1. CERMICS, École des Ponts and Inria Paris, 6 and 8 avenue Blaise Pascal, 77455 Marne-la-Vallée, France.
2. CEREMADE, University of Paris-Dauphine, PSL University, 75016 Paris, France & ENS/PSL University, Département de Mathématiques et Applications, F-75005, Paris, France.