O’KEEFE, E. S. Verification of a Conjecture of Th. Skolem. MATHEMATICA SCANDINAVICA, [S. l.], v. 9, p. 80–82, 1961. DOI: 10.7146/math.scand.a-10624. Disponível em: https://www.mscand.dk/article/view/10624. Acesso em: 3 may. 2024.