1. G. A. Jn. Baker. Einstein numbers. Amer. Math. Monthly, 61(1):39–41, 1954.
2. J. F. Barrett. Special relativity and hyperbolic geometry. Univ. Sunderland, Sunderland, UK, 1998. Physical Interpretations of Relativity Theory. Proceedings, London, UK, 11–14 Sept. 1998.
3. N. A. Černikov. Connection between the theory of relativity and Lobačevskii geometry. In Gravitation and Theory of Relativity, No. 2 (Russian), pages 9–22. Izdat. Kazan. Univ., Kazan, 1965.
4. Albert Einstein. Zur Elektrodynamik Bewegter Körper [on the electrodynamics of moving bodies]. Ann. Physik (Leipzzg), 17:891–921, 1905.
5. Tomás Feder. Strong near subgroups and left gyrogroups. J. Algebra, 259(1):177–190, 2003.