1. Department of Mathematics, Faculty Sciences, King Abdulaziz University, Jeddah, Saudi Arabia
2. Politecnico di Torino, 10129 Torino, Italy
3. Cadi Ayyad University, Ecole Nationale des Sciences Appliquées, Marrakech, Morocco
4. Department of Applied Mathematics, Dong Hua University, Shanghai 200051, P. R. China
5. Institut für Mathematik, Universität Paderborn, 33098 Paderborn, Germany