1. M. K. Aouf:
Mathematics Department, Faculty of Science, Mansoura University, Mansoura, Egypt
2. R. M. El-Ashwah:
Mathematics Department, Faculty of Science, Mansoura University, New Damietta, Egypt
3. S. M. El-Deeb:
Mathematics Department, Faculty of Science, Mansoura University, New Damietta, Egypt