1. School of Mathematics & Information Technology, Nanjing Xiaozhuang University, Nanjing 211171, China
2. Department of Mathematics, Gyeongsang National University, Jinju 660-701, Republic of Korea
3. Department of Mathematics and RINS, Gyeongsang National University, Jinju 660-701, Republic of Korea
4. School of Mathematics and Information Science, Beifang University of Nationalities, Yinchuan 750021, China