1. CEREMADE, Université; Paris-Dauphine, Paris 75016, France.
2. Department of Mathematics, Chinese University of Hong Kong, Shatin, N.T., Hong Kong.
3. CMAP, Ecole Polytechnique, 91128 Palaiseau Cedex 91128, France.
4. FAM, Fakultät für Mathematik und Geoinformation, Vienna University of Technology, A-1040 Vienna, Austria.