1. Korea University, Department of Mathematics, A nam-dong Sungbuk-gu, Seoul 136- 701, Korea
2. Mokpo National University, Department of Mathematics, Muan-gu, Chonnam 534- 729, Korea
3. Changwon National University, Department of Statistics, 9 Sarimdong, Changwon 641-773, Korea
4. University of Adelaide, Department of Applied Mathematics, Adelaide 5005, SA, Australia