1. CEREMADE (CNRS UMR n° 7534), PSL Research University, Université Paris-Dauphine, Place de Lattre de Tassigny, 75775 Paris 16, France
2. Department of Mathematics, Imperial College London, Huxley Building, 180 Queen’s Gate, London SW7 2AZ, United Kingdom
3. School of Mathematics, Georgia Institute of Technology, Skiles Building, Atlanta, Georgia 30332-0160, USA