Almost Representations

TL;DR

This paper demonstrates that approximate group representations into the unitary group of bounded operators on an infinite-dimensional Hilbert space can be closely approximated by genuine representations, with a counterexample showing certain conditions are necessary.

Contribution

It establishes a stability result for approximate representations of amenable groups into operator algebras, extending to separable amenable C*-algebras, and provides a counterexample for the fullness condition.

Findings

Approximate representations can be approximated by true homomorphisms.

A counterexample shows the necessity of the fullness condition.

The results extend to separable amenable C*-algebras.

Abstract

Let $H$ be an infinite dimensional separable Hilbert space, $B(H)$ the $C^*$-algebra of all bounded linear operators on $H,$ $U(B(H))$ the unitary group of $B(H)$ and ${\cal K}\subset B(H)$ the ideal of compact operators. Let $G$ be a countable discrete amenable group. We prove the following: For any $\epsilon>0,$ any finite subset ${\cal F}\subset G,$ and $0<\sigma\le 1,$ there exists $\delta>0,$ finite subsets ${\cal G}\subset G$ and ${\cal S}\subset {\bf C}[G]$ satisfying the following property: For any map $\phi: G\to U(B(H))$ such that $$ \|\phi(fg)-\phi(f)\phi(g)\|<\delta\,\,\,for\,\, all\,\, f,g\in {\cal G}\,\,\, and \,\,\, \|\pi\circ \tilde \phi(x)\|\ge \sigma \|x\|\,\,\, for\,\, all\,\, x\in {\cal S}, $$ there is a group homomorphism $h: G\to U(B(H))$ such that $$ \|\phi(f)-h(f)\|<\epsilon\,\,\, for\,\,\, all\,\,\, f\in {\cal F}, $$ where $\tilde \phi$ is the linear extension of $\phi$ on the group ring ${\bf C}[G]$ and $\pi: B(H)\to B(H)/{\cal K}$ is the quotient map. A counterexample is given that the fullness condition above cannot be removed. We actually prove a more general result for separable amenable $C^*$-algebras.