K-orbit closures and Barbasch-Evens-Magyar varieties

TL;DR

This paper introduces Barbasch-Evens-Magyar varieties, showing they resolve singularities of symmetric orbit closures and possess natural symplectic structures, with explicit descriptions in type A.

Contribution

It defines Barbasch-Evens-Magyar varieties, proves their isomorphism to known smooth varieties, and explores their geometric and combinatorial properties, including stratification and moment polytopes.

Findings

Barbasch-Evens-Magyar varieties resolve certain singularities.

They inherit natural symplectic structures with Hamiltonian torus actions.

Explicit description of moment polytopes in type A.

Abstract

We define the Barbasch-Evens-Magyar varieties. We show they are isomorphic to the smooth varieties defined in [D.~Barbasch-S.~Evens '94] that map generically finitely to symmetric orbit closures, thereby giving resolutions of singularities in certain cases. Our definition parallels [P.~Magyar '98]'s construction of the Bott-Samelson varieties [H.~C.~Hansen '73, M.~Demazure '74]. From this alternative viewpoint, one deduces a graphical description in type $A$, stratification into closed subvarieties of the same kind, and determination of the torus-fixed points. Moreover, we explain how these manifolds inherit a natural symplectic structure with Hamiltonian torus action. We then express the moment polytope in terms of the moment polytope of a Bott-Samelson variety.