Affiliation: | 1. Department of Mathematics and Telecommunication Mathematics Research Center, Korea University, 1, Anam-dong, Sungbuk-ku, Seoul 136-701, Republic of Korea;2. Department of Mathematics Education, Chungbuk National University 12, Gaeshin-dong, Heungduk-ku, Cheongju, Chungbuk 361-763, Republic of Korea |