R. V. Hrushevoi, “On conditions under which the sum of self-adjoint operators with given integer spectra is a scalar operator”, Ukr. Mat. Zhurn., vol. 60, no. 4, pp. 470–477, Apr. 2008, Accessed: Apr. 23, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/3169