CHENG, X.; ZHANG, Z. Existence of positive solutions to systems of nonlinear integral or differential equations. Topological Methods in Nonlinear Analysis, [S. l.], v. 34, n. 2, p. 267–277, 2009. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2009.042. Acesso em: 15 apr. 2024.