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
.