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. Ukr. Mat. Zhurn. 1988, 40 (4), 411-416.