PERERA, K.; SCHECHTER, M. Sandwich pairs in $p$-Laplacian problems. Topological Methods in Nonlinear Analysis, [S. l.], v. 29, n. 1, p. 29–34, 2007. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2007.002. Acesso em: 22 jan. 2022.