BEREZANSKY, Yu. M.; BRASCHE, J. F. Generalized selfadjoint operators and their singular perturbations. Methods of Functional Analysis and Topology, [S. l.], v. 8, n. 4, p. 1–14, 2002. Disponível em: https://zen.imath.kiev.ua/index.php/mfat/article/view/197. Acesso em: 17 dec. 2025.