaches to the problem of generating notations for ordinals: the papers of Bachmann [2], Gerber [5], Feferman [3], and Isles [6] provide examples of one and those of Ackermann [1], Takeuti [10], Kino [7], Schütte [9], and Pfeiffer [8] the other. In both of these the normal forms produced can be regarded as being finite trees “based on” some ordinal π.

In this paper we demonstrate the hereditary undecidability of finite planar graphs. In §2 we introduce the preliminary logical notions used and outline the Rabin–Scott method of semantic embedding. This method is illustrated in §3 by proving the undecidability of the theory of two finite equivalence relations of a special type. In §4 we give a proof of the main theorem by embedding these equivalence relations into finite planar graphs.

The basic idea is first to form a graph which codes a pair of these relations and then to take a representative of it and “squish” it to the plane. This “squishing” requires the introduction of crossings; and edges of the original graph become paths in the new one. To distinguish the original edges we place two different types of “diamonds” about crossing points. We can then uncode our new graphs to recover the equivalence relations by means of simple first-order incidence properties.

The nonrecursiveness of an arithmetical set can be measured in at least two ways. One way is to consider the level of the set in the arithmetical hierarchy. Another is to consider the degree of unsolvability of the set; more precisely to look at the set of degrees recursive in the degree of the set. The Hierarchy Theorem [1] indicates a correspondence between these two ways of considering the nonrecursiveness of an arithmetical set. The results below indicate some of the contrasts between these two points of view. (It should perhaps be mentioned that, although the results are stated in terms of degrees and levels in the arithmetical hierarchy, they can also be stated, in view of the Hierarchy Theorem, strictly in terms of degrees.)

We use the same terminology and notations as in Part I of this paper [3], but numerals in square brackets refer to the references at the end of the present part. The word “space” will be used in the sense of “subspace of ŪF”. A. G. Hamilton [6] proved that any two α-bases of any α-space are recursively equivalent (see also [5]). This means that dimαV, introduced in [3] only for an isolic α-space V, can be defined for anyα-space V. Several results of [3] can therefore be strengthened. These and some other improvements are listed in §8. It is proved in §9 that every α-subspace of an isolic α-space is again an isolic α-space.

“From two integers k, l one passes immediately to kl; this process leads in a few steps to numbers which are far larger than any occurring in experience, e.g., 67(257729).

Intuitionism, like ordinary mathematics, claims that this number can be represented by an arabic numeral. Could not one press further the criticism which intuitionism makes of existential assertions and raise the question: What does it mean to claim the existence of an arabic numeral for the foregoing number, since in practice we are not in a position to obtain it?

For the terminology and motivation of this note, my earlier paper [3] should be consulted. Here I make a slight change by requiring the Intuitionist Propositional Logic H to be given in terms of axiom schemata rather than axioms.

Definition. An axiom schema F is essentially negative iff each schematic letter appearing in F is negated.

Thus the schemata (¬P ∨ ¬¬P), (¬¬(¬P ∨ ¬Q)→(¬P ∨ ¬Q)) are essentially negative, whereas (¬P ∨ P) and (¬¬P→P) are not.

Lemma. Let F be an essentially negative axiom schema. Then F yields at most finitely many intuitionistically nonequivalent axioms whose atoms are chosen from a fixed finite set.

A significant portion of the study of large cardinals in set theory centers around the concept of “partition relation”. To best capture the basic idea here, we introduce the following notation: for x and y sets, κ an infinite cardinal, and γ an ordinal less than κ, we let [x]γ denote the collection of subsets of x of order-type γ and abbreviate with the partition relation for each function F frominto y there exists a subset C of κ of cardinality κ such that (such that for each α < γ) the range of F on [С]γ ([С]α) has cardinality 1. Now although each infinite cardinal κ satisfies the relation for each n and m in ω (F. P. Ramsey [8]), a connection with large cardinals arises when one asks, “For which uncountable κ do we have κ → (κ)2?” Indeed, any uncountable cardinal κ which satisfies κ → (κ)2 is strongly inaccessible and weakly compact (see [9]). As another example one can look at the improvements of Scott's original result to the effect that if there exists a measurable cardinal then there exists a nonconstructible set. Indeed, if κ is a measurable cardinal then κ → (κ)< ω, and as Solovay [11] has shown, if there exists a cardinal κ such that κ → (κ)< ω3 (κ → (ℵ1)< ω, even) then there exists a nonconstructible set of integers.

A relational structure is called weakly atomic-compact if and only if every set Σ of atomic formulas (taken from the first-order language of the similarity type of augmented by a possibly uncountable set of additional variables as “unknowns”) is satisfiable in whenever every finite subset of Σ is so satisfiable. This notion (as well as some related ones which will be mentioned in §4) was introduced by J. Mycielski as a generalization to model theory of I. Kaplansky's notion of an algebraically compact Abelian group (cf. [5], [7], [1], [8]).

In 1936 Alonzo Church proposed the following thesis: Every effectively computable number-theoretic function is general recursive. The classical mathematician can easily give examples of nonrecursive functions, e.g. by diagonalizing a list of all general recursive functions. But since no such function has been found which is effectively computable, there is as yet no classical evidence against Church's Thesis.

The intuitionistic mathematician, following Brouwer, recognizes at least two notions of function: the free-choice sequence (or ordinary number-theoretic function, thought of as the ever-finite but ever-extendable sequence of its values) and the sharp arrow (or effectively definable function, all of whose values can be specified in advance).