YEHORCHENKO, I. A. Differential invariants, hidden and conditional symmetry. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 73, n. 8, p. 1023–1033, 2021. DOI: 10.37863/umzh.v73i8.6377. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/6377. Acesso em: 5 dec. 2025.