1. Department of Applied Probability and Statistics, School of Mathematics and Statistics, Northwestern Polytechnical University, Xi’an, Shaanxi 710129, P. R. China
2. School of Applied Science, Taiyuan University of Science and Technology, Taiyuan, Shanxi 030024, P. R. China
3. Department of Applied Mathematics, School of Sciences, Xi’an Polytechnic University, Xi’an, Shaanxi 710699, P. R. China