ELLENTUCK, Erik. On the Degrees of Universal Regressive Isols. Mathematica Scandinavica, Berkeley, CA, United States, v. 32, p. 145–164, 1973. DOI: 10.7146/math.scand.a-11451. Disponível em: https://journals.msp.org/mscand/article/view/2155. Acesso em: 25 jul. 2026.