A. Y. Konstantinov, “Point Spectrum of Singularly Perturbed Self-Adjoint Operators”, Ukr. Mat. Zhurn., vol. 57, no. 5, pp. 654–658, May 2005, Accessed: Feb. 16, 2026. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/3631