1. Department of Mathematics, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan
2. Équipe-Project SALSA-LIP6/INRIA Paris-Rocquencourt, Université Pierre et Marie Curie, 4 place Jussieu, Boite Courier 169, F-75252 Paris Cedex 05, France
3. Department of Mathematics, Rikkyo University, Nishi-Ikebukuro, Tokyo 171-8501, Japan