AzizovT. Y., GinsburgY. P., & 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. Retrieved from https://umj.imath.kiev.ua/index.php/umj/article/view/5766