ERNSTRÖM, Lars. The spectrum associated to a radical operation. Mathematica Scandinavica, Berkeley, CA, United States, v. 73, p. 5–14, 1993. DOI: 10.7146/math.scand.a-12451. Disponível em: https://journals.msp.org/mscand/article/view/1214. Acesso em: 24 jul. 2026.