We study the partition relation X →w [Y]p2 that is a weakening of the usual partition relation X → [Y]ρ2. Our main result asserts that if κ is an uncountable strongly compact cardinal and 𝔡κ≤λ < κ, then Iκ,λ+ →w [Iκ,λ+]λ< κ2 does not hold.

By using finite support iteration of Suslin c.c.c forcing notions we construct several models which satisfy some ♢-like principles while other cardinal invariants are larger than ω₁.

We use κ-free but not Whitehead Abelian groups to construct Abstract Elementary Classes (AEC) which satisfy the amalgamation property but fail various conditions on the locality of Galois-types. We introduce the notion that an AEC admits intersections. We conclude that for AEC which admit intersections, the amalgamation property can have no positive effect on locality: there is a transformation of AEC’s which preserves non-locality but takes any AEC which admits intersections to one with amalgamation. More specifically we have: Theorem 5.3. There is an AEC with amalgamation which is not (ℵ₀,ℵ₁)-tame but is (2^ℵ₀,∞)-tame; Theorem 3.3. It is consistent with ZFC that there is an AEC with amalgamation which is not (≤ℵ₂,≤ℵ₂)-compact.

A monadic formula ψ(Y) is a selector for a formula φ(Y) in a structure ℳ if there exists a unique subset P of ℳ which satisfies ψ and this P also satisfies φ. We show that for every ordinal α≥ ω^ω there are formulas having no selector in the structure (α, <). For α ≤ ω₁, we decide which formulas have a selector in (α, <), and construct selectors for them. We deduce the impossibility of a full generalization of the Büchi-Landweber solvability theorem from (ω, <) to (ω^ω, <). We state a partial extension of that theorem to all countable ordinals. To each formula we assign a selection degree which measures “how difficult it is to select”. We show that in a countable ordinal all non-selectable formulas share the same degree.

We show that many singular cardinals λ above a strongly compact cardinal have regular ultrafilters D that violate the finite square principle □^fin_λ,D introduced in [3]. For such ultrafilters D and cardinals λ there are models of size λ for which M^λ/D is not λ⁺⁺-universal and elementarily equivalent models M and N of size λ for which M^λ/D and N^λ/D are non-isomorphic. The question of the existence of such ultrafilters and models was raised in [1].

Let (M,𝒳) ⊨ ACA₀ be such that P_𝒳, the collection of all unbounded sets in 𝒳, admits a definable complete ultrafilter and let T be a theory extending first order arithmetic coded in 𝒳 such that M thinks T is consistent. We prove that there is an end-extension N ⊨ T of M such that the subsets of M coded in N are precisely those in 𝒳. As a special case we get that any Scott set with a definable ultrafilter coding a consistent theory T extending first order arithmetic is the standard system of a recursively saturated model of T.

In a previous paper with M. Džamonja, class forcings were given which fixed the complexity (a universality covering number) for certain types of structures of size λ together with the value of 2^λ for every regular λ. As part of a programme for examining when such global results can be true in an inner model, we build generics for these class forcings.

Some 40 years ago, Dana Scott proved that every countable Scott set is the standard system of a model of PA. Two decades later, Knight and Nadel extended his result to Scott sets of size ω₁. Here, I show that assuming the Proper Forcing Axiom (PFA), every A-proper Scott set is the standard system of a model of PA. I define that a Scott set 𝔛 is proper if the quotient Boolean algebra 𝔛/Fin is a proper partial order and A-proper if 𝔛 is additionally arithmetically closed. I also investigate the question of the existence of proper Scott sets.

This article aims to classify those reducts of expansions of (ℚ, <) by unary predicates which eliminate quantifiers, and in particular to show that, up to interdefinability, there are only finitely many for a given language. Equivalently, we wish to classify the closed subgroups of Sym(ℚ) containing the group of all automorphisms of (ℚ, <) fixing setwise certain subsets. This goal is achieved for expansions by convex predicates, yielding expansions by constants as a special case, and for the expansion by a dense, co-dense predicate. Partial results are obtained in the general setting of several dense predicates.

