A finitary criterion for selfless tracial C*-algebras

Abstract

We study the class of selfless C*-probability spaces introduced by Robert. It is known that a selfless tracial algebra has strict comparison and a unique trace. We prove that for separable tracial C*-algebras, selflessness is equivalent to approximate selflessness, a finitary condition: for every finite set $F$, every $N \geq 1$ and $\varepsilon > 0$ there exists a unitary $u$ with $|τ(u^k)| < \varepsilon$ ($1 \leq |k| \leq N$) and $|τ(w)| < \varepsilon$ for all alternating words $w$ of length $\leq N$ built from centered elements of $F$ and powers $u^n$ ($|n| \leq N$). The equivalence is established using a diagonalisation argument in the tracial ultrapower. As an application, we give a concise proof that countable groups with a topologically-free extreme boundary are C*-selfless. We also discuss the relation to nuclearity and $\mathcal{Z}$-stability.