1. Liaoning Normal University, Department of Mathematics, Dalian, Liaoning 116029, China
2. Dalian Hongwen Middle School, Dalian, Liaoning 116029, China
3. Gyeongsang National University, Department of Mathematics and RINS, Jinju 660-701, Republic of Korea
4. Dong-A University, Department of Mathematics, Pusan 614-714, Republic of Korea