Aslanova, N. M., and M. Bairamogly. “Distribution of Eigenvalues and Trace Formula for the Sturm–Liouville Operator Equation”. Ukrains’kyi Matematychnyi Zhurnal, vol. 62, no. 7, July 2010, pp. 867–877, https://umj.imath.kiev.ua/index.php/umj/article/view/2920.