It is shown that for every (Turing) degree 0 <a <0' there is a minimal degree m <0' such that a∨ m=0' (and therefore a∧ m=0).

We present and study the category of formal topologies and some of its variants. Two main results are proven. The first is that, for any inductively generated formal cover, there exists a formal topology whose cover extends in the minimal way the given one. This result is obtained by enhancing the method for the inductive generation of the cover relation by adding a coinductive generation of the positivity predicate. Categorically, this result can be rephrased by saying that inductively generated formal topologies are coreflective into inductively generated formal covers. The second result is that unary formal covers are exponentiable in the category of inductively generated formal covers and hence, thanks to the coreflection, unary formal topologies are exponentiable in the category of inductively generated formal topologies. From a localic point of view the exponentiability of unary formal topologies means that algebraic dcpos are exponentiable in the category of open locales. But, the coreflection theorem states that open locales are coreflective in locales and hence, as a consequence of well-known impredicative results on exponentiable locales, it allows to prove that locally compact open locales are exponentiable in the category of open locales.

We develop an elimination theory for addition and the Frobenius map over rings of polynomials. As a consequence we show that if F is a countable, recursive and perfect field of positive characteristic p, with decidable theory, then the structure of addition, the Frobenius map x→ x^p and the property ‘x∈ F', over the ring of polynomials F[T], has a decidable theory.

In this paper we re-examine the semantics of classical higher-order logic with the purpose of clarifying the role of extensionality. To reach this goal, we distinguish nine classes of higher-order models with respect to various combinations of Boolean extensionality and three forms of functional extensionality. Furthermore, we develop a methodology of abstract consistency methods (by providing the necessary model existence theorems) needed to analyze completeness of (machine-oriented) higher-order calculi with respect to these model classes.

In Reverse Mathematics, the axiom system DNR, asserting the existence of diagonally non-recursive functions, is strictly weaker than WWKL₀ (weak weak König's Lemma).

We use finite model theory (in particular, the method of FM-truth definitions, introduced in [MM01] and developed in [K04], and a normal form result akin to those of [Ste93] and [G97]) to prove:

Let m ≥ 2. Then:

(A) If there exists k such that NP ⊆ σ_mTIME(n^k) ∩ Π_mTIME(n^k), then for every r there exists k_r such that P^NP[nr] ⊆ σ_mTIME(n^k_r) ∩ Π_mTIME(n^k_r);

(B) If there exists a superpolynomial time-constructible function f such that NTIME(f) ⊆ Σ^p_m ∪ Π^p_m, then additionally P^NP[nr] ⊈ Σ^p_m ∪ Π^p_m.

This strengthens a result by Mocas [M96] that for any r, P^NP[nr] ⊈ NEXP.

In addition, we use FM-truth definitions to give a simple sufficient condition for the σ¹₁ arity hierarchy to be strict over finite models.

A set X is prime bounding if for every complete atomic decidable (CAD) theory T there is a prime model 𝔞 of T decidable in X. It is easy to see that X = 0' is prime bounding. Denisov claimed that every X ≤_T 0' is not prime bounding, but we discovered this to be incorrect. Here we give the correct characterization that the prime bounding sets X ≤_T 0' are exactly the sets which are not low₂. Recall that X is low₂ if X''≤_T 0''. To prove that a low₂ set X is not prime bounding we use a 0'-computable listing of the array of sets { Y: Y≤_T X } to build a CAD theory T which diagonalizes against all potential X-decidable prime models 𝔞 of T. To prove that any non-low₂ X is indeed prime bounding, we fix a function f ≤_T X that is not dominated by a certain 0'-computable function that picks out generators of principal types. Given a CAD theory T, we use f to eventually find, for every formula φ(\overline{x}) consistent with T, a principal type which contains it, and hence to build an X-decidable prime model of T. We prove the prime bounding property equivalent to several other combinatorial properties, including some related to the limitwise monotonic functions which have been introduced elsewhere in computable model theory.

We show that Mitchell order on downward closed extenders below rank-to-rank type is wellfounded.

We prove that there is no sw-complete c.e. real, negatively answering a question in [6].

We consider two theories of “bad fields” constructed by B.Poizat using Hrushovski's amalgamation and show that these theories have natural models representable as the field of complex numbers with a distinguished subset given as a union of countably many real analytic curves. One of the two examples is based on the complex exponentiation and the proof assumes Schanuel's conjecture.

We explore some topics in the model theory of sheaves of modules. First we describe the formal language that we use. Then we present some examples of sheaves obtained from quivers. These, and other examples, will serve as illustrations and as counterexamples. Then we investigate the notion of strong minimality from model theory to see what it means in this context. We also look briefly at the relation between global, local and pointwise versions of properties related to acyclicity.

We show that if d∈ GH₁ then 𝒟(≤ d) has the complementation property, i.e., for all a < d there is some b <d such that a∧ b=0 and a∨ b=d.

In a simple theory with elimination of finitary hyperimaginaries if tp(a) is real and analysable over a definable set Q, then there exists a finite sequence (a_i | i≤ n^*)⊆ dcl^eq(a) with a_n^*=a such that for every i≤ n^*, if p_i=tp(a_i/{a_j | j<i}) then Aut(p_i/Q) is type-definable with its action on p_i^𝒞. A unidimensional simple theory eliminates the quantifier ∃^∞ and either interprets (in 𝒞^eq) an infinite type-definable group or has the property that ACL(Q)=𝒞 for every infinite definable set Q.

We study the expansion of stable structures by adding predicates for arbitrary subsets. Generalizing work of Poizat-Bouscaren on the one hand and Baldwin-Benedikt-Casanovas-Ziegler on the other we provide a sufficient condition (Theorem 4.7) for such an expansion to be stable. This generalization weakens the original definitions in two ways: dealing with arbitrary subsets rather than just submodels and removing the ‘small' or ‘belles paires' hypothesis. We use this generalization to characterize in terms of pairs, the ‘triviality' of the geometry on a strongly minimal set (Theorem 2.5). Call a set A benign if any type over A in the expanded language is determined by its restriction to the base language. We characterize the notion of benign as a kind of local homogenity (Theorem 1.7). Answering a question of [8] we characterize the property that M has the finite cover property over A (Theorem 3.9).

The authors show, by means of a finitary version □^fin_{λ, D} of the combinatorial principle □^b*_λ of [7], the consistency of the failure, relative to the consistency of supercompact cardinals, of the following: for all regular filters D on a cardinal λ, if M_i and N_i are elementarily equivalent models of a language of size ≤ λ, then the second player has a winning strategy in the Ehrenfeucht-Fraïssé game of length λ⁺ on ∏_i M_i/D and ∏_i N_i/D. If in addition 2^λ=λ⁺ and i <λ implies |M_i|+|N_i|≤ λ⁺ this means that the ultrapowers are isomorphic. This settles negatively conjecture 18 in [2].

Working in the theory “ZF + There is a nontrivial elementary embedding j: V → V ”, we show that a final segment of cardinals satisfies certain square bracket finite and infinite exponent partition relations. As a corollary to this, we show that this final segment is composed of Jonsson cardinals. We then show how to force and bring this situation down to small alephs. A prototypical result is the construction of a model for ZF in which every cardinal μ ≥ ℵ₂ satisfies the square bracket infinite exponent partition relation μ → [μ]^ω_ℵ2. We conclude with a discussion of some consistency questions concerning different versions of the axiom asserting the existence of a nontrivial elementary embedding j : V → V. By virtue of Kunen's celebrated inconsistency result, we use only a restricted amount of the Axiom of Choice.