[1] Univ Cattolica Sacro Cuore, Dipartimento Matemat & Fis, Via Garzetta 48, I-25133 Brescia, Italy
[2] Univ Udine, Dipartimento Sci Matematiche Informat & Fis, Via Sci 206, I-33100 Udine, Italy
[3] Univ Birmingham, Sch Math, Watson Bldg, Birmingham B15 2TT, England