[1] Dipartimento Matemat & Informat U Dini, Viale Morgagni 67-A, I-50134 Florence, Italy
[2] Weierstrass Inst Appl Anal & Stochast, Mohrenstr 39, D-10117 Berlin, Germany
[3] Univ Milan, Dipartimento Matemat F Enriques, I-20133 Milan, Italy
[4] Univ Pavia, Dipartimento Matemat F Casorati, Via Ferrata 1, I-27100 Pavia, Italy