1. Department of Mathematics, Faculty of Science, University of Mansoura, Mansoura 35516, Egypt
2. Department of Mathematics, Teachers' College in Abha, King Khalid University, Abha, P.O. Box 249, Saudi Arabia
3. Department of Applied Mathematics, Pukyong National University, Pusan 608-737, South Korea
4. Department of Mathematics, Faculty of Science, King AbdulAziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia