Affiliation: | Département de Mathématique, UniversitéLibre de Bruxelles, Campus Plaine C.P.218/1, 1050 Bruxelles, Belgium;Department of Mathematical Physics, University College, Dublin 4, Ireland;Istituto di Matematiche Applicate 'U,DINI', Universitádi Pisa, via Bonanno, 25B, Pisa, Italy |