1. Department of Mathematics; Southeast University; Nanjing 210096 China
2. School of Automation and the State Key Laboratory of Digital Manufacturing Equipment and Technology; Huazhong University of Science and Technology; Wuhan 430074 China
3. Department of Automation; Tianjin University; Tianjin 300072 China
4. School of Automation; Northwestern Polytechnical University; Xi'an 710129 China