Institution: | Ecole Normale Supérieure, Département de Mathématique et d'Informatique, 45 rue d'Ulm, 75230 Paris Cedex 05, France ; Department of Mathematics, University of Crete, 714 09 Heraklion, Crete, and Institute of Applied and Computational Mathematics, FORTH, 711 10 Heraklion, Crete, Greece |