AzizovT. Ya., GinsburgYu. P., and LangerH. 1994. “On M. G. Krein’s Papers in the Theory of Spaces With an Indefinite Metric”. Ukrains’kyi Matematychnyi Zhurnal 46 (1-2), 5–17. https://umj.imath.kiev.ua/index.php/umj/article/view/5766.