Urmanchev, V. I. “Qualitative Investigation of Discontinuous Dynamical Systems on a Plane by the Method of Pointwise Mappings”. Ukrains’kyi Matematychnyi Zhurnal, vol. 56, no. 1, Jan. 2004, pp. 108-1, https://umj.imath.kiev.ua/index.php/umj/article/view/3732.