1. Operator Theory and Applications Research Group, Department of Mathematics, Faculty of Science; King Abdulaziz University; P.O. Box 80203 Jeddah 21589 Saudi Arabia
2. Pedagogical Department, Mathematics and Informatics; National and Capodistrian University of Athens; 4, Agamemnonos Str., Aghia Paraskevi Athens 15342 Attica Greece