1. Department of Mathematics, Faculty of Science, King Abdulaziz University, P. O. Box 80203, Jeddah 21589, Saudi Arabia
2. Department of Mathematics, Faculty of Science, Al-Azhar University, Assiut Branch, Assiut, Egypt
3. Department of Mathematics, College of Science, Taif University, P. O. Box 888, Taif 21974, Saudi Arabia