FEDORCHUK, V. M.; FEDORCHUK, V. I. On functional bases of the first-order differential invariants for non-conjugate subgroups of the Poincaré group P(1,4). Annales Universitatis Paedagogicae Cracoviensis Studia Mathematica, [S. l.], v. 7, p. 41–50, 2008. Disponível em: https://studmath.uken.krakow.pl/article/view/13268. Acesso em: 31 oct. 2026.