CHENG, R.; XU, J.; ZHANG, D. On the existence of periodic solutions for a class of non-autonomous differential delay equations. Topological Methods in Nonlinear Analysis, [S. l.], v. 35, n. 1, p. 139–154, 2010. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2010.009. Acesso em: 25 apr. 2024.