Sheaves and concepts : a model-theoretic interpretation of Grothendieck topoi
Simplicity of algebras requires to investigate almost all operations
Simplicity simplified.
Singular properties of Morley rank
Small profinite m-stable groups
A small profinite m-stable group has an open abelian subgroup of finite ℳ-rank and finite exponent.
Solution to a problem of Gandy's
Some automorphisms of natural numbers in the alternative set theory
Some categories of models.
Some combinatorial principles defined in terms of elementary submodels
We give an equivalent, but simpler formulation of the axiom SEP, which was introduced in [9] in order to capture some of the combinatorial behaviour of models of set theory obtained by adding Cohen reals to a model of CH. Our formulation shows that many of the consequences of the weak Freese-Nation property of 𝒫(ω) studied in [6] already follow from SEP. We show that it is consistent that SEP holds while 𝒫(ω) fails to have the (ℵ₁,ℵ ₀)-ideal property introduced in [2]. This answers a question...
Some consequences of the normality of the space of models
Some decidable congruences of free monoids
Let be the free monoid over a finite alphabet . We prove that a congruence of generated by a finite number of pairs , where and , is always decidable.
Some decidable theories with finitely many covers which are decidable and algorithmically found
In any recursive algebraic language, I find an interval of the lattice of equational theories, every element of which has finitely many covers. With every finite set of equations of this language, an equational theory of this interval is associated, which is decidable with decidable covers that can be algorithmically found. If the language is finite, both this theory and its covers are finitely based. Also, for every finite language and for every natural number n, I construct a finitely based decidable...
Some embedding theorems.
Some model theory for monotone quantifiers.
Some model theory of simple algebraic groups over algebraically closed fields
Some model theory of SL(2,ℝ)
We study the action of G = SL(2,ℝ), viewed as a group definable in the structure M = (ℝ,+,×), on its type space . We identify a minimal closed G-flow I and an idempotent r ∈ I (with respect to the Ellis semigroup structure * on ). We also show that the “Ellis group” (r*I,*) is nontrivial, in fact it is the group with two elements, yielding a negative answer to a question of Newelski.
Some (non-)elimination results for curves in geometric structures
We show that the first order structure whose underlying universe is ℂ and whose basic relations are all algebraic subsets of ℂ² does not have quantifier elimination. Since an algebraic subset of ℂ² is either of dimension ≤ 1 or has a complement of dimension ≤ 1, one can restate the former result as a failure of quantifier elimination for planar complex algebraic curves. We then prove that removing the planarity hypothesis suffices to recover quantifier elimination: the structure with the universe...
Some observations on Uniform Reduction for properties invariant on the range of definable relations
Some preservation theorems
Some problem in elementary arithmetics