V. Khrabustovskyi, “Eigenfunction expansions associated with an operator differential equation non-linearly depending on a spectral parameter”, MFAT, vol. 20, no. 1, pp. 68–91, Mar. 2014, Accessed: Dec. 16, 2025. [Online]. Available: https://zen.imath.kiev.ua/index.php/mfat/article/view/568