1. Department of Mathematics; Harbin Institute of Technology; 92 West Dazhi Street Nan Gang District Harbin 150001 China
2. Université de Lyon, Université Lyon 1, CNRS UMR 5208, Institut Camille Jordan 43 blvd du 11 novembre 1918, F-69622 Villeurbanne-Cedex, France & Inria Team Dracula, Inria Center Grenoble Rhone-Alpes; France