YavorskiiYu. M. “Estimation of Eigenvalues of Self-Adjoint Operators”. Ukrains’kyi Matematychnyi Zhurnal 46, no. 5 (May 25, 1994): 642–648. Accessed July 27, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/5725.