1. Department of Mathematics, College of Science, King Saud University, P. O. Box 2455, Riyadh 11451, Saudi Arabia
2. Modern Academy For Engineering And Technology, Maadi, Egypt
3. Mathematics Department, Faculty of Science, Tanata University, Tanta, Egypt
4. I.I.S. Lagrange, Via L. Modignani 65, Milan 20161, Italy