Mallet-Paret, John, Roger D. Nussbaum, and Panagiotis Paraskevopoulos. 1994. “Periodic Solutions for Functional Differential Equations With Multiple State-Depend Time Lags”. Topological Methods in Nonlinear Analysis 3 (1):101-62. https://apcz.umk.pl/TMNA/article/view/TMNA.1994.006.