Here “group” means abelian group. Compact connected groups contain δ-subgroups, that is, compact totally disconnected subgroups with torus quotients, which are essential ingredients in the important Resolution Theorem, a description of compact groups. Dually, full free subgroups of discrete torsion-free groups of finite rank are studied in order to obtain a comprehensive picture of the abundance of δ-subgroups of a protorus. Associated concepts are also considered.
Lewis, W., Loth, P., & Mader, A. (2020). Free subgroups with torsion quotients and profinite subgroups with torus quotients. Rendiconti del Seminario Matematico della Universita di Padova, 144, 177-195. Doi: 10.4171/RSMUP/64