1. School of Mathematics and Systems Science and LMIB, Beihang University, Beijing 100191, China and School of Statistics and Mathematics, Inner Mongolia University of Finance and Economics, Hohhot 010070, China
2. School of Mathematics and Systems Science and LMIB, Beihang University, Beijing 100191, China
3. College of Sciences, Beijing Forestry University, Beijing 100083, China
4. MOE Key Laboratory of Bioinformatics, Zhou Pei-Yuan Center for Applied Mathematics, Tsinghua University, Beijing 100084, China