1. School of Mathematics and Statistics, Henan University, Kaifeng, Henan 475004, P. R. China
2. Guangxi Center for Mathematical Research, Guangxi University, Nanning, Guanxi 530004, P. R. China
3. Institute of Mathematics, Academy of Mathematics and Systems Science, Chinese Academy of Sciences, Beijing 100190, P. R. China
4. School of Mathematical Sciences, University of Chinese Academy of Sciences, Beijing 100049, P. R. China