Holonomy in pseudo-Hermitian geometry

TL;DR

This paper investigates the holonomy of sub-Riemannian structures on contact manifolds, especially pseudo-Hermitian structures, providing classifications and relations to Riemannian holonomy and symmetric structures.

Contribution

It offers a classification of holonomy algebras for pseudo-Hermitian structures and explores their relation to Riemannian holonomy and symmetric sub-Riemannian structures.

Findings

Holonomy algebras classified for pseudo-Hermitian structures.

Relation established between sub-Riemannian and Riemannian holonomy.

Characterization of symmetric sub-Riemannian contact structures.

Abstract

We study the holonomy that is associated to a sub-Riemannian structure defined on the kernel of a global contact form. This includes the holonomy of Schouten's horizontal connection as well as of the adapted connection, both canonical invariants of the structure. Under a condition on the torsion of the structure, we show that they are either equal or that the former is a codimension one normal subgroup of the latter. Furthermore, we establish a close relation to Riemannian holonomy, which yields a complete holonomy classification in the torsion-free case. For the main result we focus on the special case of pseudo-Hermitian structures and give a classification of holonomy algebras for both the Schouten and the adapted connection. Based on this, we derive a classification of symmetric sub-Riemannian structures and of of those holonomy groups that admit parallel spinors. Finally we exhibit a relation between locally symmetric sub-Riemannian contact structures and locally homogeneous Riemannian structures.