Affiliation: | a Department of Mathematical Sciences, Hirosaki University, Hirosaki 036-8560, Japan b Department of Mathematics, Osaka Kyoiku University, Kashiwara, Osaka 582-8582, Japan c Bon Agency Co., Ltd., 30 Higashigoryo-cho, Higashikujo, Minami-ku, Kyoto 601-8028, Japan |