Spaces of operator-valued functions measurable with respect to the strong operator topology

Abstract

Let $X$ and $Y$ be Banach spaces and $(Ω,Σ,μ)$ a finite measure space. In this note we introduce the space $L^p[μ;L(X,Y)]$ consisting of all (equivalence classes of) functions $Φ:Ω\mapsto L(X,Y)$ such that $ω\mapsto Φ(ω)x$ is strongly $μ$-measurable for all $x\in X$ and $ω\mapsto Φ(ω)f(ω)$ belongs to $L^1(μ;Y)$ for all $f\in L^{p'}(μ;X)$, $1/p+1/p'=1$. We show that functions in $L^p[μ;Ł(X,Y)]$ define operator-valued measures with bounded $p$-variation and use these spaces to obtain an isometric characterization of the space of all $L(X,Y)$-valued multipliers acting boundedly from $L^p(μ;X)$ into $L^q(μ;Y)$, $1\le q< p<\infty$.