Taekyun Kim,

Department of Mathematics, Kwangwoon University, Seoul 139-701, Republic of Korea.
E-mail : tkkim@kw.ac.kr

Dae San Kim,

Department of mathematics, Sogang University, Seoul 121-742, Republic of Korea.
E-mail : dskim@sogang.ac.kr

Hong Kyung Pak

Jangjeon Research Institute for Mathematical Sciences & Physics, Cheonma town 225-106, Sinmae-Dong,
Suseong-Gu, Daegu, 706-781, Republic of Korea.
E-Mail : ascm2001@yahoo.co.kr (or hkpak@dhu.ac.kr