D. S. Bihun, O. O. Pokutnyi, and E. V. Panasenko, “Autonomous nonlinear boundary-value problems for the Lyapunov equation in Hilbert space”, Ukr. Mat. Zhurn., vol. 73, no. 7, pp. 867–878, Jul. 2021, doi: 10.37863/umzh.v73i7.6691.