1. Mathematics Department, Faculty of Science, 35516 Mansoura, Egypt
2. Mathematics Department, Faculty of Science, Al-Ain P. O. Box 17551, U.A.E
3. Max Planck Institut für Physik Komplexer Systeme, Nöthnitzer Str. 38, D-01187 Dresden, Germany
4. Mathematics Department, Faculty of Education, 45111 El-Arish, Egypt