Nilpotent group C*-algebras as compact quantum metric spaces

TL;DR

This paper demonstrates that for certain nilpotent group C*-algebras, the metric derived from a length function aligns with the weak-* topology, establishing them as compact quantum metric spaces.

Contribution

It proves that nilpotent group C*-algebras with polynomial growth length functions form compact quantum metric spaces, extending Connes' approach to a broader class of groups.

Findings

Metrics from length functions match weak-* topology on state space

Applies to finitely generated nilpotent-by-finite groups

Validates the quantum metric space structure for these algebras

Abstract

Let $L$ be a length function on a group $G$, and let $M_L$ denote the operator of pointwise multiplication by $L$ on $\ell^2(G)$. Following Connes, $M_L$ can be used as a "Dirac" operator for the reduced group C*-algebra $C_r^*(G)$. It defines a Lipschitz seminorm on $C_r^*(G)$, which defines a metric on the state space of $C_r^*(G)$. We show that for any length function of a strong form of polynomial growth on a discrete group, the topology from this metric coincides with the weak-$*$ topology (a key property for the definition of a "compact quantum metric space"). In particular, this holds for all word-length functions on finitely generated nilpotent-by-finite groups.