CODDINGTON, Earl A. On maximal symmetrie ordinary differential operators. MATHEMATICA SCANDINAVICA, [S. l.], v. 4, p. 22–28, 1956. DOI: 10.7146/math.scand.a-10452. Disponível em: https://tidsskrift.dk/math/article/view/10452. Acesso em: 14 aug. 2026.