1. Department of Mathematics and Physics, Dalian Jiaotong University, Dalian, Liaoning, 116028, P.R. China
2. Department of Applied Mathematics, Dalian University of Technology, Dalian, Liaoning, 116024, P.R. China
3. Department of Mathematics, Hubei Institute for Nationalities, Enshi, Hubei, 445000, P.R. China