DUDKIN, M. Ye. Invariant symmetric restrictions of a self-adjoint operator. I. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 50, n. 5, p. 623–631, 1998. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4900. Acesso em: 5 dec. 2025.