BARANNIK , L. F.; LAGNO , V. I.; FUSHCHICH , V. I. Subalgebras of the Poincare algebra $AP (2, 3)$ and symmetry reduction of the nonlinear ultrahуperbolic d’Alambert equation. I. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 40, n. 4, p. 411–416, 1988. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/9173. Acesso em: 7 dec. 2025.