Almost maximally almost-periodic group topologies determined by T-sequences

Abstract

A sequence $\{a_n\}$ in a group $G$ is a {\em $T$-sequence} if there is a Hausdorff group topology $\tau$ on $G$ such that $a_n\stackrel{\tau }{⟶}0$. In this paper, we provide several sufficient conditions for a sequence in an abelian group to be a $T$-sequence, and investigate special sequences in the Prüfer groups $\mathbb{Z}(p^{\infty })$. We show that for $p\ne 2$, there is a Hausdorff group topology $\tau$ on $\mathbb{Z}(p^{\infty })$ that is determined by a $T$-sequence, which is close to being maximally almost-periodic--in other words, the von Neumann radical $\mathbf{n}(\mathbb{Z}(p^{\infty }),\tau )$ is a non-trivial finite subgroup. In particular, $\mathbf{n}(\mathbf{n}(\mathbb{Z}(p^{\infty }),\tau ))⊊\mathbf{n}(\mathbb{Z}(p^{\infty }),\tau )$. We also prove that the direct sum of any infinite family of finite abelian groups admits a group topology determined by a $T$-sequence with non-trivial finite von Neumann radical.

v2 - accepted (discussion on non-abelian case is removed, replaced by new results on direct sums of finite abelian groups)

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