Slyusarchuk, V. Y. (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