YAVORSKII, Yu. M. Estimation of eigenvalues of self-adjoint operators. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 46, n. 5, p. 642–648, 1994. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5725. Acesso em: 5 dec. 2025.