Affiliation: | 1. School of Mathematics and Statistics, Lanzhou University, Lanzhou 730000, China;2. College of Sciences, Hebei University of Science and Technology, Shijiazhuang 050018, China;3. School of Mathematics and Statistics, Jiangsu Normal University, Xuzhou 221116, China;4. Dongling School of Economics and Management, University of Science and Technology of Beijing, Beijing 100083, China |