Boyko, O. P., O. M. Martynyuk, and V. N. Pivovarchik. “On Maximal Multiplicity of Eigenvalues of Finite-Dimensional Spectral Problem on a Graph”. Methods of Functional Analysis and Topology 25, no. 2 (June 25, 2019): 104–117. Accessed December 15, 2025. https://zen.imath.kiev.ua/index.php/mfat/article/view/715.