1. L. V. Ovsiannikov, Group Analysis of Differential Equations [in Russian], Nauka, Moscow (1978)
2. English translation: Academic Press, New York (1982)
3. A. V. Pogorelov, Multidimensional Minkowski Problem [in Russian], Nauka, Moscow (1975).
4. V. M. Fedorchuk, “Symmetry reduction and some exact solutions of a nonlinear five-dimensional wave equation,” Ukr. Mat. Zh., 48, No. 4, 573–577 (1996); English translation: Ukr. Math. J., 48, No. 4, 636–640 (1996); https://doi.org/10.1007/BF02390625.
5. V. M. Fedorchuk and V. I. Fedorchuk, “On the classification of low-dimensional nonconjugate subalgebras of the Lie algebra of the Poincaré group P(1,4),” in: Symmetry and Integrability of the Equations of Mathematical Physics: Collection of Works of the Institute of Mathematics, Nats. Akad. Nauk Ukr. [in Ukrainian], Vol. 3, No. 2 (2006), pp. 301–307.