Affiliation: | a Graduate School of Mathematical Sciences, The University of Tokyo, 3-8-1 Komaba Meguro-ku, Tokyo 153-8914, Japan b Graduate School of Environmental Science, Okayama University, 3-1-1 Naka Tsushima, Okayama 700-8530, Japan c Graduate School of Information Science and Technology, The University of Tokyo, 3-8-1 Komaba Meguro-ku, Tokyo 153-8914, Japan d Knowledge Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai Nomi, Ishikawa 923-1211, Japan e Library and Information Technology Center, Gunma University, 4-2 Aramaki-machi, Maebashi 371-8510, Japan |