We develop a version of Namba forcing which is useful for constructing models with no good scale on ℵω. A model is produced in which holds for all finite n ≥ 1, but there is no good scale on ℵω; this strengthens a theorem of Cummings, Foreman, and Magidor [3] on the non-compactness of square.

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.

This paper defines and explores the properties of several effectivizations of the structure of surreal numbers. The construction of one of previously investigated systems, the metadyadics, is shown to be effectively equivalent to the construction of the surreals in . This equivalence is used to answer several open questions concerning the metadyadics. Results obtained seem to indicate that the metadyadics best capture the notion of a recursive surreal number.

We prove that Kreisel's Conjecture is true, if Peano arithmetic is axiomatised using minimality principle and axioms of identity (theory PAM). The result is independent on the choice of language of PAM. We also show that if infinitely many instances of A(x) are provable in a bounded number of steps in PAM then there exists . The results imply that PAM does not prove scheme of induction or identity schemes in a bounded number of steps.

We prove that given any first order formula ϕ in the language L′ = {+, ·, <,(fi)iЄI, (ci)iЄI}, where the fi are unary function symbols and the ci are constants, one can find an existential formula Ψ such that φ and Ψ are equivalent in any L′-structure

Following Robinson [3], we say that a first-order sentence S is persistent with respect to a set of sentences K if, whenever S is true in a model M of K, then S is true in every extension of M that is also a model of K. Robinson proved in [3] that:

(T) In order that the sentence S be persistent with respect to the set K it is necessary and sufficient that there be some existential sentence Y such that the sentence S↔Y is deducible from K.