HRUSHEVOI, R. V. On conditions under which the sum of self-adjoint operators with given integer spectra is a scalar operator. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 60, n. 4, p. 470–477, 2008. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/3169. Acesso em: 27 apr. 2025.