Kovalev, A.M., and V.N. Nespirnyy. “Method of Additional Functions in Stability Theory for Nonautonomous Systems of Differential Equations”. Reports of the National Academy of Sciences of Ukraine, no. 8 (March 1, 2025): 20–27. Accessed May 10, 2026. https://nasu-periodicals.org.ua/index.php/dp/article/view/16605.