In 2004 Csima, Hirschfeldt, Knight, and Soare [1] showed that a set A ≤T 0’ is nonlow₂ if and only if A is prime bounding, i.e., for every complete atomic decidable theory T, there is a prime model ℳ computable in A. The authors presented nine seemingly unrelated predicates of a set A, and showed that they are equivalent for Δ⁰₂ sets. Some of these predicates, such as prime bounding, and others involving equivalence structures and abelian p-groups come from model theory, while others involving meeting dense sets in trees and escaping a given function come from pure computability theory. As predicates of A, the original nine properties are equivalent for Δ⁰₂ sets; however, they are not equivalent in general. This article examines the (degree-theoretic) relationship between the nine properties. We show that the nine properties fall into three classes, each of which consists of several equivalent properties. We also investigate the relationship between the three classes, by determining whether or not any of the predicates in one class implies a predicate in another class.

An important technique in large cardinal set theory is that of extending an elementary embedding j:M→ N between inner models to an elementary embedding j^*:M[G]→ N[G^*] between generic extensions of them. This technique is crucial both in the study of large cardinal preservation and of internal consistency. In easy cases, such as when forcing to make the GCH hold while preserving a measurable cardinal (via a reverse Easton iteration of α-Cohen forcing for successor cardinals α), the generic G^* is simply generated by the image of G. But in difficult cases, such as in Woodin’s proof that a hypermeasurable is sufficient to obtain a failure of the GCH at a measurable, a preliminary version of G^* must be constructed (possibly in a further generic extension of M[G]) and then modified to provide the required G^*. In this article we use perfect trees to reduce some difficult cases to easy ones, using fusion as a substitute for distributivity. We apply our technique to provide a new proof of Woodin’s theorem as well as the new result that global domination at inaccessibles (the statement that d(κ) is less than 2^κ for inaccessible κ, where d(κ) is the dominating number at κ) is internally consistent, given the existence of 0^#.

There is a simple technique, due to Dragalin, for proving strong cut-elimination for intuitionistic sequent calculus, but the technique is constrained to certain choices of reduction rules, preventing equally natural alternatives. We consider such a natural, alternative set of reduction rules and show that the classical technique is inapplicable. Instead we develop another approach combining two of our favorite tools—Klop’s ι-translation and perpetual reductions. These tools are of independent interest and have proved useful in a variety of settings; it is therefore natural to investigate, as we do here, what they have to offer the field of sequent calculus.

We prove constructively that the weak König lemma and quantifier-free number—number choice imply that every pointwise continuous function from Cantor space into Baire space has a modulus of uniform continuity.

We consider a modal language for affine planes, with two sorts of formulas (for points and lines) and three modal boxes. To evaluate formulas, we regard an affine plane as a Kripke frame with two sorts (points and lines) and three modal accessibility relations, namely the point-line and line-point incidence relations and the parallelism relation between lines. We show that the modal logic of affine planes in this language is not finitely axiomatisable.

We provide a short proof that if $\kappa$ is a regular cardinal with $\kappa \leq \mathfrak{c}$, then $\left(\begin{matrix} \kappa \\ \omega \end{matrix}\right) \rightarrow \left(\begin{matrix} \kappa~~~\alpha \\ \omega~~~\omega \end{matrix} \right)^{1,1}$ for any ordinal $\alpha$ < min {$\mathfrak{p},\kappa$}. In particular, $\left(\begin {matrix} \mathfrak{p} \\ \omega \end{matrix}\right) \rightarrow \left(\begin{matrix} \mathfrak{p}~~~\alpha \\ \omega~~~\omega \end{matrix}\right)^{1,1}$ for any ordinal $\alpha < \mathfrak{p}$. This generalizes an unpublished result of E. Szemerédi that Martin’s axiom implies that $\left(\begin{matrix} \mathfrak{c} \\ \omega \end{matrix}\right) \rightarrow \left(\begin{matrix} \mathfrak{c}~~~\kappa \\ \omega~~~\omega \end{matrix}\right)^{1,1}$. for any cardinal $\kappa$ with $\kappa < \mathfrak{c}$.

