Affiliation: | (1) School of Mathematics, University of Edinburgh, JCMB King’s Buildings, Edinburgh, EH9 3JZ, United Kingdom;(2) Department of Mathematics, Sun Yat-Sen University, Guangzhou, Guangdong, 510275, P. R. China;(3) Present address: Département de Mathématiques et Applications, école Normale Supérieure, 45 rue d’Ulm, F-75230 Paris Cedex 05, France |