Palyutkin, V. G. “Distribution of Eigenvalues of the Sturm-Liouville Problem With Slowly Increasing Potential”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 48, no. 6, June 1996, pp. 813-25, https://umj.imath.kiev.ua/index.php/umj/article/view/5281.