Petryshyn, R. I., et al. “Construction of an Integral Manifold of a Multifrequency Oscillation System With Fixed Times of Pulse Action”. Ukrains’kyi Matematychnyi Zhurnal, vol. 55, no. 5, May 2003, pp. 641-62, https://umj.imath.kiev.ua/index.php/umj/article/view/3940.