Institution: | aInstitute for Mathematics, Physics and Mechanics,University of Ljubljana, P. O. Box 2964, 1001 Ljubljana, Slovenia bDepartment of Differential Geometry, Faculty of Mechanics and Mathematics,Moscow State University, Moscow 119992, Russia cDipartimento di Matematica, Università degli Studi di Modena e Reggio Emilia, Via Campi 213/B, Modena 41100, Italy |