Slyusarchuk, V. Yu. “Equations in a Hilbert Space Whose Solution Sets Are Invariant With Respect to a Group Isomorphic to a One-Parameter Group of Unitary Operators”. Ukrains’kyi Matematychnyi Zhurnal 72, no. 1 (January 15, 2020): 86-99. Accessed June 14, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/687.