HU, T.- kai. On the Bases of Free Algebras. MATHEMATICA SCANDINAVICA, [S. l.], v. 16, p. 25–28, 1965. DOI: 10.7146/math.scand.a-10743. Disponível em: https://www.mscand.dk/article/view/10743. Acesso em: 2 may. 2024.