Stratified Langlands duality in the $A_n$ tower

TL;DR

This paper explores the structure of extended quotients related to Langlands duality in the $A_n$ tower, revealing homotopy equivalences and cohomological isomorphisms through algebraic and topological analysis.

Contribution

It provides a detailed description of the extended quotients as bundles over unions of tori and establishes a homotopy equivalence reflecting Langlands duality in the $A_n$ setting.

Findings

Extended quotients are described as bundles over unions of tori.

Homotopy equivalence between $T_k // W$ and its dual $T_{n/k} // W$ is established.

Cohomology of the extended quotients is shown to be stratified and isomorphic under duality.

Abstract

Let $\mathbf{S}_k$ denote a maximal torus in the complex Lie group $\mathbf{G} = \mathrm{SL}_n(\mathbb{C})/C_k$ and let $T_k$ denote a maximal torus in its compact real form $\mathrm{SU}_n(\mathbb{C})/C_k$, where $k$ divides $n$. Let $W$ denote the Weyl group of $\mathbf{G}$, namely the symmetric group $\mathfrak{S}_n$. We elucidate the structure of the extended quotient $\mathbf{S}_k // W$ as an algebraic variety and of $T_k // W$ as a topological space, in both cases describing them as bundles over unions of tori. Corresponding to the invariance of $K$-theory under Langlands duality, this calculation provides a homotopy equivalence between $T_k // W$ and its dual $T_{\frac{n}{k}} // W$. Hence there is an isomorphism in cohomology for the extended quotients which is stratified as a direct sum over conjugacy classes of the Weyl group. We use our formula to compute a number of examples.