S. A. Kuzhel' and L. P. Nizhnik, “Singularly perturbed self-adjoint operators in scales of Hilbert spaces”, Ukr. Mat. Zhurn., vol. 59, no. 6, pp. 723–743, Jun. 2007, Accessed: Feb. 17, 2026. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/3341