JONSSON, B. Algebraic Extensions of Relational Systems. MATHEMATICA SCANDINAVICA, [S. l.], v. 11, p. 179–205, 1962. DOI: 10.7146/math.scand.a-10665. Disponível em: https://www.mscand.dk/article/view/10665. Acesso em: 5 may. 2024.