Projections, multipliers and decomposable maps on noncommutative $\mathrm{L}^p$-spaces

TL;DR

This paper develops a noncommutative analogue of the absolute value for operators on noncommutative L^p-spaces, proving the equivalence of regular and decomposable norms and analyzing classes of multipliers.

Contribution

It introduces a noncommutative absolute value, proves the equality of regular and decomposable norms, and characterizes regular multipliers on noncommutative L^p-spaces.

Findings

Regular and decomposable norms are identical.

Bounded projections onto multiplier subspaces preserve positivity.

Existence of Fourier multipliers not approximable by regular operators.

Abstract

We introduce a noncommutative analogue of the absolute value of a regular operator acting on a noncommutative $\mathrm{L}^p$-space. We equally prove that two classical operator norms, the regular norm and the decomposable norm are identical. We also describe precisely the regular norm of several classes of regular multipliers. This includes Schur multipliers and Fourier multipliers on some unimodular locally compact groups which can be approximated by discrete groups in various senses. A main ingredient is to show the existence of a bounded projection from the space of completely bounded $\mathrm{L}^p$ operators onto the subspace of Schur or Fourier multipliers, preserving complete positivity. On the other hand, we show the existence of bounded Fourier multipliers which cannot be approximated by regular operators, on large classes of locally compact groups, including all infinite abelian locally compact groups. We finish by introducing a general procedure for proving positive results on selfadjoint contractively decomposable Fourier multipliers, beyond the amenable case.