K-Invariants in the universal enveloping algebra of the Desitter Group. Alfredo Brega; Juan Tirao — 1987 Manuscripta mathematica
A Transversality property of a derivation of the universal enverloping algebra U (k), for SO (n,1) and SU (n,1). Alfredo Brega; Juan Tirao — 1992 Manuscripta mathematica