BAC, P. H. An approach to expanding relational algebra. Journal of Computer Science and Cybernetics, [S. l.], v. 1, n. 4, p. 20–23, 2015. DOI: 10.15625/1813-9663/1/4/6701. Disponível em: https://vjs.ac.vn/index.php/jcc/article/view/6701. Acesso em: 5 may. 2024.