Boichuk О. A., and S. A. Krivosheya. 1998. “Criterion of the Solvability of Matrix Equations of the Lyapunov Type”. Ukrains’kyi Matematychnyi Zhurnal 50 (8): 1021–1026. https://umj.imath.kiev.ua/index.php/umj/article/view/4851.