A Cotangent Model for the Next-to-Minimal Orbit of E7

Abstract

Let $\mathcal O_{2A_1}$ be the next-to-minimal nilpotent orbit of the complex simple Lie algebra of type $E_7$. Let $\mathfrak p_7$ be the maximal parabolic subalgebra corresponding to the simple root $\alpha_7$, and let $\mathfrak u^+$ be its nilpotent radical. We prove that the affinization $T^*(\mathcal O_{2A_1}\cap\mathfrak u^+)^{\mathrm{aff}}$ of the cotangent bundle of $\mathcal O_{2A_1}\cap\mathfrak u^+$ is isomorphic to the nilpotent orbit closure $\overline{\mathcal O}_{2A_1}$ as affine varieties.