1. School of Mathematics and Computing Science, Guilin University of Electronic Technology, Guilin, Guangxi 541002, P. R. China
2. SUSTech International Center for Mathematics, Southern University of Science and Technology, Shenzhen, Guangdong 518055, P. R. China
3. School of Mathematics and Statistics, Guangxi Normal University Guilin, Guangxi 541004, P. R. China