DUDKIN, M. Ye. Invariant symmetric restrictions of a self-adjoint operator. II. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 50, n. 6, p. 781–791, 1998. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4888. Acesso em: 17 apr. 2025.