1. Department of Mathematics, Jeonju University, Chon-ju 560-759, Korea
2. Department of Mathematics, Hanyang University, Haengdang-Dong, Seongdong-Gu, Seoul 133-791, Korea
3. Department of Applied Mathematics, Hanyang University, 1271 Sa-1 dong, Sangnok-gu, Ansan, Gyeonggi-do 426-791, Korea
4. Department of Mathematical and Computer Science, University of Wisconsin-Whitewater, Whitewater, WI 53190, USA