1. School of Computer and Information Technology , Shanxi University, Taiyuan 030006, Shanxi, China
2. Complex Systems Research Center , Shanxi University, Taiyuan 030006, Shanxi, China
3. Department of Mathematics , North University of China, Taiyuan 030051, Shanxi, China
4. Department of Mathematics , University of Miami, Coral Gables, FL 33146, USA