1. Mathematics Postdoctoral Research Center, Hebei Normal University, Shijiazhuang, Heibei, China
2. Department of Mathematics, Wilfrid Laurier University, Waterloo, Ontario, Canada; and School of Mathematical Sciences, Hebei Normal University, Shijiazhuang, Hebei, China
3. School of Mathematics and Statistics, Xinyang Normal University, Xinyang, Henan, China