Lê, Thiếu Tráng. “Using Some Principles of Discrete Mathematics in Counting Problems for Olympic Students”. SCIENTIFIC JOURNAL OF TAN TRAO UNIVERSITY 4, no. 10 (April 6, 2021): 12–21. Accessed June 30, 2022. https://tckh.daihoctantrao.edu.vn/index.php/sjttu/article/view/242.