A topological zero-one law and elementary equivalence of finitely generated groups

Abstract

Let G denote the space of finitely generated marked groups. We give equivalent characterizations of closed subspaces S⊆G satisfying the following zero-one law: for any sentence σ in the infinitary logic Lω1,ω, the set of all models of σ in S is either meager or comeager. In particular, we prove that the zero-one law holds for certain natural spaces associated to hyperbolic groups and their generalizations. As an application, we show that generic torsion-free lacunary hyperbolic groups are elementarily equivalent; the same claim holds for lacunary hyperbolic groups without non-trivial finite normal subgroups. Our paper has a substantial expository component. We give streamlined proofs of some known results and survey ideas from topology, logic, and geometric group theory relevant to our work. We also discuss some open problems.