1. Department of Mathematics; University of Mostaganem; Box 227 Mostaganem 27000 Algeria
2. Dipartimento di Matematica e Informatica; Università di Catania; Viale Andrea Doria 6 - 95125 Catania Italy
3. Department of Mathematics, Faculty of Sciences; University of Oran; Box 1524 Oran 31000 Algeria