We will study some aspects of the local structure of models of certain C-minimal theories. We will prove (theorem 19) that, in a sufficiently saturated C-minimal structure in which the algebraic closure has the exchange property and which is locally modular, we can construct an infinite type-definable group around any non trivial point (a term to be defined later).

In this paper a method for the replacement, in formulas, of strong quantifiers by functions is introduced that can be considered as an alternative to Skolemization in the setting of constructive theories. A constructive extension of intuitionistic predicate logic that captures the notions of preorder and existence is introduced and the method, orderization, is shown to be sound and complete with respect to this logic. This implies an analogue of Herbrand’s theorem for intuitionistic logic. The orderization method is applied to the constructive theories of equality and groups.

We investigate the relationship between lowness for Schnorr randomness and Schnorr triviality. We show that a real is low for Schnorr randomness if and only if it is Schnorr trivial and hyperimmune free.

Much previous study has been done on the degree spectra of prime models of a complete atomic decidable theory. Here we study the analogous questions for homogeneous models. We say a countable model 𝒜 has a d-basis if the types realized in 𝒜 are all computable and the Turing degree d can list Δ₀⁰-indices for all types realized in 𝒜. We say 𝒜 has a d-decidable copy if there exists a model ℬ≅𝒜 such that the elementary diagram of ℬ is d-computable. Goncharov, Millar, and Peretyat’kin independently showed there exists a homogeneous 𝒜 with a 0-basis but no decidable copy. We prove that any homogeneous 𝒜 with a 0’-basis has a low decidable copy. This implies Csima’s analogous result for prime models. In the case where all types of the theory T are computable and 𝒜 is a homogeneous model with a 0-basis, we show 𝒜 has copies decidable in every nonzero degree. A degree d is 0-homogeneous bounding if any automorphically nontrivial homogeneous 𝒜 with a 0-basis has a d-decidable copy. We show that the nonlow₂ Δ₂⁰ degrees are 0-homogeneous bounding.

The purpose of this note is to demonstrate that a weak form of club guessing on ω₁ implies the existence of an Aronszajn line with no Countryman suborders. An immediate consequence is that the existence of a five element basis for the uncountable linear orders does not follow from the forcing axiom for ω-proper forcings.

Denef and Loeser defined a map from the Grothendieck ring of sets definable in pseudo-finite fields to the Grothendieck ring of Chow motives, thus enabling to apply any cohomological invariant to these sets. We generalize this to perfect, pseudo algebraically closed fields with pro-cyclic Galois group. In addition, we define some maps between different Grothendieck rings of definable sets which provide additional information, not contained in the associated motive. In particular we infer that the map of Denef-Loeser is not injective.

When restricted to proving Σ_i^q formulas, the quantified propositional proof system G_i^* is closely related to the Σ_i^b theorems of Buss’s theory S₂ⁱ. Namely, G_i^* has polynomial-size proofs of the translations of theorems of S₂ⁱ, and S₂ⁱ proves that G_i^* is sound. However, little is known about G_i^* when proving more complex formulas. In this paper, we prove a witnessing theorem for G_i^* similar in style to the KPT witnessing theorem for T₂ⁱ. This witnessing theorem is then used to show that S₂ⁱ proves G_i^* is sound with respect to Σ_i+1^q formulas. Note that unless the polynomial-time hierarchy collapses S₂ⁱ is the weakest theory in the S₂ hierarchy for which this is true. The witnessing theorem is also used to show that G₁^* is p-equivalent to a quantified version of extended-Frege for prenex formulas. This is followed by a proof that G_i p-simulates G_i+1^* with respect to all quantified propositional formulas. We finish by proving that S₂ can be axiomatized by S₂¹ plus axioms stating that the cut-free version of G₀^* is sound. All together this shows that the connection between G_i^* and S₂ⁱ does not extend to more complex formulas.

In this paper we give a new proof of the characterization of the closed fragment of the provability logic of Heyting’s Arithmetic. We also provide a characterization of the closed fragment of the provability logic of Heyting’s Arithmetic plus Markov’s Principle and Heyting’s Arithmetic plus Primitive Recursive Markov’s Principle.