Non hypoellipticité analytique pour des opérateurs à caractéristiques doubles
We give a short overview of sub-Laplacians on Carnot groups starting from a result by Caccioppoli dated 1934. Then we show that sub-Laplacians on Carnot groups of step one arise in studying curvature problems for manifolds. We restrict our presentation to the cases of the Webster-Tanaka curvature problem for the sphere and of the Levi-curvature equation for strictly pseudoconvex functions.
The article studies a second-order linear differential operator of the type
In the euclidean setting the celebrated Aleksandrov-Busemann-Feller theorem states that convex functions are a.e. twice differentiable. In this paper we prove that a similar result holds in the Heisenberg group, by showing that every continuous –convex function belongs to the class of functions whose second order horizontal distributional derivatives are Radon measures. Together with a recent result by Ambrosio and Magnani, this proves the existence a.e. of second order horizontal derivatives for...