Makarov, V. L. “Exact and Approximate Solutions of Spectral Problems for the Schrödinger operator on (−∞,∞) With Polynomial Potential”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 70, no. 1, Jan. 2018, pp. 79-93, https://umj.imath.kiev.ua/index.php/umj/article/view/1543.