1. College of Science, Huazhong Agricultural University, Wuhan 430070, P. R. China
2. School of Computer Science and Information Technology, Zhejiang Wanli University, Ningbo 315100, P. R. China
3. Institute of Mathematics, Zhejiang Wanli University, Ningbo 315100, P. R. China
4. Department of Mathematics, Ningbo University, Ningbo 315211, P. R. China