1. School of Mathematics, China University of Mining and Technology, Xuzhou, Jiangsu 221116, P. R. China
2. State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology, Xuzhou Jiangsu 221116, P. R. China
3. Department of Mathematics, Faculty of Science, King Abdulaziz University, P. O. Box 80257, Jeddah 21589, Saudi Arabia