KONSTANTINOV, A. Yu. Point Spectrum of Singularly Perturbed Self-Adjoint Operators. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 57, n. 5, p. 654–658, 2005. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/3631. Acesso em: 5 dec. 2025.