1. Department of Mathematics, Lingnan Normal University, Zhanjiang, Guangdong, People's Republic of China
2. Department of Mathematics, Yancheng Teachers University, Yancheng, Jiangsu, People's Republic of China
3. Department of Mathematics, Taizhou University, Linhai, Zhejiang, People's Republic of China
4. SMU college of art and science, Shanghai maritime University, Shanghai, People's Republic of China