LÊ, T. T. Using some principles of discrete mathematics in counting problems for Olympic students. SCIENTIFIC JOURNAL OF TAN TRAO UNIVERSITY, [S. l.], v. 4, n. 10, p. 12–21, 2021. DOI: 10.51453/2354-1431/2018/242. Disponível em: https://tckh.daihoctantrao.edu.vn/index.php/sjttu/article/view/242. Acesso em: 30 jun. 2022.