Othman Echi
Saudi Arabia
King Fahd University of Petroleum and Minerals
King Fahd University of Petroleum and Minerals, Department of Mathematics & Statistics, P.O. Box 5046, Dhahran 31261, Saudi Arabia.
Sami Lazaar
Tunisia
University Tunis-El Manar
Department of Mathematics, University Tunis-El Manar, Faculty of Sciences of Tunis "Campus Universitaire", 2092 Tunis, Tunisia
