1. Department of Mathematics and Physical Science, Institute of Engineering and Technology, Nanhua University, Hengyang, Hunan 421001, P. R. China, Institute of Mathematics and Physics, Xinjiang University, Wulumuqi, Xinjiang 830046, P. R. China, and Department of Control Science and Engineering, Huazhong University of Science & Technology, Wuhan, Hubei 430074, P. R. China