Generic pipe dreams, lower-upper varieties, and Schwartz-MacPherson classes

TL;DR

This paper introduces a unified approach to calculating equivariant cohomology classes of lower-upper varieties using generic pipe dreams, connecting classical formulas and deriving new degree and characteristic class formulas.

Contribution

It provides a new formula for equivariant cohomology classes of lower-upper varieties via generic pipe dreams, unifying and extending existing Schubert polynomial formulas.

Findings

Derived a formula for the degree of the nth commuting variety as a sum of powers of 2.

Connected generic pipe dreams to the Segre-Schwarz-MacPherson classes and orbit classes.

Reproduced classical and bumpless pipe dream formulas as limits of the new approach.

Abstract

We recall the lower-upper varieties from [Knutson '05] and give a formula for their equivariant cohomology classes, as a sum over generic pipe dreams. We recover as limits the classic and bumpless pipe dream formulae for double Schubert polynomials. As a byproduct, we obtain a formula for the degree of the $n$th commuting variety as a sum of powers of 2. Generic pipe dreams also appear in the Segre-Schwarz-MacPherson analogue of the AJS/Billey formula, and when computing the Chern-Schwarz-MacPherson class of the orbit $B_- w B_+ \subseteq Mat_{k\times n}$ or of a double Bruhat cell $B_-u B_+ \cap B_+ v B_-$.