1. Department of Mathematics, Faculty of Mathematical Sciences, University of Guilan, Rasht, Iran
2. Department of Mathematics, Faculty of Mathematical Sciences, University of Mazandaran, Babolsar, Iran
3. School of Electronics and Information Engineering, Wuhan Donghu University, Wuhan 430212, People's Republic of China
4. Department of Mathematics, Howard University, Washington, DC 20059, USA
5. Department of Physics and Engineering, Delaware State University, Dover, Delaware 19901-2277, USA
6. Department of Mathematical Sciences, Delaware State University, Dover, Delaware 19901-2277, USA
7. Department of Mathematics, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
8. Science Program, Texas A and M University at Qatar, P. O. Box 23874, Doha, Qatar