Affiliation: | 1. Department of Mathematics, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul, 136-701, Republic of Korea;2. Department of Mathematics Education, Chungbuk National University, 52 Naesudong-ro, Heungdeok-gu, Cheongju, Chungbuk, 361-763, Republic of Korea |