A new class of $({\cal H}^k,1)$-rectifiable subsets of metric spaces

TL;DR

This paper introduces a new class of rectifiable sets in metric spaces using continuously metric differentiable curves of degree k, extending rectifiability concepts to include non-horizontal curves in Carnot-Carathéodory spaces.

Contribution

It defines $({\cal H}^k,1)$-rectifiable sets via a novel class of curves and proves a density result, generalizing Euclidean rectifiability to more complex metric spaces.

Findings

Introduces continuously metric differentiable curves of degree k.

Establishes a density theorem for $({\cal H}^k,1)$-rectifiable sets.

Provides an integral formula for Hausdorff measures along these curves.

Abstract

The main motivation of this paper arises from the study of Carnot-Carath\'eodory spaces, where the class of 1-rectifiable sets does not contain smooth non-horizontal curves; therefore a new definition of rectifiable sets including non-horizontal curves is needed. This is why we introduce in any metric space a new class of curves, called continuously metric differentiable of degree $k$, which are H\"older but not Lipschitz continuous when $k>1$. Replacing Lipschitz curves by this kind of curves we define $({\cal H}^k,1)$-rectifiable sets and show a density result generalizing the corresponding one in Euclidean geometry. This theorem is a consequence of computations of Hausdorff measures along curves, for which we give an integral formula. In particular, we show that both spherical and usual Hausdorff measures along curves coincide with a class of dimensioned lengths and are related to an interpolation complexity, for which estimates have already been obtained in Carnot-Carath\'eodory spaces.