Institution: | a Department of Mathematics and Computer Science, College of National Science, Vietnam National University HoChiMinh City, 227 Nguyen Van Cu Street, District 5, Ho Chi Minh City, Vietnamb MAPMO, UMR 6628, Bâtiment de Mathématiques, Université d'Orléans, BP 6759, Orléans Cedex 2, Francef1alpham@worldonline.frf1 |