1. School of Mathematical Sciences, South China University of Technology, Guangzhou, Guangdong 510641, P. R. China
2. School of Mathematics and Computing Science, Guilin University of Electronic Technology, Guilin, Guangxi 541004, P. R. China
3. School of Mathematical and Computational Science, Sun Yat-Sen University, Guangzhou, Guangdong 510275, P. R. China