Slyusarchuk, V. Yu. 1995. “Necessary and Sufficient Conditions for Oscillation of Solutions of Nonlinear Pulse Systems With Multiplicatively Separated Right-Hand Side”. Ukrains’kyi Matematychnyi Zhurnal 47 (3), 381–389. https://umj.imath.kiev.ua/index.php/umj/article/view/5427.