Du, Bo, Jianxin Zhao, and Weigao Ge. “Periodic Solutions for a Neutral Differential Equation With Variable Parameter”. Topological Methods in Nonlinear Analysis 33, no. 2 (June 1, 2009): 275–283. Accessed July 8, 2025. https://apcz.umk.pl/TMNA/article/view/TMNA.2009.017.