EHRENFEUCHT, A.; FUHRKEN, G. On Models with Undefinable Elements. Mathematica Scandinavica, Berkeley, CA, United States, v. 28, p. 325–328, 1971. DOI: 10.7146/math.scand.a-11027. Disponível em: https://journals.msp.org/mscand/article/view/2270. Acesso em: 25 jul. 2026.