BOONE, W. W.; ROGERS, H. ,. Jr. On a Problem of J.H.C. Whitehead and a Problem of Alonzo Church. Mathematica Scandinavica, Berkeley, CA, United States, v. 19, p. 185–192, 1966. DOI: 10.7146/math.scand.a-10808. Disponível em: https://journals.msp.org/mscand/article/view/2534. Acesso em: 25 jul. 2026.