Duality for ideals of Lipschitz maps

Abstract

We develop a systematic approach to the study of duality for ideals of Lipschitz maps from a metric space to a Banach space, inspired by the classical theory that relates ideals of operators and tensor norms for Banach spaces, by using the Lipschitz tensor products previously introduced by the same authors. We first study spaces of Lipschitz maps, from a metric space to a dual Banach space, that can be represented canonically as the dual of a Lipschitz tensor product endowed with a Lipschitz cross-norm. We show that several known examples of ideals of Lipschitz maps (Lipschitz maps, Lipschitz $p$-summing maps and maps admitting a Lipschitz factorization through a subset of an $L_p$ space) admit such a representation, and more generally we characterize when a space of Lipschitz maps from a metric space to a dual Banach space is in canonical duality with a Lipschitz cross-norm. Furthermore, we give conditions on the Lipschitz cross-norm that are almost equivalent to the space of Lipschitz maps having an ideal property. We introduce a concept of operators which are approximable with respect to one of these ideals of Lipschitz maps, and identify them in terms of tensor-product notions. Finally, we also prove a Lipschitz version of the representation theorem for maximal operator ideals. This allows us to relate Lipschitz cross-norms to ideals of Lipschitz maps taking values in general Banach spaces, and not just dual spaces.