COSTA, David G. A First Encounter with Variational Methods in Differential Equations. CUBO, A Mathematical Journal, [S. l.], v. 4, n. 1, p. 248–278, 2002. Disponível em: https://cubo.ufro.cl/index.php/cubo/article/view/1733. Acesso em: 12 oct. 2025.