1. School of Information Science and Engineering, Hangzhou Normal University, Hangzhou 311121, China.
2. Department of Computer Science, Brunel University London, Uxbridge UB8 3PH, U.K..
3. Department of Mathematics, Harbin University of Science and Technology, Harbin 150080, China.
4. School of Information Science and Engineering, Hangzhou Normal University, Hangzhou 311121, China (e-mail: weiguouk@hotmail.com)