1. B.D. Boe, Homomorphisms between generalized Verma modules, Trans. Amer. Math. Soc. 288 (1985) no. 2, 791–799.
2. B.D. Boe and D.H. Collingwood, Intertwining operators between holomorphically induced modules, Pacific J. Math. 124 (1986), 73–84.
3. N. Bourbaki, Groupes et algébres de Lie, chapitres 4, 5 et 6, Hermann, 1968.
4. V.K. Dobrev, Canonical construction of differential operators intertwining representations of real semisimple Lie groups, Rep. Math. Phys. 25 (1988), 159–181.
5. V.K. Dobrev, Invariant differential operators for non-compact Lie algebras parabolically related to conformal Lie algebras, J. High Energy Phys. (2013), no. 2, 015, 41 pages.