Extensions of topological Abelian groups and three-space problems

Abstract: A twisted sum in the category of topological abelian groups is a short exact sequence $0\to Y\to X \to Z\to 0$ where all maps are assumed to be continuous and open onto their images. The twisted sum splits if it is equivalent to $0\to Y\to Y \times Z \to Z\to 0$. \par We study the class $\ST$ of topological groups $G$ for which every twisted sum $0\to \T\to X \to G\to 0$ splits. We prove that this class contains locally precompact groups, sequential direct limits of locally compact groups and topological groups with $\mathcal{L}_\infty$ topologies. We also prove that it is closed by taking open and dense subgroups, quotients by dually embedded subgroups and coproducts. As a technique to find further examples of groups in $\ST$ we use the relation of this class with the existence of quasi-characters on $G$ and with 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 of $K$-space, which were interpreted for topological groups by Cabello.