1. Ph.D. Candidate, Dept. of Management Science and Technology, Graduate school of Engineering, Tohoku Univ., 6-6-11-816 Aoba-yama, Sendai 980-8579, Japan.
2. Professor, Dept. of Management Science and Technology, Graduate school of Engineering, Tohoku Univ., 6-6-11-816 Aoba-yama, Sendai 980-8579, Japan (corresponding author).