Slyusarchuk, V. Y. “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, Vol. 72, no. 1, Jan. 2020, pp. 86-99, https://umj.imath.kiev.ua/index.php/umj/article/view/687.