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An Application of Spherical Geometry to Hyperkähler Slices

Part of: Linear algebraic groups and related topics Special varieties Global differential geometry

Published online by Cambridge University Press:  24 February 2020

Peter Crooks  and
Maarten van Pruijssen
Peter Crooks
Affiliation:
Department of Mathematics, Northeastern University, 360 Huntington Ave., Boston, MA02115, USA e-mail: p.crooks@northeastern.edu
Maarten van Pruijssen
Affiliation:
Department of Mathematics, Radboud Universiteit Nijmegen, P.O. Box 9010, 6500 GL Nijmegen, Netherlands e-mail: m.vanpruijssen@math.ru.nl
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Abstract

This work is concerned with Bielawski’s hyperkähler slices in the cotangent bundles of homogeneous affine varieties. One can associate such a slice with the data of a complex semisimple Lie group $G$, a reductive subgroup $H\subseteq G$, and a Slodowy slice $S\subseteq \mathfrak{g}:=\text{Lie}(G)$, defining it to be the hyperkähler quotient of $T^{\ast }(G/H)\times (G\times S)$ by a maximal compact subgroup of $G$. This hyperkähler slice is empty in some of the most elementary cases (e.g., when $S$ is regular and $(G,H)=(\text{SL}_{n+1},\text{GL}_{n})$, $n\geqslant 3$), prompting us to seek necessary and sufficient conditions for non-emptiness.

We give a spherical-geometric characterization of the non-empty hyperkähler slices that arise when $S=S_{\text{reg}}$ is a regular Slodowy slice, proving that non-emptiness is equivalent to the so-called $\mathfrak{a}$-regularity of $(G,H)$. This $\mathfrak{a}$-regularity condition is formulated in several equivalent ways, one being a concrete condition on the rank and complexity of $G/H$. We also provide a classification of the $\mathfrak{a}$-regular pairs $(G,H)$ in which $H$ is a reductive spherical subgroup. Our arguments make essential use of Knop’s results on moment map images and Losev’s algorithm for computing Cartan spaces.

Keywords

hyperkähler quotientSlodowy slicespherical geometry

MSC classification

Primary: 20G20: Linear algebraic groups over the reals, the complexes, the quaternions
Secondary: 53C26: Hyper-Kähler and quaternionic Kähler geometry, “special” geometry 14M17: Homogeneous spaces and generalizations
Type
Article
Information
Canadian Journal of Mathematics , Volume 73 , Issue 3 , June 2021 , pp. 687 - 716
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Copyright
© Canadian Mathematical Society 2020

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Footnotes

The first author is supported by the Natural Sciences and Engineering Research Council of Canada [PDF–516638–2018].

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