1. Department of Mathematics, Naruto University of Education, 748, Nakashima, Takashima, Naruto-cho, Naruto-shi 772-8502, Japan
2. Department of Mathematics and RINGS, Gyeongsang National University, Jinju 660-701, Korea
3. Department of Mathematics and Institute of Natural Sciences, Kyung Hee University, Suwon 446-701, Korea