1. Département de Mathématiques, Université d'Evry Val d'Essonne, Bboulevard F. Mitterrand, 91025 Evry Cedex, France
2. Division of General Education-Mathematics, Kwang-Woon University, Seoul 139-701, Republic of Korea
3. Department of Wireless Communications Engineering, Kwangwoon University, Seoul 139-701, Republic of Korea
4. Department of Mathematics Education, Kyungpook National University, Taegu 702-701, Republic of Korea