CODDINGTON, Earl 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://tidsskrift.dk/math/article/view/10451. Acesso em: 13 aug. 2026.