1. School of Mathematics and Statistics, Xi'an Jiaotong University, Xi'an, Shaanxi, People's Republic of China
2. School of Science, Inner Mongolia University of Science and Technology, Baotou, Inner Mongolia, People's Republic of China
3. Department of Mathematics, School of Sciences, Hangzhou Dianzi University, Hangzhou, People's Republic of China
4. Faculty of Science, Jiangsu University, Zhenjiang, Jiangsu, People's Republic of China