Orlov, I.V., E.V. Bozhonok, and E.M. Kuzmenko. “Necessary Conditions for the K-Extremum of a Variational Functional in Sobolev Spaces over Multi-Dimensional Domains”. Reports of the National Academy of Sciences of Ukraine, no. 4 (February 17, 2025): 19–24. Accessed April 19, 2026. https://nasu-periodicals.org.ua/index.php/dp/article/view/15773.