1. Institut für Theoretische Physik der Universität, 69120 Heidelberg, Germany
2. Department of Physics, Shinshu University, Matsumoto 390‐8621, Japan
3. Toba National College of Maritime Technology, Toba 517‐8501, Japan
4. Department of Comprehensive Management, Matsumoto University, Matsumoto 390‐1295, Japan