Schur and Fourier multipliers of an amenable group acting on non-commutative Lp-spaces

TL;DR

This paper investigates the relationship between Fourier and Schur multipliers on non-commutative Lp-spaces associated with locally compact groups, establishing norm inequalities and conditions for amenability.

Contribution

It extends known results by proving norm equality for amenable groups and identifying obstructions for non-amenable groups in the context of non-commutative Lp-spaces.

Findings

Norm of Schur multiplier is bounded by the Lp-Fourier multiplier norm.

Equality of norms holds for amenable groups.

Obstruction to extending results to non-amenable groups identified.

Abstract

Consider a completely bounded Fourier multiplier phi of a locally compact group G, and take 1 <= p <= infinity. One can associate to phi a Schur multiplier on the Schatten classes S_p(L^2 G), as well as a Fourier multiplier on Lp(LG), the non-commutative Lp-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 > 2 is an even integer, we show the following. If there exists a map between Lp(LG) and an ultraproduct of Lp(M) \otimes 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.