Couchouron, J.-F., and M. I. Kamenskiĭ. “Averaging Method for Neutral Differential Equations in Finite Dimension”. Topological Methods in Nonlinear Analysis, vol. 35, no. 2, Apr. 2010, pp. 221-33, https://apcz.umk.pl/TMNA/article/view/TMNA.2010.013.