The set of limits of Riemann integral sums of a multifunction and Banach space geometry

Abstract

Let $X$ be a Banach space and $F:[0,1]\to 2^X\setminus \{\varnothing \}$ be a bounded multifunction. We study properties of the set $I(F)$ of limits in Hausdorff distance of Riemann integral sums of $F$. The main results are: (1) $I(F)$ is convex in the case of finite-dimensional $X$; (2) $I(F)=I(\mathrm{conv}F)$ in B-convex spaces or for compact-valued multifunctions; (3) $I(F)$ consists of convex sets whenever $X$ is B-convex; (4) $I(F)$ is star-shaped (thus non-empty) for compact-valued multifunctions in separable spaces. (5) For each infinite-dimensional Banach space there is a bounded multifunction with empty $I(F)$.

12 pages