Intrinsic random walks and sub-Laplacians in sub-Riemannian geometry

Abstract

On a sub-Riemannian manifold we define two type of Laplacians. The \emph{macroscopic Laplacian} $\Delta_\omega$, as the divergence of the horizontal gradient, once a volume $\omega$ is fixed, and the \emph{microscopic Laplacian}, as the operator associated with a sequence of geodesic random walks. We consider a general class of random walks, where \emph{all} sub-Riemannian geodesics are taken in account. This operator depends only on the choice of a complement $\mathbf{c}$ to the sub-Riemannian distribution, and is denoted $L^c$. We address the problem of equivalence of the two operators. This problem is interesting since, on equiregular sub-Riemannian manifolds, there is always an intrinsic volume (e.g. Popp's one $P$) but not a canonical choice of complement. The result depends heavily on the type of structure under investigation. On contact structures, for every volume $\omega$, there exists a unique complement $c$ such that $\Delta_\omega=L^c$. On Carnot groups, if $H$ is the Haar volume, then there always exists a complement $c$ such that $\Delta_H=L^c$. However this complement is not unique in general. For quasi-contact structures, in general, $\Delta_P \neq L^c$ for any choice of $c$. In particular, $L^c$ is not symmetric w.r.t. Popp's measure. This is surprising especially in dimension 4 where, in a suitable sense, $\Delta_P$ is the unique intrinsic macroscopic Laplacian. A crucial notion that we introduce here is the N-intrinsic volume, i.e. a volume that depends only on the set of parameters of the nilpotent approximation. When the nilpotent approximation does not depend on the point, a N-intrinsic volume is unique up to a scaling by a constant and the corresponding N-intrinsic sub-Laplacian is unique. This is what happens for dimension smaller or equal than 4, and in particular in the 4-dimensional quasi-contact structure mentioned above.