1. Department of Mathematics, Sogang University, Seoul, Republic of Korea
2. Department of Mathematics, Kwangwoon University, Seoul, Republic of Korea
3. Hanrimwon, Kwangwoon University, Seoul, Republic of Korea
4. Division of General Education, Kwangwoon University, Seoul, Republic of Korea
5. Department of Mathematics Education, Kyungpook National University, Taegu, Republic of Korea