Cheng, R., Xu, J. and Zhang, D. (2010) “On the existence of periodic solutions for a class of non-autonomous differential delay equations”, Topological Methods in Nonlinear Analysis, 35(1), pp. 139–154. Available at: https://apcz.umk.pl/TMNA/article/view/TMNA.2010.009 (Accessed: 25 April 2024).