Orlov, I., Bozhonok, E. and Kuzmenko, E. 2025. 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. 4 (Feb. 2025), 19–24. DOI:https://doi.org/10.15407/dopovidi2014.04.019.