KUZHEL', S. A.; NIZHNIK, L. P. Singularly perturbed self-adjoint operators in scales of Hilbert spaces. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 59, n. 6, p. 723–743, 2007. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/3341. Acesso em: 6 dec. 2025.