Malamud , M. M. “Certain Classes for Expansions of the Hermitian Operator With Gaps”. Ukrains’kyi Matematychnyi Zhurnal 44, no. 2 (February 28, 1992): 215–233. Accessed December 6, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/7836.