[1] Hokkaido Univ, Grad Sch Informat Sci & Technol, Kita Ku, Kita 14,Nishi 9, Sapporo, Hokkaido 0600814, Japan
[2] Hokkaido Univ Sci, Dept Informat & Comp Sci, 7 Jo 15-4-1 Maeda, Sapporo, Hokkaido 0068585, Japan
[3] Univ Aizu, Dept Comp Sci & Engn, Aizu Wakamatsu, Fukushima 9658580, Japan