1. Department of Mathematics, Liaoning Normal University, Dalian, Liaoning 116029, China
2. Dalian No. 39 Middle School Dalian, Liaoning 116001, China
3. Department of Mathematics, Changwon National University, Changwon 641-773, Republic of Korea
4. Department of Mathematics and RINS, Gyeongsang National University, Jinju 660-701, Republic of Korea