1. College of Mathematics, Jilin University, Changchun, Jilin, China
2. School of Mathematics and Statistics, Center for Mathematics and Interdisciplinary Sciences, Northeast Normal University, Changchun, Jilin, China
3. College of Mathematics, Jilin University, Changchun, Jilin, China; and School of Mathematics and Statistics, Center for Mathematics and Interdisciplinary Sciences, Northeast Normal University, Changchun, Jilin, China