Pivovarchik, V. N., O. M. Martynyuk, and O. P. Boyko. “On a Generalization of the Three Spectral Inverse Problem”. Methods of Functional Analysis and Topology 22, no. 1 (March 25, 2016): 74–80. Accessed December 16, 2025. https://zen.imath.kiev.ua/index.php/mfat/article/view/628.