Locally compact modules over abelian groups and compactly generated metabelian groups

Abstract

We perform a general study of the structure of locally compact modules over compactly generated abelian groups. We obtain a devissage result for such modules of the form "compact-by-sheer-by-discrete", and then study more specifically the sheer part. The main typical example of a sheer module is a polycontractable module, i.e., a finite direct product of modules, each of which is contracted by some group element. We show that every sheer module has a "large" polycontractable submodule, in a suitable sense. We apply this to the study of compactly generated metabelian groups. For instance, we prove that they always have a maximal compact normal subgroup, and we extend the Bieri-Strebel characterization of compactly presentable metabelian groups from the discrete case to this more general setting.