Affiliation: | a Laboratoire de Mathématiques, CNRS UMR 6620, Université Blaise Pascal (Clermont-Ferrand 2), F-63177 Aubière cedex, France b Department of Mathematics, College of Sciences, Hohai University, Nanjing 210098, Jiangsu, China c School of Mathematical Sciences, Fudan University, Shanghai 200433, China |