Smooth and strongly smooth points in symmetric spaces of measurable operators

Abstract

We investigate the relationships between smooth and strongly smooth points of the unit ball of an order continuous symmetric function space E, and of the unit ball of the space of τ-measurable operators $${E(\mathcal{M},\tau)}$$ associated to a semifinite von Neumann algebra $${(\mathcal{M}, \tau)}$$ . We prove that x is a smooth point of the unit ball in $${E(\mathcal{M}, \tau)}$$ if and only if the decreasing rearrangement μ(x) of the operator x is a smooth point of the unit ball in E, and either μ(∞; f) = 0, for the function $${f\in S_{E^{\times}}}$$ supporting μ(x), or s(x *) = 1. Under the assumption that the trace τ on $${\mathcal{M}}$$ is σ-finite, we show that x is strongly smooth point of the unit ball in $${E(\mathcal{M}, \tau)}$$ if and only if its decreasing rearrangement μ(x) is a strongly smooth point of the unit ball in E. Consequently, for a symmetric function space E, we obtain corresponding relations between smoothness or strong smoothness of the function f and its decreasing rearrangement μ(f). Finally, under suitable assumptions, we state results relating the global properties such as smoothness and Frechet smoothness of the spaces E and $${E(\mathcal{M},\tau)}$$ .