1. School of Mathematics and Statistics, Central South University, Changsha, Hunan, P.R. China
2. Department of Mathematics, College of Science, King Saud University, Riyadh, Saudi Arabia
3. Department of Chemistry, Faculty of Science, King Khalid University, Abha, Saudi Arabia
4. Department of Mathematics, Women University of Azad Jammu and Kashmir, Bagh, Pakistan
5. School of Mathematics and Statistics, Shandong University, Weihai, P.R. China