Samoilenko, A. M., V. G. Samoilenko, and V. S. Sobchuk. 1999. “On Periodic Solutions of the Equation of a Nonlinear Oscillator With Pulse Influence”. Ukrains’kyi Matematychnyi Zhurnal 51 (6): 827–834. https://umj.imath.kiev.ua/index.php/umj/article/view/4669.