SlyusarchukV. Y. “Necessary and Sufficient Conditions for Oscillation of Solutions of Nonlinear Pulse Systems With Multiplicatively Separated Right-Hand Side”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 47, no. 3, Mar. 1995, pp. 381–389, https://umj.imath.kiev.ua/index.php/umj/article/view/5427.