G. V. Lukashenko and G. M. Gubreev, “About nilpotent $C_0$-semigroups of operators in the Hilbert spaces and criteria for similarity to the integration operator”, MFAT, vol. 14, no. 1, pp. 60–66, Mar. 2008, Accessed: Feb. 04, 2026. [Online]. Available: https://zen.imath.kiev.ua/index.php/mfat/article/view/370