SCHUR AND FOURIER MULTIPLIERS OF AN AMENABLE GROUP ACTING ON NON- COMMUTATIVE L-p-SPACES

Consider a completely bounded Fourier multiplier phi of a locally compact group G, and take 1 <= p <= infinity l. One can associate to phi a Schur multiplier on the Schatten classes S-p(L(2)G), as well as a Fourier multiplier on L-p(LG), the non-commutative L-p-space of the group von Neumann algebra of G. We prove that the completely bounded norm of the Schur multiplier is not greater than the completely bounded norm of the Lp- Fourier multiplier. When G is amenable we show that equality holds, extending a result by Neuwirth and Ricard to non-discrete groups. For a discrete group G and in the special case when p not equal 2 is an even integer, we show the following. If there exists a map between Lp(LG) and an ultraproduct of L-p(M) circle times S-p(L(2)G) that intertwines the Fourier multiplier with the Schur multiplier, then G must be amenable. This is an obstruction to extend the Neuwirth-Ricard result to non-amenable groups.