1. School of Mathematics and Statistics, Gansu Key Laboratory of Applied Mathematics and Complex Systems, Lanzhou University, Lanzhou, Gansu 730000, P. R. China
2. Department of Mathematics and Department of Computer Science, Georgetown University, Washington D.C. 20057, USA
3. Department of Mathematics, Fu Jen Catholic University, Taipei 242, Taiwan
4. School of Mathematical Sciences, Beijing Normal University, Laboratory of Mathematics and Complex Systems, Ministry of Education, Beijing 100875, P. R. China
5. Department of Mathematics, Linyi University, Linyi 276005, P. R. China