1. Graduate School of Computer Science and Engineering, The University of Aizu, Tsuruga, Ikki-machi, Aizuwakamatsu 965-8580, Japan
2. Department of Applied Physics and Physico-Informatics, Keio University, Yokohama 223-8522, Japan
3. School of Computer Science and Technology, Harbin Institute of Technology, Harbin 150001, China