1. M. K. Aouf:
Department of Mathematics, Faculty of Science, Mansoura University, Mansoura, Egypt
2. A. Shamandy:
Department of Mathematics, Faculty of Science, Mansoura University, Mansoura, Egypt
3. A. O. Mostafa:
Department of Mathematics, Faculty of Science, Mansoura University, Mansoura, Egypt
4. F. Ei-Emam:
Department of Mathematics, Faculty of Science, Mansoura University, Mansoura, Egypt