Kuzhel’S. A., and NizhnikL. P. “Singularly Perturbed Self-Adjoint Operators in Scales of Hilbert Spaces”. Ukrains’kyi Matematychnyi Zhurnal 59, no. 6 (June 25, 2007): 723–743. Accessed July 27, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3341.