TkachB. P. “Conditional Stability of Quasitoroidal Manifolds of Systems of Partial Differential Equations”. Ukrains’kyi Matematychnyi Zhurnal 50, no. 3 (March 25, 1998): 401–408. Accessed July 27, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/4939.