Slyusarchuk, V. Yu. “Generalization of the Kneser Theorem on Zeros of Solutions of the Equation $y" + P (t) Y = 0$”. *Ukrains’kyi Matematychnyi Zhurnal* 59, no. 4 (April 25, 2007): 571–576. Accessed December 9, 2023. https://umj.imath.kiev.ua/index.php/umj/article/view/3332.