Extending Topological Abelian Groups by the Unit Circle

Abstract

A twisted sum in the category of topological Abelian groups is a short exact sequence0→Y→X→Z→0where all maps are assumed to be continuous and open onto their images. The twisted sum splits if it is equivalent to0→Y→Y×Z→Z→0. We study the classSTG𝕋of topological groupsGfor which every twisted sum0→𝕋→X→G→0splits. We prove that this class contains Hausdorff locally precompact groups, sequential direct limits of locally compact groups, and topological groups withℒ∞topologies. We also prove that it is closed by taking open and dense subgroups, quotients by dually embedded subgroups, and coproducts. As means to find further subclasses ofSTG𝕋, we use the connection between extensions of the form0→𝕋→X→G→0and quasi-characters onG, as well as three-space problems for topological groups. The subject is inspired on some concepts known in the framework of topological vector spaces such as the notion of𝒦-space, which were interpreted for topological groups by Cabello.