1. Department of Mathematics, Higher School of Sci. and Tech. of Hammam-Sousse, University of Sousse, Sousse, Tunisia
2. Department of Mathematics, Faculty of Sciences of Tunis, University of Tunis El-Manar, Tunis, Tunisia
3. Department of Mathematics, Higher Institute of Appl. Sci. and Tech. of Gabes, University of Gabes, Gabes, Tunisia