SHAPOVAL, N. The point symmetry group of a system of free second-order equations . Reports of the National Academy of Sciences of Ukraine, Kyiv, Ukraine, n. 6, p. 32–36, 2025. DOI: 10.15407/dopovidi2014.06.032. Disponível em: https://nasu-periodicals.org.ua/index.php/dp/article/view/16538. Acesso em: 19 apr. 2026.