Discrete subsets and convergent sequences in topological groups

摘要

We show that (1) assuming there is no rapid ultrafilter, every countable nondiscrete maximally almost periodic topological group contains a discrete nonclosed subset which is a convergent sequence in some weaker totally bounded group topology, (2) every infinite Abelian totally bounded topological group contains such a discrete subset (in ZFC), and (3) if an extremally disconnected topological group contains such a discrete subset, then there is a selective ultrafilter. (C) 2015 Elsevier B.V. All rights reserved.