SvistunO. P. “On the Existence of Piecewise-Continuous Separatrix Curves for a Certain Class of Pulse Systems”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 46, no. 5, May 1994, pp. 580–585, https://umj.imath.kiev.ua/index.php/umj/article/view/5720.