ELOE, Paul W. Positive Operators and Maximum Principles for Ordinary Differential Equations. CUBO, A Mathematical Journal, [S. l.], v. 7, n. 2, p. 237–260, 2005. Disponível em: https://cubo.ufro.cl/index.php/cubo/article/view/1551. Acesso em: 9 jan. 2026.