1. Department of Applied Mathematics, Northwestern Polytechnical University, 710072, Xi’An, P. R. China
2. Université Côte d’Azur, GREDEG, CNRS, Valbonne, France
3. Faculty of Electrical Engineering, Mathematics and Computer Science, Department of Applied Mathematics, University of Twente, P. O. Box 217, 7500 AE Enschede, The Netherlands