Slyusarchuk, V. Yu. “Strengthening of the Kneser Theorem on Zeros of Solutions of the Equation $u″ + q(t)u = 0$ Using One Functional Equation”. Ukrains’kyi Matematychnyi Zhurnal 62, no. 12 (December 25, 2010): 1705–1714. Accessed April 28, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/2993.