[1] Univ Florence, Dipartimento Matemat Ulisse Dini, Viale Morgagni 67-A, I-50134 Florence, Italy
[2] Univ Southern Calif, Dept Math, Los Angeles, CA 90089 USA
[3] Univ Western Australia, Sch Math & Stat, Ctr Math Symmetry & Computat, Crawley, WA 6009, Australia
[4] Univ Milano Bicocca, Dipartimento Matemat & Applicaz, Via Cozzi 55, I-20125 Milan, MI, Italy