1. Department of Mathematics and MOE-LSC, Shanghai Jiao Tong University, Shanghai, China. E-mails: {ycli, ygwang}@sjtu.edu.cn
2. Laboratoire de Mathématiques, CNRS UMR 6620, Université Blaise Pascal (Clermont-Ferrand 2), Aubière cedex, France. E-mail: peng@math.univ-bpclermont.fr