1. Laboratoire Paul Painlevé (U.M.R. CNRS 8524), U.F.R. de Mathématiques, Université Lille 1, 59655 Villeneuve d’Ascq Cedex, France
2. Institut de Mathematiques de Toulouse UMR5219, Université de Toulouse CNRS, 31062 Toulouse Cedex 9, France
3. Department of Mathematics, Sichuan Normal University, Chengdu 610066, China and School of Mathematical Sciences, University of Electronic Science and Technology of China, Chengdu, Sichuan 611731, China