DEAL, E. R. A Quasi-Spectral Operator. Mathematica Scandinavica, Berkeley, CA, United States, v. 16, p. 29–32, 1965. DOI: 10.7146/math.scand.a-10744. Disponível em: https://journals.msp.org/mscand/article/view/2581. Acesso em: 25 jul. 2026.