1. Department of Mathematics, Liaoning Normal University, Dalian, Liaoning 116029, China
2. Suizhong No. 1 Senior High School, Huludao, Liaoning 125200, China
3. Department of Mathematics and RINS, Gyeongsang National University, Jinju 52828, Republic of Korea
4. Department of Mathematics, Dong-A University, Busan 49315, Republic of Korea