On uniformly convex functions

TL;DR

This paper investigates discrete uniform convexity in functions, showing it is preserved under convex envelope formation and applying this to Banach space renorming, coercivity, and compactness measures.

Contribution

It introduces methods to transfer discrete uniform convexity to convex envelopes and applies these to Banach space renorming and compactness characterization.

Findings

Convex envelope inherits discrete uniform convexity.

Provides a sharp estimate for the distance to Lipschitz convex functions.

Establishes equivalence of measures for non-super weakly compact sets.

Abstract

Non-convex functions that yet satisfy a condition of uniform convexity for non-close points can arise in discrete constructions. We prove that this sort of discrete uniform convexity is inherited by the convex envelope, which is the key to obtain other remarkable properties such as the coercivity. Our techniques allow to retrieve Enflo's uniformly convex renorming of super-reflexive Banach spaces as the regularization of a raw function built from trees. Among other applications, we provide a sharp estimation of the distance of a given function to the set of differences of Lipschitz convex functions. Finally, we prove the equivalence of several natural fashions to quantify the non-super weakly compactness of a subset of a Banach space.