Samoilenko, A. M., Samoilenko, V. G., & Sobchuk, V. S. (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