Konstantinov, A. Yu. “Point Spectrum of Singularly Perturbed Self-Adjoint Operators”. Ukrains’kyi Matematychnyi Zhurnal 57, no. 5 (May 25, 2005): 654–658. Accessed May 6, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3631.