1. School of Mathematics and Statistics, Changshu Institute of Technology, Changshu, 215500 Jiangsu, P. R. China
2. School of Mathematics, China University of Mining and Technology, Xuzhou, 221116 Jiangsu, P. R. China
3. State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology, Xuzhou, 221116 Jiangsu, P. R. China
4. Department of Mathematics, Faculty of Science, King Abdulaziz University, P. O. Box 80257, Jeddah 21589, Saudi Arabia