Bantsur, N. R., and E. P. Trofimchuk. 1998. “On the Existence and Stability of Periodic and Almost Periodic Solutions of Quasilinear Equations With Maxima”. Ukrains’kyi Matematychnyi Zhurnal 50 (6): 747–754. https://umj.imath.kiev.ua/index.php/umj/article/view/4884.