1. Department of Mathematics, Honghe University, Mengzi, Yunnan, People's Republic of China
2. College of Science, Hunan University of Technology and Business, Changsha, Hunan, People's Republic of China
3. Key Laboratory of Hunan Province for Statistical Learning and Intelligent Computation, Hunan University of Technology and Business, Changsha, Hunan, People's Republic of China
4. Department of Mathematics, University of Craiova, Craiova, Romania
5. School of Mathematics and Physics, North China Electric Power University, Beijing, People's Republic of China
6. Department of Mathematics and Statistics, Yunnan University, Kunming, Yunnan, People's Republic of China