BALSLEV, E. Perturbation of Ordinary Differential Operators. MATHEMATICA SCANDINAVICA, [S. l.], v. 11, p. 131–148, 1962. DOI: 10.7146/math.scand.a-10659. Disponível em: https://www.mscand.dk/article/view/10659. Acesso em: 1 may. 2024.