1. Department of Mathematics, The University of Kansas, Lawrence, KS 66045, U.S.A.
2. Instituto de Fisica Teorica – UNESP, R. Pamplona 145, 01405-900 São Paulo, Brazil
3. Department of Aeronautics and Astronautics, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501