Affiliation: | (1) Sepp Herbergerstrasse 12, D-8056 Neufahrn;(2) Dipartimento di Matematica, Universita di Genova, Via L. B. Alberti 4, 16132 Genova, Italy;(3) Mathematical Institute of the Hungarian Academy of Sciences, Reáltanoda U. 13-15, H-1053, Budapest |