Slyusarchuk, V. Yu. 2010. “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 (12), 1705–1714. https://umj.imath.kiev.ua/index.php/umj/article/view/2993.