Convergence of Peter--Weyl Truncations of Compact Quantum Groups

TL;DR

This paper studies how quantum metric spaces associated with coamenable compact quantum groups can be approximated by finite-dimensional truncations using Peter--Weyl decompositions, with convergence in a quantum Gromov--Hausdorff sense.

Contribution

It establishes the convergence of a net of truncated quantum metric spaces to the original space using bi-invariant Lip-norms and operator system projections.

Findings

Net of projections converges strongly to identity on L^2(G)

Quantum metric spaces from projections converge in Gromov--Hausdorff distance

Constructs approximate inverses of compression maps in Lip-norm

Abstract

We consider a coamenable compact quantum group $\mathbb{G}$ as a compact quantum metric space if its function algebra $\mathrm{C}(\mathbb{G})$ is equipped with a Lip-norm. By using a projection $P$ onto direct summands of the Peter--Weyl decomposition, the $\mathrm{C}^*$-algebra $\mathrm{C}(\mathbb{G})$ can be compressed to an operator system $P\mathrm{C}(\mathbb{G})P$, and there are induced left and right coactions on this operator system. Assuming that the Lip-norm on $\mathrm{C}(\mathbb{G})$ is bi-invariant in the sense of Li, there is an induced bi-invariant Lip-norm on the operator system $P\mathrm{C}(\mathbb{G})P$ turning it into a compact quantum metric space. Given an appropriate net of such projections which converges strongly to the identity map on the Hilbert space $\mathrm{L}^2(\mathbb{G})$, we obtain a net of compact quantum metric spaces. We prove convergence of such nets in terms of Kerr's complete Gromov--Hausdorff distance. An important tool is the choice of an appropriate state whose induced slice map gives an approximate inverse of the compression map $\mathrm{C}(\mathbb{G}) \ni a \mapsto PaP$ in Lip-norm.