1. Department of Mathematics, Faculty of Science, University Ibn Tofail, Kenitra, Morocco
2. Department of Mathematics, Faculty of Science and Technology Box 509-Boutalamine, University Moulay Ismaïl, Errachidia, Morocco
3. Department of Mathematics, Faculty of Science and Technology of Fez, Box 2202, University S. M. Ben Abdellah Fez, Fez, Morocco