Locally Lipschitz Functions and Bornological Derivatives

Abstract

We study the relationships between Gateaux, weak Hadamard and Frechet differentiability and their bornologies for Lipschitz and for convex functions. In particular, Frechet and weak Hadamard differentiabily coincide for all Lipschitz functions if and only if the space is reflexive (an earlier paper of the first two authors shows that these two notions of differentiability coincide for continuous convex functions if and only if the space does not contain a copy of $\ell_1$). We also examine when Gateaux and weak Hadamard differentiability coincide for continuous convex functions. For instance, spaces with the Dunford-Pettis (Schur) property can be characterized by the coincidence of Gateaux and weak Hadamard (Frechet) differentiabilty for dual norms.