1. Department of Mathematics, School of Information, Renmin University of China; Beijing 100872 China
2. Department of Mathematics and Statistics; Georgetown University; Washington D.C. 20057 USA
3. Graduate Institute of Applied Science and Engineering; Fu Jen Catholic University; Taipei 242 Taiwan
4. Hubei Key Laboratory of Applied Mathematics, Faculty of Mathematics and Statistics; Hubei University; Wuhan 430062 China
5. Laboratory of Mathematics and Complex Systems (Ministry of Education of China), School of Mathematical Sciences; Beijing Normal University; Beijing 100875 China