Molyboga, V. M. “Estimates for Periodic Eigenvalues of the Differential Operator $(-1)^md^2m Dx^2m+V$ With $V$-Distribution”. Methods of Functional Analysis and Topology 9, no. 2 (June 25, 2003): 163–178. Accessed December 17, 2025. https://zen.imath.kiev.ua/index.php/mfat/article/view/212.