A Localized Besicovitch-Federer Projection Theorem

TL;DR

This paper extends the classical projection theorem by establishing a localized version, providing a new rectifiability criterion based on the behavior of Hausdorff measure under Lipschitz perturbations.

Contribution

It introduces a localized Besicovitch-Federer projection theorem and a rectifiability criterion based on measure semi-continuity under Lipschitz maps.

Findings

Hausdorff measure is lower semi-continuous under Lipschitz perturbations for rectifiable sets.

A converse to the projection theorem is established, characterizing rectifiability.

The results connect measure behavior with geometric structure of sets.

Abstract

The classical Besicovitch-Federer projection theorem implies that the d-dimensional Hausdorff measure of a set in Euclidean space with non-negligible d-unrectifiable part will strictly decrease under orthogonal projection onto almost every d-dimensional linear subspace. In fact, there exist maps which are arbitrarily close to the identity in the C^0 topology which have the same property. A converse holds as well, yielding the following rectifiability criterion: under mild assumptions, a set is rectifiable if and only if its Hausdorff measure is lower semi-continuous under bounded Lipschitz perturbations.