REN, J.; ZHAI, C. Some properties of sets, fixed point theorems in ordered product spaces and applications to a nonlinear system of fractional differential equations. Topological Methods in Nonlinear Analysis, [S. l.], v. 49, n. 2, p. 625–645, 2017. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2016.095. Acesso em: 22 jul. 2024.