1. Department of Mathematics and Computer Science, Transilvania University of Brasov, Brasov, Romania
2. Academy of Romanian Scientists, Bucharest, Romania
3. Department of Mechanical and Aerospace Engineering, Politecnico di Torino, Torino, Italy
4. Department of Mathematics, Faculty of Science, Zagazig University, Zagazig, Egypt
5. Department of mathematics, Faculty of Science, King Abdulaziz University, Saudi Arabia
6. Department of mathematics, Faculty of Science, Sohag University, Sohag, Egypt