Samoilenko, A. M., et al. “On Periodic Solutions of the Equation of a Nonlinear Oscillator With Pulse Influence”. Ukrains’kyi Matematychnyi Zhurnal, vol. 51, no. 6, June 1999, pp. 827–834, https://umj.imath.kiev.ua/index.php/umj/article/view/4669.