1. College of Mathematics and System Science, Shenyang Normal University, Shenyang 110034, P. R. China
2. Mathematics School and Institute of Jilin University, Jilin University, Changchun 130012, Jilin, P. R. China
3. Key Laboratory of Applied Statistics of MOE, School of Mathematics and Statistics, Northeast Normal University, Changchun 130024, Jilin, P. R. China