1. School of Mathematical Sciences, Beihang University, Beijing 100083, P. R. China
2. School of Mathematics and Information Science, Shandong Technology and Business University, Yantai, Shandong Province 264003, P. R. China
3. Graduate School of Systems and Information Engineering, University of Tsukuba, Tennodai 1-1-1, Tsukuba 305-8573, Japan