Vust's theorem and higher level Schur-Weyl duality for types B, C and D

Abstract: Let $G$ be a complex linear algebraic group, $\mathfrak{g}=\Lie(G)$ its Lie algebra and $e\in\mathfrak{g}$ a nilpotent element. Vust's theorem says that in case of $G=\GL(V)$, the algebra $\mbox{End}_{G_e}(V^{\otimes d})$, where $G_e\subset G$ is the stabilizer of $e$ under the adjoint action, is generated by the image of the natural action of $d$-th symmetric group $\mathfrak{S}_d$ and the linear maps ${1^{\otimes (i-1)}\otimes e\otimes1^{\otimes (d-i)}|i=1,\ldots,d}$. In this paper, we generalize this theorem to $G=\O(V)$ and $\SP(V)$ for nilpotent element $e$ with $\overline{G\cdot e}$ being normal. As an application, we study the higher Schur-Weyl duality in the sense of \cite{BK2} for types $B$, $C$ and $D$, which establishes a relationship between $W$-algebras and degenerate affine braid algebras.