Aslanova, N. M., and M. Bairamogly. 2010. “Distribution of Eigenvalues and Trace Formula for the Sturm–Liouville Operator Equation”. Ukrains’kyi Matematychnyi Zhurnal 62 (7): 867–877. https://umj.imath.kiev.ua/index.php/umj/article/view/2920.