1. Department of Mathematics and Computer Science, University of Perugia, via Vanvitelli 1, 06123 Perugia, Italy
2. Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, Italy
3. Department of Mathematics and Computer Science Ulisse Dini, University of Florence, viale Morgagni 67/A, 50134 Florence, Italy
4. Department of Mathematics and Computer Science, Eindhoven University of Technology, P. O. Box 513, 5600 MB Eindhoven, The Netherlands
5. Department of Mathematics, Physics and Computer Science, University of Modena and Reggio Emilia, via G. Campi 213/b, 41125 Modena, Italy