The Coxeter Flag Variety

TL;DR

This paper introduces the $c$-Coxeter flag variety, exploring its geometric structure, cohomology, and connections to noncrossing partitions, providing explicit descriptions and paving methods.

Contribution

It defines the $c$-Coxeter flag variety, constructs an affine paving, and describes its cohomology in terms of $c$-clusters and noncrossing partitions, linking geometry and combinatorics.

Findings

The $c$-Coxeter flag variety is the common vanishing locus of certain generalized Plücker coordinates.

An explicit affine paving of $ ext{CFl}_c$ is constructed.

The cohomology ring is described as permuted quasisymmetric coinvariants in type A.

Abstract

For a Coxeter element $c$ in a Weyl group $W$, we define the $c$-Coxeter flag variety $\operatorname{CFl}_c\subset G/B$ as the union of left-translated Richardson varieties $w^{-1}X^{wc}_w$. This is a complex of toric varieties whose geometry is governed by the lattice $\operatorname{NC}(W,c)$ of $c$-noncrossing partitions. We show that $\operatorname{CFl}_c$ is the common vanishing locus of the generalized Pl\"ucker coordinates indexed by $W\setminus\operatorname{NC}(W,c)$. We also construct an explicit affine paving of $\operatorname{CFl}_c$ and identify the $T$-weights of each cell in terms of $c$-clusters. This paving gives a GKM description of $H^\bullet(\operatorname{CFl}_c)$ and $H^\bullet_{T_{ad}}(\operatorname{CFl}_c)$ in terms of the induced Cayley subgraph on $\operatorname{NC}(W,c)$, and we show these rings are naturally isomorphic for different choices of $c$. In type $\mathrm{A}$, this recovers the quasisymmetric flag variety for a special $c$, and for general $c$ we show the cohomology ring has a presentation as permuted quasisymmetric coinvariants.