HU, Tah-kai. On the Bases of Free Algebras. Mathematica Scandinavica, Berkeley, CA, United States, v. 16, p. 25–28, 1965. DOI: 10.7146/math.scand.a-10743. Disponível em: https://journals.msp.org/mscand/article/view/2582. Acesso em: 25 jul. 2026.