Institution: | a 4-24 Furue-higashi-machi, Nishi-ku, Hiroshima 733-0872, Japanb Department of Mathematics, Graduate School of Science, Hiroshima University, Higashi-Hiroshima 739-8521, Japanc Faculty of Education and Welfare Science, Oita University, Dannoharu Oita-city 870-1192, Japand Department of Mathematics, Graduate School of Education, Hiroshima University, Higashi-Hiroshima 739-8524, Japan |