1. Department of Mathematics, Pohang University of Science and Technology, San 31, Hyoja-Dong, Namgu, Pohang, Kyungbuk, 790-784, Korea
2. Department of Mathematics Education, Kyungnam University, Wolyong-Dong 449, Changwon-Si, Kyungnam 631-701, Korea
3. KIAS Hoegiro 87 (207-43 Cheongnyangni-Dong), Dongdaemun-Gu, Seoul 130-722, Korea