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Projections of the minimal nilpotent orbit in a simple Lie algebra and secant varieties
Published online by Cambridge University Press: 13 July 2023
Abstract
Let G be a simple algebraic group with ${\mathfrak g}={\textrm{Lie }} G$ and
${\mathcal O}_{\textsf{min}}\subset{\mathfrak g}$ the minimal nilpotent orbit. For a
${\mathbb Z}_2$-grading
${\mathfrak g}={\mathfrak g}_0\oplus{\mathfrak g}_1$, let
$G_0$ be a connected subgroup of G with
${\textrm{Lie }} G_0={\mathfrak g}_0$. We study the
$G_0$-equivariant projections
$\varphi\,:\,\overline{{\mathcal O}_{\textsf{min}}}\to {\mathfrak g}_0$ and
$\psi:\overline{{\mathcal O}_{\textsf{min}}}\to{\mathfrak g}_1$. It is shown that the properties of
$\overline{\varphi({\mathcal O}_{\textsf{min}})}$ and
$\overline{\psi({\mathcal O}_{\textsf{min}})}$ essentially depend on whether the intersection
${\mathcal O}_{\textsf{min}}\cap{\mathfrak g}_1$ is empty or not. If
${\mathcal O}_{\textsf{min}}\cap{\mathfrak g}_1\ne\varnothing$, then both
$\overline{\varphi({\mathcal O}_{\textsf{min}})}$ and
$\overline{\psi({\mathcal O}_{\textsf{min}})}$ contain a 1-parameter family of closed
$G_0$-orbits, while if
${\mathcal O}_{\textsf{min}}\cap{\mathfrak g}_1=\varnothing$, then both are
$G_0$-prehomogeneous. We prove that
$\overline{G{\cdot}\varphi({\mathcal O}_{\textsf{min}})}=\overline{G{\cdot}\psi({\mathcal O}_{\textsf{min}})}$. Moreover, if
${\mathcal O}_{\textsf{min}}\cap{\mathfrak g}_1\ne\varnothing$, then this common variety is the affine cone over the secant variety of
${\mathbb P}({\mathcal O}_{\textsf{min}})\subset{\mathbb P}({\mathfrak g})$. As a digression, we obtain some invariant-theoretic results on the affine cone over the secant variety of the minimal orbit in an arbitrary simple G-module. In conclusion, we discuss more general projections that are related to either arbitrary reductive subalgebras of
${\mathfrak g}$ in place of
${\mathfrak g}_0$ or spherical nilpotent G-orbits in place of
${\mathcal O}_{\textsf{min}}$.
MSC classification
- Type
- Research Article
- Information
- Mathematical Proceedings of the Cambridge Philosophical Society , Volume 175 , Issue 3 , November 2023 , pp. 595 - 624
- Copyright
- © The Author(s), 2023. Published by Cambridge University Press on behalf of Cambridge Philosophical Society
Footnotes
This research was funded by RFBR, project no. 20-01-00515.
References
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