Norming and dense sets of extreme points of the unit ball in spaces of bounded Lipschitz functions

On spaces of finite signed Borel measures on a metric space one has introduced the Fortet-Mourier and Dudley norms, by embedding the measures into the dual space of the Banach space of bounded Lipschitz functions, equipped with different - but equivalent - norms: the FM -norm and the BL -norm, respectively. The norm of such a measure is then obtained by maximising the value of the measure when applied by integration to extremal functions of the unit ball. We introduce Lipschitz extension operators, essentially based on those defined by McShane, and investigate their properties. A remarkable one is that non -trivial extreme points are mapped to non -trivial extreme points of FM- and BL -norm unit balls. Using these extension operators, we define suitable 'small' subsets of extremal functions that are weak -star dense in the full set of extreme points of the unit ball, for any underlying metric space. For connected metric spaces, we additionally find a larger set of extremal functions for the BL -norm, similar to such a set that was defined previously by J. Johnson for the FM -norm. This set is then also weak -star dense in the extremal functions. These results may open an avenue to obtaining computational approaches for the Dudley norm on signed Borel measures. (c) 2024 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons .org /licenses /by /4 .0/).