Lуkоvа О. В. “Extension of a Theorem of N. N. Bogolyubov to the Case of a Hilbert Space ”. Ukrains’kyi Matematychnyi Zhurnal, vol. 18, no. 5, Aug. 1966, pp. 53-59, https://umj.imath.kiev.ua/index.php/umj/article/view/8440.