Equivariant quantization of Poisson homogeneous spaces and Kostant's problem
Let $\mathfrak g$ be a finite dimensional split semisimple Lie algebra and $λ$ a weight of $\mathfrak g$. Let $F$ be the algebra of quantized regular functions on the connected simply connected group $G$ corresponding to $\mathfrak g$. In the present paper we introduce a certain subspace $F'$ of $F$ (which is not necessary a subalgebra of $F$) and endow it with an associative $\star$-product using the so-called reduced fusion element. We prove that the algebra $(F',\star)$ is isomorphic to $(L(λ))_{fin}$, where $L(λ)$ is the irreducible highest weight $\check{U}_q\mathfrak g$-module and "$fin$" stands for the subalgebra of the locally finite elements with respect to the adjoint action of $\check{U}_q\mathfrak g$. The introduced $\star$-product has some limiting properties what enables us to prove Kostant's problem for $\check{U}_q\mathfrak g$ in certain cases. We remind the reader that this means that $(L(λ))_{fin}$ coincides with the image of $\check{U}_q\g$ in $L(λ)$. We also note that if $λ$ is such that $<λ,α_i^\vee>=0$ for some simple roots $α_i$ and generic otherwise, then $(F,\star)$ is a $\check{U}_q\mathfrak g$-invariant quantization of the Poisson homogeneous space $G/K$, where $K$ is the stabilizer of $λ$.