BRINCK, Inge. Self-Adjointness and Spectra of Sturm-Liouville Operators. MATHEMATICA SCANDINAVICA, [S. l.], v. 7, p. 219–240, 1959. DOI: 10.7146/math.scand.a-10575. Disponível em: https://www.mscand.dk/article/view/10575. Acesso em: 5 dec. 2025.