Affiliation: | 1. Dip. Mat. Castelnuovo, Univ. di Roma La Sapienza, Rome, Italy;2. Univ. Paris Diderot-Paris 7, IMJ - PRG CNRS UMR 7586, Bât. Sophie Germain, Case 7012, 75205 Paris Cedex 13, France;3. Department of Mathematics, UC Berkeley, 970 Evans Hall, Berkeley, CA 94720, USA |