The Lusin theorem and horizontal graphs in the Heisenberg group

Abstract

In this paper we prove that every collection of measurable functions $f_{\alpha }$, $|\alpha |=m$ coincides a.e. with $m$th order derivatives of a function $g\in C^{m-1}$ whose derivatives of order $m-1$ may have any modulus of continuity weaker than that of a Lipschitz function. This is a stronger version of earlier results of Lusin, Moonens-Pfeffer and Francos. As an application we construct surfaces in the Heisenberg group with tangent spaces being horizontal a.e.