LуkоvаО. В. “Extension of a Theorem of N. N. Bogolyubov to the Case of a Hilbert Space ”. Ukrains’kyi Matematychnyi Zhurnal 18, no. 5 (August 31, 1966): 53-59. Accessed July 27, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/8440.