[1] Univ Florence, Dipartimento Matemat U Dini, I-50134 Florence, Italy
[2] Univ Milan, Dipartimento Matemat F Enriques, I-20133 Milan, Italy
[3] Univ Valencia, Dept Algebra, Fac Matemat, E-46100 Valencia, Spain
[4] Univ Padua, Dipartimento Matemat Pura & Applicata, I-35121 Padua, Italy