KEISLER, H. J. On Some Results of Jonnsson and Tarski Concerning Free Algebras. MATHEMATICA SCANDINAVICA, [S. l.], v. 9, p. 102–106, 1961. DOI: 10.7146/math.scand.a-10628. Disponível em: https://www.mscand.dk/article/view/10628. Acesso em: 20 may. 2024.