[1] Sapienza Univ Roma, Dip Matemat, Ple Aldo Moro 2, I-00185 Rome, Italy
[2] ENS Lyon, UMPA, 46 Allee Italie, F-69364 Lyon 7, France
[3] IUF, 46 Allee Italie, F-69364 Lyon 7, France
[4] Cimat, Guanajuato 36000, Mexico
[5] UPMC, IMJ PRG Projet Anal Algebr, 4 Pl Jussieu,Case 247, F-75252 Paris 5, France