Affiliation: | (1) Department of Mathematics, Niigata University, 950-21 Niigata, Japan;(2) RIMS Kyoto University, 606 Kyoto, Japan;(3) Department of Mathematics Ryukyu University, 903-01 Okinawa, Japan;(4) Musashi Institute of Technology, Tamatsuzumi Setagaya, 158 Tokyo, Japan |