Mitsuo Kanemitsu
Department of Mathematics, Aichi University of Education, Kariya, Aichi, 448-8542, Japan
e-mail : mkanemit@auecc.aichi-edu.ac.jp

Ry^uki Matsuda
Department of Mathematical Sciences, Ibaraki University, Bunkyo, Mito, Ibaraki, 310-8512, Japan
e-mail : rmazda@mx.ibaraki.ac.jp

Nobuharu Onoda
Department of Mathematics, University of Fukui, Bunkyo, Fukui, 910-8507, Japan
e-mail : onoda@edu00.f-edu.fukui-u.ac.jp

Takasi Sugatani
Department of Mathematics, Toyama University, Gofuku, Toyama, 930-8555,
Japan
e-mail : sugatani@sci.toyama-u.ac.jp