1.
Kuzhel' SA, Nizhnik LP. Singularly perturbed self-adjoint operators in scales of Hilbert spaces. Ukr. Mat. Zhurn. [Internet]. 2007 Jun. 25 [cited 2025 Dec. 6];59(6):723–743. Available from: https://umj.imath.kiev.ua/index.php/umj/article/view/3341