Mikhailets, V. A., and V. M. Molyboga. “Uniform Estimates for the Semi-Periodic Eigenvalues of the Singular Differential Operators”. Methods of Functional Analysis and Topology 10, no. 4 (December 25, 2004): 30–57. Accessed December 16, 2025. https://zen.imath.kiev.ua/index.php/mfat/article/view/257.