Institution: | a Université Jean Monnet (Saint-Etienne), Mathématiques, 23 rue du Dr. Paul Michelon, 42023 Saint-Etienne Cedex 2, France b University of Ostrava, Department of Mathematics, 30. Dubna 22, 701 03 Ostrava 1, Czech Republic c Department of Mathematics University J. Selye, PO Box 54, 94501 Komárno, Slovak Republic |