1. M. K. Aouf:
Mathematics Department, Faculty of Science, Mansoura University, Mansoura 33516, Egypt
2. R. M. El-Ashwah:
Mathematics Department, Faculty of Science, Mansoura University, Mansoura 33516, Egypt
3. E. E. Ali:
Mathematics Department, Faculty of Science, Mansoura University, Mansoura 33516, Egypt; Mathematics Department, Faculty of Science, University of Hail, Kingdome of Saudi Arabia