Sequences from zero entropy noncommutative toral automorphisms and Sarnak Conjecture

Abstract

In this paper we study $C^*$-algebra version of Sarnak Conjecture for noncommutative toral automorphisms. Let $A_\Theta$ be a noncommutative torus and $\alpha_\Theta$ be the noncommutative toral automorphism arising from a matrix $S\in GL(d,\mathbb{Z})$. We show that if the Voiculescu-Brown entropy of $\alpha_{\Theta}$ is zero, then the sequence $\{\rho(\alpha_{\Theta}^nu)\}_{n\in \mathbb{Z}}$ is a sum of a nilsequence and a zero-density-sequence, where $u\in A_\Theta$ and $\rho$ is any state on $A_\Theta$. Then by a result of Green and Tao, this sequence is linearly disjoint from the M\"obius function.