Geometry of Carnot--Carathéodory Spaces, Differentiability and Coarea Formula

Abstract

We give a simple proof of Gromov's Theorem on nilpotentization of vector fields, and exhibit a new method for obtaining quantitative estimates of comparing geometries of two different local Carnot groups in Carnot--Carathéodory spaces with $C^{1,\alpha }$-smooth basis vector fields, $\alpha \in [0,1]$. From here we obtain the similar estimates for comparing geometries of a Carnot--Carathéodory space and a local Carnot group. These two theorems imply basic results of the theory: Gromov type Local Approximation Theorems, and for $\alpha >0$ Rashevski\vı-Chow Theorem and Ball--Box Theorem, etc. We apply the obtained results for proving $hc$-differentiability of mappings of Carnot--Carathéodory spaces with continuous horizontal derivatives. The latter is used in proving the coarea formula for some classes of contact mappings of Carnot--Carathéodory spaces.

94 pages