PerestyukN. A., and TkachA. B. 1997. “Periodic Solutions of a Weakly Nonlinear System of Partial Differential Equations With Pulse Influence”. Ukrains’kyi Matematychnyi Zhurnal 49 (4), 601–605. https://umj.imath.kiev.ua/index.php/umj/article/view/5038.