Similarity degree of Fourier algebras (1511.03423v3)

Abstract: We show that for a locally compact group $G$, amongst a class which contains amenable and small invariant neighbourhood groups, that its Fourier algebra $A(G)$ satisfies a completely bounded version Pisier's similarity property with similarity degree at most $2$. Specifically, any completely bounded homomorphism $\pi: A(G)\to B(H)$ admits an invertible $S$ in $B(H)$ for which $|S||S^{-1}|\leq ||\pi||_{cb}^2$ and $S^{-1}\pi(\cdot)S$ extends to a $$-representation of the $C^$-algebra $C_0(G)$. This significantly improves some results due to Brannan and Samei (J. Funct. Anal. 259, 2010) and Brannan, Daws and Samei (M\"{u}nster J. Math 6, 2013). We also note that $A(G)$ has completely bounded similarity degree $1$ if and only if it is completely isomorphic to an operator algebra if and only if $G$ is finite.