1. University of Tunis El Manar, National Engineering School of Tunis, Laboratory of Engineering Mathematics EPT, BP 743, La Marsa 2078, Tunisia
2. Université Ibn Zohr, BP 32/S, CP 80000 Agadir, Morocco
3. LMAH, FR-CNRS-3335, Université du Havre Normandie, 25 Rue Ph. Lebon, BP 540, 76058 Le Havre Cedex, (Normandie) France