[1] Univ Paris 09, CNRS, CEREMADE, UMR 7534, F-75775 Paris 16, France
[2] Paris Est ENPC, CERMICS, F-77455 Marne La Vallee 2, France
[3] Ecole Polytech, CMAP INRIA Futurs, Projet Commands, F-91128 Palaiseau, France
[4] UMA, ENSTA, F-75739 Paris 15, France