1. Department of Mathematics, Faculty of Sciences (I), Lebanese University, Beirut, Lebanon
2. Laboratoire de Mathématiques et Applications, UMR CNRS 7348, Université de Poitiers, TSA 61125, 11 Boulevard Marie et Pierre Curie, 86073 Poitiers Cedex 9, France
3. Mathematics Laboratory, Doctoral School of Sciences and Technology, Lebanese University, Beirut, Lebanon