Institution: | (1) Istituto Matematico U. Dini, Universitá degli Studi di Firenze, I-50134 Firenze, Italy;(2) Dept. of Physics and Astronomy, Vrije Universiteit, 1081 HV Amsterdam, The Netherlands;(3) Present address: Dept. of Mathematics, Virginia Polytechnic Institute & State University, 24061 Blacksburg, Virginia, USA |