Department of Applied Mathematics, Northwestern Polytechnical University, Xi'an 710072 2Department of Automatic Control, Northwestern Polytechnical University, Xi'an 710072;
Department of Applied Mathematics, Northwestern Polytechnical University, Xi'an 710072 2Department of Automatic Control, Northwestern Polytechnical University, Xi'an 710072;
Department of Automatic Control, Northwestern Polytechnical University, Xi'an 710072;