LevinB. Y., and MirochnikL. Y. “Estimation of the Solutions of the Sturm-Liouville Equation”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 46, no. 3, Mar. 1994, pp. 244–278, https://umj.imath.kiev.ua/index.php/umj/article/view/5760.