1. School of Computer Science and Engineering, University of Aizu, Ikki, Aizu‐Wakamatsu 965‐8580, Japan
2. Graduate School of Science and Engineering, Yamaguchi University, 755‐8611 Yamaguchi, Japan
3. Institute of Theoretical Physics, Charles University, V Holesovickach 2, 180 00 Praha 8, Czech Republic