1. Department of Mathematics, Heriot-Watt University>, Edinburgh EH14 4AS, UK ()
2. Dipartimento di Fisica ‘;E. R. Caianiello’, Universita di Salerno, Via S. Allende, 84081 Baronissi (SA), Italy ()
3. Department of Mathematical Sciences, Norwegian University of Science and Technology, Alfred Getz vei 1, 7491 Trondheim, Norway ()