CODDINGTON, E. A. On self-adjoint ordinary differential operators. MATHEMATICA SCANDINAVICA, [S. l.], v. 4, p. 9–21, 1956. DOI: 10.7146/math.scand.a-10451. Disponível em: https://www.mscand.dk/article/view/10451. Acesso em: 5 may. 2024.