1. School of Information Science and Engineering, Central South University, Changsha 410083, China
2. Department of Computer Science, St. Francis Xavier University, Antigonish, NS, Canada, B2G 2W5
3. School of Symbiotic Systems Science and Technology, Fukushima University, Fukushima 960-1296, Japan
4. School of Computer Science and Engineering, The University of Aizu, Aizuwakamatsu 965-8580, Japan