DU, B.; ZHAO, J.; GE, W. Periodic solutions for a neutral differential equation with variable parameter. Topological Methods in Nonlinear Analysis, [S. l.], v. 33, n. 2, p. 275–283, 2009. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2009.017. Acesso em: 28 apr. 2024.