1. Department of Mathematics and Physics, National Defense Academy, Hashirimizu, Yokosuka 239, Japan
2. Institute for Nuclear Study, University of Tokyo, 3-2-1 Midori-cho, Tanashi-shi, Tokyo 188, Japan
3. Faculty of Science, Department of Physics, Ochanomizu University, 1-1, Otsuka 2, Bunkyo-ku, Tokyo 112, Japan