1. M. K. Aouf:
Department of Mathematics, Faculty of Science, Mansoura University, Mansoura 35516, Egypt
2. A. A. Shamandy:
Department of Mathematics, Faculty of Science, Mansoura University, Mansoura 35516, Egypt
3. A. O. Mostafa:
Department of Mathematics, Faculty of Science, Mansoura University, Mansoura 35516, Egypt
4. A. K. Wagdy:
Department of Mathematics, Faculty of Science, Mansoura University, Mansoura 35516, Egypt