HIRSCHFELD, Rudolf A. Operators with thin spectral sets. Mathematica Scandinavica, Berkeley, CA, United States, v. 39, p. 367–370, 1976. DOI: 10.7146/math.scand.a-11669. Disponível em: https://journals.msp.org/mscand/article/view/1976. Acesso em: 25 jul. 2026.