Slyusarchuk, V. E. 1996. “Refinement of Kneser Theorem on Zeros of Solutions of the Equation $y" + p(x)y = 0$”. Ukrains’kyi Matematychnyi Zhurnal 48 (4), 520-24. https://umj.imath.kiev.ua/index.php/umj/article/view/5314.