Singular Riemannian metrics, sub-rigidity vs rigidity
  We analyze sub-Riemannian and lightlike metrics from the point of view of their rigidity as geometric structures. Following Cartan's and Gromov's formal definitions, they are never rigid, yet, in generic cases, they naturally give rise to rigid geometric structures!? 
Samir Bekkara (USTO), Abdelghani Zeghib (UMPA-ENSL)
math.DG