1. Department of Mathematics, University of Oslo, Postboks 1053 Blindern, 0316 Oslo, Norway
2. Department of Physics, University of Oslo, Postboks 1048 Blindern, 0317 Oslo, Norway
3. Department of Computational Science, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan