1. Université de Poitiers, Laboratoire de Mathématiques et Applications, UMR CNRS 7348 – SP2MI, Boulevard Marie et Pierre Curie – Téléport 2, F-86962 Chasseneuil Futuroscope Cedex, France. E-mails: ahmad.makki@math.univ-poitiers.fr, alain.miranville@math.univ-poitiers.fr, madalina.petcu@math.univ-poitiers.fr
2. Lebanese American University, Departement of Computer Science and Mathematics, Beirut, Lebanon
3. The Institute of Mathematics of the Romanian Academy, Bucharest, Romania
4. The Institute of Statistics and Applied Mathematics of the Romanian Academy, Bucharest, Romania