1. M. K. Aouf:
Mathematics Department, Faculty of Science, Mansoura University, Mansoura, Egypt
2. A. Shamandy:
Mathematics Department, Faculty of Science, Mansoura University, Mansoura, Egypt
3. R. M. El-Ashwah:
Mathematics Department, Faculty of Science, Mansoura University, Mansoura, Egypt
4. E. E. Ali:
Mathematics Department, Faculty of Science, Mansoura University, Mansoura, Egypt