The stationary set splitting game

The stationary set splitting game is a game of perfect information of length ω₁ between two players, unsplit and split, in which unsplit chooses stationarily many countable ordinals and split tries to continuously divide them into two stationary pieces. We show that it is possible in ZFC to force a winning strategy for either player, or for neither. This gives a new counterexample to Σ²₂ maximality with a predicate for the nonstationary ideal on ω₁, and an example of a consistently undetermined game of length ω₁ with payoff de.nable in the second‐order monadic logic of order. We also show that the determinacy of the game is consistent with Martin's Axiom but not Martin's Maximum. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On definability of addition in Lie algebras

An answer is obtained to I. V. Arzhantsev’s question on definability of the structure of a semisimple Lie algebra by the multiplicative groupoid of L.     

<Emphasis Type="Italic">G</Emphasis>-linear sets and torsion points in definably compact groups

Let G be a definably compact group in an o-minimal expansion of a real closed field. We prove that if dim(G\X) < dim G for some definable then X contains a torsion point of G. Along the way we develop a general theory for the so-called G-linear sets, and investigate definable sets which contain abstract subgroups of G. M. Otero was Partially supported by GEOR MTM2005-02568 and Grupos UCM 910444.     

The logic of integration

We develop a model theoretic framework for studying algebraic structures equipped with a measure. The real line is used as a value space and its usual arithmetical operations as connectives. Integration is used as a quantifier. We extend some basic results of pure model theory to this context and characterize measurable sets in terms of zero-sets of formulas.      

On the definability of verbal subgroups

We show that if G is a group of finite Morley rank, then the verbal subgroup <w(G)> is of finite width, where w is a concise word. As a byproduct, we show that if G is any abelian-by-finite group, then G ⁿ =<x ⁿ (G)> is definable. Received: 15 March 1996 / Published online: 18 July 2001     

Lattice definability of semisimple jordan algebras

In this paper we prove that two finite-dimensional linear Jordan algebras over an algebraically closed field with isothermic lattices of subalgebras must bi isothemic if one of them is semisimple non-isothermic to F. As a corollary of this fact, we prove that two unital Jordan algebras with isothermic lattices of subalgebras must have the same dimension when the ground field is algebraically closed of characteristic zero. Through this work we see similar results in more general fields for particular families of simple Jordan algebras.     

The first order definability of graphs: Upper bounds for quantifier depth

Let D(G) denote the minimum quantifier depth of a first order sentence that defines a graph G up to isomorphism in terms of the adjacency and equality relations. Call two vertices of G similar if they have the same adjacency to any other vertex and denote the maximum number of pairwise similar vertices in G by σ(G). We prove that σ(G)+1?D(G)?max{σ(G)+2,(n+5)/2}, where n denotes the number of vertices of G. In particular, D(G)?(n+5)/2 for every G with no transposition in the automorphism group. If G is connected and has maximum degree d, we prove that for a constant . A linear lower bound for graphs of maximum degree 3 with no transposition in the automorphism group follows from an earlier result by Cai, Fürer, and Immerman [An optimal lower bound on the number of variables for graph identification, Combinatorica 12(4) (1992) 389-410]. Our upper bounds for D(G) hold true even if we allow only definitions with at most one alternation in any sequence of nested quantifiers.In passing we establish an upper bound for a related number D(G,G^′), the minimum quantifier depth of a first order sentence which is true on exactly one of graphs G and G^′. If G and G^′ are non-isomorphic and both have n vertices, then D(G,G^′)?(n+3)/2. This bound is tight up to an additive constant of 1. If we additionally require that a sentence distinguishing G and G^′ is existential, we prove only a slightly weaker bound D(G,G^′)?(n+5)/2.     

Functoriality and uniformity in Hrushovski's groupoid-cover correspondence

The correspondence between definable connected groupoids in a theory T and internal generalised imaginary sorts of T, established by Hrushovski in [“Groupoids, imaginaries and internal covers,” Turkish Journal of Mathematics, 2012], is here extended in two ways: First, it is shown that the correspondence is in fact an equivalence of categories, with respect to appropriate notions of morphism. Secondly, the equivalence of categories is shown to vary uniformly in definable families, with respect to an appropriate relativisation of these categories. Some elaborations on Hrushovki's original constructions are also included.     

Witnessing numbers of Shelah Cardinals

We consider minimal ranks of extenders associated with Shelah cardinals by introducing witnessing numbers. Using these numbers we shall investigate effects of Shelah cardinals above themselves. MSC: 03E55.     

The question of the definability of Abelian groups by the centers of their endomorphism rings

We obtain necessary and sufficient conditions for an Abelian group in the class of completely decomposable torsion-free Abelian groups of a chosen finite rank to be definable in this class by the center of the endomorphism ring of the group.