1. Center for Applied Mathematics, Tianjin University, Weijin Road, Tianjin 300072, P. R. China
2. School of Information Science and Engineering, Central South University, Lushan South Road, Changsha, Hunan 410083, P. R. China
3. Department of Mathematics of Qinghai Nationalities University, Bayi Road, Xining, Qinghai 810000, P. R. China