Fedorchuk, V. M., and V. I. Fedorchuk. “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 7 (November 8, 2008): 41–50. Accessed October 31, 2026. https://studmath.uken.krakow.pl/article/view/13268.