1. Postgraduate Student, School of Mathematics and Computer Sciences, Guangxi Univ. for Nationalities, Nanning, Guangxi 530006, P.R. China.
2. Professor, School of Mathematics and Computer Sciences, Guangxi Univ. for Nationalities, Nanning, Guangxi 530006, P.R. China (corresponding author).
3. Professor, State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan Univ., Chengdu, Sichuan 610065, P.R. China.