1. Department of Mechanical Engineering and Science, Kyoto University, Katsura, 615-8540 Kyoto, Japan
2. Dipartimento di Matematica, Università di Roma La Sapienza, Piazzale Aldo Moro 5, 00185 Roma, Italy
3. Zentrum Mathematik, TU München, Boltzmannstrasse 3, 85748 Garching, Germany