On model-theoretic connected components in some group extensions

TL;DR

This paper investigates model-theoretic connected components in certain group extensions defined by 2-cocycles, providing characterizations and new examples where the smallest type-definable subgroup differs from the smallest invariant subgroup.

Contribution

It offers a characterization of connected components in group extensions via 2-cocycles and introduces new examples, including the universal cover of SL2(R), where these components differ.

Findings

Characterization of connected components in group extensions using 2-cocycles.

Identification of new classes of groups with differing type-definable and invariant subgroups.

First example of a group with this property by Conversano and Pillay, extended to SL2(Z).

Abstract

We analyze model-theoretic connected components in extensions of a given group by abelian groups which are defined by means of 2-cocycles with finite image. We characterize, in terms of these 2-cocycles, when the smallest type-definable subgroup of the corresponding extension differs from the smallest invariant subgroup. In some situations, we also describe the quotient of these two connected components. Using our general results about extensions of groups together with Matsumoto-Moore theory or various quasi-characters considered in bounded cohomology, we obtain new classes of examples of groups whose smallest type-definable subgroup of bounded index differs from the smallest invariant subgroup of bounded index. This includes the first known example of a group with this property found by Conversano and Pillay, namely the universal cover of SL2(R) (intepreted in a monster model), as well as various examples of different nature, e.g. some central extensions of free groups or of fundamental groups of closed orientable surfaces. We also obtain a variant of the example of Conversano and Pillay for SL2(Z) instead of SL2(R), which (as most of our examples) was not accessible by the previously known methods.