Hi,

I'm trying to get a better understanding of multiplicities in geometric quantization, and so I've been concentrating on a specific simple case: let $\mathcal{O}\subset\mathfrak{g}^*$ be an integral coadjoint orbit of a compact semisimple Lie group $G$ with KKS symplectic form $\omega$, defined by $\omega_\mu(-\mathrm{ad}_\xi^*\mu,-\mathrm{ad}_\zeta^*\mu)=\mu([\xi,\zeta])$, and compatible line bundle-connection pair $(L,\nabla)$. Let $(L^*,\nabla^*)$ be the dual bundle, with base $(\mathcal{O}^-,-\omega)$. Put a totally complex polarization $F$ on $\mathcal{O}$, with corresponding complex structure $\mathcal{J}_F$, and the complex conjugate polarization $F^*$ on $\mathcal{O}^-$, with corresponding complex structure $\mathcal{J} _{F^*} = -\mathcal{J}_F$. The momentum map corresponding to the diagonal coadjoint $G$-action on $\mathcal{O}\times\mathcal{O}^-$ is $\mathrm{J}(\mu,\nu)=\mu-\nu$. The covariantly constant sections of $L$ form an irrep, and those of $L^*$ the dual irrep, so the $G$-invariant sections of $L\otimes L^*$ should then be one-dimensional, and I'm trying to verify this explicitly. Following some ideas in Guillemin and Sternberg's 1982 paper "Geometric quantization and multiplicities of group representations", I using the complex structure to to extend the $G$-action on $\mathcal{O}\times\mathcal{O}^-$ to a $G^\mathbb{C}$-action, by defining the infinitesimal generator corresponding to $i\xi\in i\mathfrak{g}$ to be $(i\xi)_{\mathcal{O}\times\mathcal{O^-}}=\mathcal{J}\xi_{\mathcal{O}\times\mathcal{O^-}}$, (where $\mathcal{J}$ is the combined complex structure) and exponentiating.

I want to show that the so-defined $G^\mathbb{C}$-action is transitive on the whole of $\mathcal{O}\times\mathcal{O}^-$, but I'm having trouble doing so (and maybe it's not?). The problem is that the action isn't free everywhere, and I don't really understand how it spans the tangent space at a general point $(\mu,\nu)$. I think it would help if I understood what $\ker T_{(\mu,\nu)}\mathrm{J}$ was everywhere. For example, if I can show that $\ker T_{(\mu,\nu)}\mathrm{J}\subset \mathfrak{g}\cdot(\mu,\nu)$, then since $\left(\mathfrak{g}\cdot(\mu,\nu)\right)^\omega = \ker T_{(\mu,\nu)}\mathrm{J}$, $\;\mathfrak{g}\cdot(\mu,\nu)$ will be coisotropic, and so $(i\mathfrak{g})\cdot(\mu,\nu)$ will be transversal to it (by general properties of complex structures). Then its just comes down to counting dimensions. But maybe this is the wrong track.

Any help appreciated. Thanks.