On $T$-characterized subgroups of compact Abelian groups

TL;DR

This paper characterizes $T$-characterized subgroups of infinite compact Abelian groups, linking them to $G_\delta$-subgroups and group topologies, and explores their properties and existence in various group settings.

Contribution

It provides a complete characterization of $T$-characterized subgroups in terms of topological and dual group properties, and constructs examples with specific subgroup properties.

Findings

Closed $T$-characterized subgroups are exactly the $G_\delta$-subgroups with certain topological dual conditions.

All closed subgroups are $T$-characterized if and only if the group is metrizable and connected.

Existence of $T$-characterized subgroups that are not $F_{\sigma}$-subgroups in groups of infinite exponent.

Abstract

We say that a subgroup $H$ of an infinite compact Abelian group $X$ is {\it $T$-characterized} if there is a $T$-sequence $\mathbf{u} =\{u_n \}$ in the dual group of $X$ such that $H=\{x\in X: \; (u_n, x)\to 1 \}$. We show that a closed subgroup $H$ of $X$ is $T$-characterized if and only if $H$ is a $G_\delta$-subgroup of $X$ and the annihilator of $H$ admits a Hausdorff minimally almost periodic group topology. All closed subgroups of an infinite compact Abelian group $X$ are $T$-characterized if and only if $X$ is metrizable and connected. We prove that every compact Abelian group $X$ of infinite exponent has a $T$-characterized subgroup which is not an $F_{\sigma}$-subgroup of $X$ that gives a negative answer to Problem 3.3 in [10].