1. School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, P. R. China
2. Key Laboratory of Mathematics and Information Network, (Beijing University of Posts and Telecommunications), Ministry of Education, Beijing 100876, P. R. China
3. School of Mathematics and Information Science, Shandong Technology and Business University, Yantai 264003, Shandong Province, P. R. China
4. School of Mathematics and System Science, Beihang University, Beijing 100191, P. R. China