1. School of Mathematics and Statistics, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China
2. School of Electrical Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China
3. State Power Economic Research Institute, Beijing 10052, China
4. School of Mechanical Engineering, Northwestern Polytechnical University, Xian, Shaanxi 710072, China