JONSSON, Bjarni; TARSKI, Alfred. On Two Properties of Free Algebras. Mathematica Scandinavica, Berkeley, CA, United States, v. 9, p. 95–101, 1961. DOI: 10.7146/math.scand.a-10627. Disponível em: https://journals.msp.org/mscand/article/view/2717. Acesso em: 25 jul. 2026.