HIRSCHFELD, R. A. Operators with thin spectral sets. MATHEMATICA SCANDINAVICA, [S. l.], v. 39, p. 367–370, 1976. DOI: 10.7146/math.scand.a-11669. Disponível em: https://www.mscand.dk/article/view/11669. Acesso em: 14 apr. 2025.