Affiliation: | a Aso Fukuoka Junior College, Dazaifu 740, Dazaifu-shi, Fukuoka-ken 818-01, Japan b Applied Mathematics Department, Fukuoka University, Nanakuma 8-19-1, Jonan-ku, Fukuoka 814-01, Japan c Department of Mathematical Sciences, lbaraki University, Mito-shi, Ibaraki 310, Japan |