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. School of Mechanics and Civil Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, P. R. China
4. School of Mathematics and Statistics, Qujing Normal University, Qujing, Yunnan 655000, P. R. China