1. Department of Natural Sciences and Technology, University of Khanh Hoa, Nha Trang, Vietnam
2. Department of Mathematics, University of Architecture of Ho Chi Minh City, Ho Chi Minh City, Vietnam
3. MAPMO, Department of Mathematics, University of Orléans, Orléeans, France
4. Department of Mathematics and Computer Science, VNUHCM-University of Science, Ho Chi Minh City, Vietnam