1. CERMICS, Ecole des Ponts, 77455 Marne-la-Vallée, France, and Inria, 2 rue Simone Iff, 75589 Paris, France
2. Department of Mathematics, Indian Institute of Science, Bangalore, India 560012
3. Department of Mathematics, University College London, London WC1E 6BT, UK
4. Inria, 2 rue Simone Iff, 75589 Paris, France, and CERMICS, Ecole des Ponts, 77455 Marne-la-Vallée, France