1. School of Mathematics and Information Science, Nanchang Hangkong University, Nanchang 330063, China
2. College of Information Science and Technology, Beijing University of Chemical Technology, Beijing 100029, China
3. State Key Laboratory for Strength and Vibration, Xian Jiaotong University, Xian 710049, China
4. School of Engineering, University of California, Merced, Merced, CA 95343 e-mail: