O'KEEFE, Edward S. Verification of a Conjecture of Th. Skolem. Mathematica Scandinavica, Berkeley, CA, United States, v. 9, p. 80–82, 1961. DOI: 10.7146/math.scand.a-10624. Disponível em: https://journals.msp.org/mscand/article/view/2721. Acesso em: 25 jul. 2026.