1. School of Mathematics, Shanxi University, Taiyuan 030006, P. R. China
2. School of Mathematics and Statistics, Northwestern Polytechnical University, Xi’an 710072, P. R. China
3. MOE Key Laboratory of Complexity Science in Aerospace, Northwestern Polytechnical University, Xi’an 710072, P. R. China
4. Research & Development Institute, Northwestern Polytechnical University, Shenzhen, 518057, P. R. China