1. Department of Mathematics and Computer Science University of Science Ho Chi Minh City Vietnam
2. Vietnam National University Ho Chi Minh City Vietnam
3. Department of Mathematics University of Architecture Ho Chi Minh City Vietnam
4. University of Khanh Hoa Nha Trang city Vietnam
5. MAPMO, UMR 6628, bât. Mathématiques University of Orléans Orléans France