1. Department of Mathematics, Liaoning Normal University, Dalian, Liaoning 116029, China
2. Department of Mathematics, Kunming University, Kunming, Yunnan 650214, China
3. Department of Mathematics and RINS, Gyeongsang National University, Jinju 660-701, Republic of Korea
4. Department of Mathematics, Dong-A University, Pusan 614-714, Republic of Korea