Dynamical deformations of three-dimensional Lie algebras in Bianchi classification over the harmonic oscillatorPaal, Eugen; Virkepu, JüriJournal of mathematical physics2009 / p. 053523-1 - 053523-8 Geodesic multiplication and the theory of gravityKuusk, Piret; Örd, Jüri; Paal, EugenJournal of mathematical physics1994 / 1, p. 321-334