1. Institut de recherche mathématique de Rennes Université de Rennes1 Campus Beaulieu Rennes Cedex France
2. Department of Mathematics Hyogo University of Teacher Education Kato Hyogo Japan
3. Department of Applied Mathematics Waseda University Shinjuku‐ku Tokyo Japan
4. Graduate School of Mathematics Nagoya University, Furo‐cho, Chigusa‐ku Nagoya Japan
5. Department of Mathematics Graduate School of Science Hokkaido University Sapporo Hokkaido Japan