Gomilko, A. M., and V. N. Pivovarchik. 2001. “Asymptotics of Solutions of the Sturm–Liouville Equation With Respect to a Parameter”. Ukrains’kyi Matematychnyi Zhurnal 53 (6), 742-57. https://umj.imath.kiev.ua/index.php/umj/article/view/4297.