JONSSON, B.; TARSKI, A. On Two Properties of Free Algebras. MATHEMATICA SCANDINAVICA, [S. l.], v. 9, p. 95–101, 1961. DOI: 10.7146/math.scand.a-10627. Disponível em: https://www.mscand.dk/article/view/10627. Acesso em: 3 may. 2024.