NIZHNIK, L. P. On rank one singular perturbations of selfadjoint operators. Methods of Functional Analysis and Topology, [S. l.], v. 7, n. 3, p. 54–66, 2001. Disponível em: https://zen.imath.kiev.ua/index.php/mfat/article/view/157. Acesso em: 18 dec. 2025.