1. Department of Mathematics, Liaoning Normal University, Dalian, Liaoning 116029, China
2. Department of Mathematics, Changwon National University, Changwon 641-773, Republic of Korea
3. Department of Mathematics, Gyeongsang National University, Jinju 660-701, Republic of Korea
4. Research Institute of Natural Science, Gyeongsang National University, Jinju 660-701, Republic of Korea