EHRENFEUCHT, A.; FUHRKEN, G. On Models with Undefinable Elements. MATHEMATICA SCANDINAVICA, [S. l.], v. 28, p. 325–328, 1971. DOI: 10.7146/math.scand.a-11027. Disponível em: https://www.mscand.dk/article/view/11027. Acesso em: 6 may. 2024.