Affiliation: | a Faculty of Engineering, Kyushu Sangyo University, 3-1 Matsukadai 2-Chome, Higashi-ku, Fukuoka 813-8503, Japan b Department of Mathematics, Ariake National College of Technology, 150 Higashihagio-machi, Omuta-shi, Fukuoka 836-8585, Japan c Faculty of Mathematics and Computer Science, Babe?-Bolyai University, 1 M.Kog?lniceanu Str., 400084 Cluj-Napoca, Romania |