AGARWAL, Ravi P. Triple solutions of constant sign for a system of fredholm integral equations. CUBO, A Mathematical Journal, [S. l.], v. 6, n. 3, p. 1–45, 2004. Disponível em: https://cubo.ufro.cl/index.php/cubo/article/view/1631. Acesso em: 7 dec. 2025.