Approximate representability of finite abelian group actions on the Razak-Jacelon algebra

TL;DR

This paper characterizes when finite abelian group actions on certain nuclear C*-algebras are approximately representable, and classifies these actions up to conjugacy and cocycle conjugacy using invariants related to the Razak-Jacelon algebra.

Contribution

It provides a criterion for approximate representability of group actions on monotracial C*-algebras and classifies such actions via characteristic invariants and induced actions on II$_1$ factors.

Findings

Approximate representability characterized by trivial characteristic invariant.

Classification of actions up to conjugacy and cocycle conjugacy based on invariants.

Construction of explicit model actions.

Abstract

Let $A$ be a simple separable nuclear monotracial C$^*$-algebra, and let $\alpha$ be an outer action of a finite abelian group $\Gamma$ on $A$. In this paper, we show that $\alpha\otimes \mathrm{id}_{\mathcal{W}}$ on $A\otimes\mathcal{W}$ is approximately representable if and only if the characteristic invariant of $\tilde{\alpha}$ is trivial, where $\mathcal{W}$ is the Razak-Jacelon algebra and $\tilde{\alpha}$ is the induced action on the injective II$_1$ factor $\pi_{\tau_{A}}(A)^{''}$. As an application of this result, we classify such actions up to conjugacy and cocycle conjugacy. In particular, we show the following: Let $A$ and $B$ be simple separable nuclear monotracial C$^*$-algebras, and let $\alpha$ and $\beta$ be outer actions of a finite abelian group $\Gamma$ on $A$ and $B$, respectively. Assume that the characteristic invariants of $\tilde{\alpha}$ and $\tilde{\beta}$ are trivial. Then $\alpha\otimes \mathrm{id}_{\mathcal{W}}$ and $\beta\otimes \mathrm{id}_{\mathcal{W}}$ are conjugate (resp. cocycle conjugate) if and only if $\tilde{\alpha}$ on $\pi_{\tau_{A}}(A)^{''}$ and $\tilde{\beta}$ on $\pi_{\tau_{B}}(B)^{''}$ are conjugate (resp. cocycle conjugate). We also construct the model actions.