CODDINGTON, Earl A. On self-adjoint ordinary differential operators. Mathematica Scandinavica, Berkeley, CA, United States, v. 4, p. 9–21, 1956. DOI: 10.7146/math.scand.a-10451. Disponível em: https://journals.msp.org/mscand/article/view/2871. Acesso em: 25 jul. 2026.