Lowering topological entropy and asymptotic $h$-expansiveness for amenable group actions

Abstract

Let $G$ be a countably infinite discrete amenable group acting continuously on a compact metric space $X$. We study the problem of lowering topological entropy over subsets of $X$ and its connections with asymptotic $h$-expansiveness. Let $\{F_n\}$ be a tempered Følner sequence such that $e_G\in F_1\subseteq F_2\subseteq\cdots$ and $|F_n|/\log n\to\infty$. If $(X,G)$ has finite topological entropy, then, for every $h\in[0,h_{\mathrm{top}}(X,G)]$,there exists a non-empty compact set $K_h\subseteq X$ whose topological entropy, Bowen topological entropy and packing topological entropy along $\{F_n\}$ are all equal to $h$. We next consider D-hereditary lowerability, which requires every non-empty analytic (Souslin) subset to be lowerable with respect to Bowen topological entropy. We prove that every subshift over a finite alphabet is D-hereditarily lowerable along an increasing Følner sequence. Combining principal quasi-symbolic extensions with a dimensional entropy inequality for factor maps, we further prove that every asymptotically $h$-expansive $G$-system is D-hereditarily lowerable along an increasing Følner sequence. More generally, suppose that $(X,G)$ has finite topological entropy and tail entropy $a$, and let $\{F_n\}$ be an increasing Følner sequence. If an analytic set $K\subseteq X$ has Bowen topological entropy $H>a$ along $\{F_n\}$, then, for every $h\in[0,H-a]$, there exists a non-empty set $K_h\subseteq K$ such that $h\le h_{\mathrm{top}}^B(K_h,{F_n})\le h+a$. Finally, for tempered increasing Følner sequences satisfying the above growth condition, we prove that asymptotic $h$-expansiveness is equivalent to hereditary uniform lowerability.