1. Department of Mathematics and Physics, North China Electric Power University, Beijing 102206, P. R. China
2. School of Electrical and Electronic Engineering, North China Electric Power University, Beijing 102206, P. R. China
3. School of Energy Power and Mechanical Engineering, North China Electric Power University, Beijing 102206, P. R. China
4. College of Science, China University of Petroleum, Beijing 102249, P. R. China
5. School of Information, Beijing Wuzi University, Beijing 101149, P. R. China
6. School of Mathematics and Statistics, Central University of Finance and Economics, Beijing 100081, P. R. China