Word-Length Spectral Triples of $(\mathbb{Z}/2\mathbb{Z})\wr {\mathbb{F}}_d$ Are Not Metric

Abstract

Given a countable discrete group equipped with a proper length function, one can construct a natural spectral triple on its reduced group C${}^{\ast }$-algebra. A well-studied question in non-commutative metric geometry is whether the Connes pseudo-metric associated with such a triple recovers the weak${}^{\ast }$-topology on the state space, thereby yielding a compact quantum metric space in the sense of Rieffel. While this metric property is known to hold for several classes of groups - including those of polynomial growth and word-hyperbolic groups - it was widely expected that not every word-length function induces a compact quantum metric space. Despite this, no explicit counterexample has been identified to date. In this note, we provide the first family of counterexamples by proving that for every integer $d\ge 2$ the canonical spectral triple of the Lamplighter group $(\mathbb{Z}/2\mathbb{Z})\wr {\mathbb{F}}_d$, equipped with the word-length function associated with a finite symmetric generating set, fails to be a spectral metric space.

11 pages