Affiliation: | 1. Kyushu University, Fukuoka 810-8560, Japan;2. IWI-RuG, University of Groningen, Mathematics Department, Nijenborgh 9, 9747 AG Groningen, The Netherlands;3. Department of Mathematics, Faculty of Education, Nagasaki University, 1-14, Bunkyo-machi, Nagasaki 852-8521, Japan |