1. Department of Mathematics, Faculty of Sciences, Mohammed V University, P.O. Box 1014, Rabat, Morocco
2. Division of Applied Mathematics, Science and Technology Advanced Institute, Van Lang University, Ho Chi Minh, Vietnam; Faculty of Applied Technology, School of Technology, Van Lang University, Ho Chi Minh, Vietnam
3. Dipartimento di Matematica e Informatica, Università di Catania, 95125 Catania, Italy; Faculty of Fundamental Science, Industrial University of Ho Chi Minh City, Ho Chi Minh, Vietnam