YAKES, Christopher J. Essential Spectrum and Fredholm Index for Certain Composition Operators. Mathematica Scandinavica, Berkeley, CA, United States, v. 118, n. 1, p. 152–160, 2016. DOI: 10.7146/math.scand.a-23302. Disponível em: https://journals.msp.org/mscand/article/view/361. Acesso em: 24 jul. 2026.