XU, J.; WEI, Z.; DING, Y. Existence of positive solutions for a second order periodic boundary value problem with impulsive effects. Topological Methods in Nonlinear Analysis, [S. l.], v. 43, n. 1, p. 11–22, 2016. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2014.002. Acesso em: 1 feb. 2023.