Directional Differentiability of the Metric Projection in Uniformly Convex and Uniformly Smooth Banach Spaces

Abstract

Let X be a real uniformly convex and uniformly smooth Banach space and C a nonempty closed and convex subset of X. Let Pc from X to C denote the (standard) metric projection operator. In this paper, we define the Gateaux directional differentiability of Pc. We investigate some properties of the Gateaux directional differentiability of Pc. In particular, if C is a closed ball or a closed and convex cone (including proper closed subspaces), then, we give the exact representations of the directional derivatives of Pc. Finally, we define the concept of p-q uniformly convex and uniformly smooth Banach spaces. We will prove that if X is a p-q uniformly convex and uniformly smooth Banach space, then for any nonempty closed and convex subset C of X, Pc is directionally differentiable on the whole space X. The results in this paper can be immediately applied to Hilbert spaces.