CHRISTENSEN, D. J.; PIERCE, R. S. Free Products of $\alpha$-Distributive Boolean Algebras. Mathematica Scandinavica, Berkeley, CA, United States, v. 7, p. 81–105, 1959. DOI: 10.7146/math.scand.a-10564. Disponível em: https://journals.msp.org/mscand/article/view/2782. Acesso em: 8 sep. 2026.