PerestyukN. A., and TkachA. B. “Periodic Solutions of a Weakly Nonlinear System of Partial Differential Equations With Pulse Influence”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 49, no. 4, Apr. 1997, pp. 601–605, https://umj.imath.kiev.ua/index.php/umj/article/view/5038.