Slyusarchuk, V. Y. (2007). Generalization of the Kneser theorem on zeros of solutions of the equation $y" + p (t) y = 0$. Ukrains’kyi Matematychnyi Zhurnal, 59(4), 571–576. Retrieved from https://umj.imath.kiev.ua/index.php/umj/article/view/3332