∗Dept. of Mathematics, University of Pittsburgh, USA. Partially supported by the Alfred P.Sloan Research Fellowship and the National Science Foundation DMS-9622872
†Mathématiques Appliquées de Bordeaux, Université Bordeaux I, 351 cours de la Libération, 33405 Talence Cedex, France
Notes
∗Dept. of Mathematics, University of Pittsburgh, USA. Partially supported by the Alfred P.Sloan Research Fellowship and the National Science Foundation DMS-9622872
†Mathématiques Appliquées de Bordeaux, Université Bordeaux I, 351 cours de la Libération, 33405 Talence Cedex, France