AZIZOV, T. Ya.; GINSBURG, Yu. P.; LANGER, H. On M. G. Krein’s papers in the theory of spaces with an indefinite metric. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 46, n. 1-2, p. 5–17, 1994. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5766. Acesso em: 5 dec. 2025.