Relative norm averaging property and selflessness of almost periodic actions

Let $D$ be a unital separable $C^*$-algebra with a faithful state $σ$ and $G$ be a discrete group. We introduce a property called relative norm averaging property (RNA) for almost periodic $σ$-preserving actions by $G$ on $(D,σ)$. We prove that if an almost periodic $σ$-preserving action $α$ satisfies that $\overline{α(G)} \subseteq \text{Aut}(D,σ)$ is connected and nontrivial, then $α$ satisfies (RNA). As an application, we prove that if $G$ has property PHP of Ozawa and $α:G\curvearrowright (D,σ)$ is an ergodic (i.e. $D^G=\mathbb{C}$) almost periodic $σ$-preserving action with $\overline{α(G)}$ being connected, then $D\rtimes_r G$ is selfless. This includes certain paradoxical isometric actions of $\mathbb{F}_2$ on the sphere. In non-ergodic case, we prove that the relative norm averaging property implies that $D\rtimes_r G$ satisfies the $D^G$-valued selfless, if $G$ satisfies PHP.

Authors

Publication Details

Journal
arXiv (Cornell University)
Published
2026-10-26
DOI
https://doi.org/10.48550/arxiv.2610.35644
Primary Topic
Advanced Operator Algebra Research
Type
preprint
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preprint

Relative norm averaging property and selflessness of almost periodic actions

Ryo Toyota
arXiv (Cornell University)
Advanced Operator Algebra Research
preprint

Relative norm averaging property and selflessness of almost periodic actions

Ryo Toyota
preprint en

Abstract

Let $D$ be a unital separable $C^*$-algebra with a faithful state $σ$ and $G$ be a discrete group. We introduce a property called relative norm averaging property (RNA) for almost periodic $σ$-preserving actions by $G$ on $(D,σ)$. We prove that if an almost periodic $σ$-preserving action $α$ satisfies that $\overline{α(G)} \subseteq \text{Aut}(D,σ)$ is connected and nontrivial, then $α$ satisfies (RNA). As an application, we prove that if $G$ has property PHP of Ozawa and $α:G\curvearrowright (D,σ)$ is an ergodic (i.e. $D^G=\mathbb{C}$) almost periodic $σ$-preserving action with $\overline{α(G)}$ being connected, then $D\rtimes_r G$ is selfless. This includes certain paradoxical isometric actions of $\mathbb{F}_2$ on the sphere. In non-ergodic case, we prove that the relative norm averaging property implies that $D\rtimes_r G$ satisfies the $D^G$-valued selfless, if $G$ satisfies PHP.

arXiv (Cornell University)
Advanced Operator Algebra Research
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