DERKACH, V. A. On Krein space symmetric linear relations with gaps. Methods of Functional Analysis and Topology, [S. l.], v. 4, n. 2, p. 16–40, 1998. Disponível em: https://zen.imath.kiev.ua/index.php/mfat/article/view/58. Acesso em: 19 dec. 2025.