On the homomorphisms between the generalized Verma modules arising from   conformally invariant systems
  It is shown by Barchini, Kable, and Zierau that conformally invariant systems of differential operators yield explicit homomorphisms between certain generalized Verma modules. In this paper we determine whether or not the homomorphisms arising from such systems of first and second order differential operators associated to maximal parabolic subalgebras of quasi-Heisenberg type are standard. 
Toshihisa Kubo
math.RT