B. P. Tkach, “Conditional stability of quasitoroidal manifolds of systems of partial differential equations”, Ukr. Mat. Zhurn., vol. 50, no. 3, pp. 401–408, Mar. 1998, Accessed: Feb. 17, 2026. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/4939