A dynamical approach to Schur's Theorem

TL;DR

This paper extends Schur's classical theorem to topological groups using topological entropy, providing a dynamical perspective and new interpretations for groups with certain properties.

Contribution

It introduces a dynamical version of Schur's Theorem for topological groups via topological entropy, connecting group structure with dynamical complexity.

Findings

Topological entropy of endomorphisms relates to the structure of maximal almost periodic groups.

A new dynamical interpretation of Schur's Theorem for Z-groups is established.

Constructs and examples justify the interpretation as a generalization of the classical theorem.

Abstract

A classical result of Schur of 1904 shows that an infinite (discrete) group $E$ with finite central quotient $E/Z(E)$ should have finite derived subgroup $[E,E]$. Schur's Theorem has many important consequences, which have been extensively investigated in the literature. Here we focus on topological Hausdorff groups, which are not necessarily discrete groups, and show a dynamical version of Schur's Theorem via the notion of topological entropy of Adler, Konheim and McAndrew. Their perspective follows some original intuitions of Kolmogov and Sinai from the area of the dynamical systems. Firstly, we investigate the topological entropy of continuous endomorphisms of maximal almost periodic groups whose closed derived subgroup is compact. The properties of these groups were known to Takahashi in 1952 and among them we find the $\mathsf{Z}$-groups of Grosser and Moskowitz. Secondly, we give a new dynamical interpretation of the Schur's Theorem, showing that a $\mathsf{Z}$-group $G$ with continuous endomorphisms of finite topological entropy should have closed derived subgroup $\overline{[G,G]}$ with continuous endomorphisms of finite topological entropy. Finally, we illustrate a series of constructions and examples, which allow us to justify our interpretation of Schur's Theorem as generalization of the original version.