CODDINGTON, E. 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://www.mscand.dk/article/view/10452. Acesso em: 1 may. 2024.