Homomorphisms with small bound between Fourier algebras

Abstract: Inspired by Kalton and Wood's work on group algebras, we describe almost completely contractive algebra homomorphisms from Fourier algebras into Fourier-Stieltjes algebras (endowed with their canonical operator space structure). We also prove that two locally compact groups are isomorphic if and only if there exists an algebra isomorphism $T$ between the associated Fourier algebras (resp. Fourier-Stieltjes algebras) with completely bounded norm $| T |{cb} < \sqrt {3/2}$ (resp. $ | T |{cb} < \sqrt {5}/2$). We show similar results involving the norm distortion $| T | | T ^{-1} |$ with universal but non-explicit bound. Our results subsume Walter's well-known structural theorems and also Lau's theorem on second conjugate of Fourier algebras.