Affiliation: | (1) Oshima National College of Maritime Technology, Komatsu, Oshima-cho, Oshima-gun, Yamaguchi 742-2193, Japan. e-mail: abe@c.oshima-k.ac.jp, JP;(2) Department of Mathematical Science, Faculty of Science, Kumamoto University, Kumamoto 860-0862, Japan. e-mail: wagami@math.sci.kumamoto-u.ac.jp, JP |