The equivariant degree and an enriched count of rational cubics

Abstract

We define the equivariant degree and local degree of a proper $G$-equivariant map between smooth $G$-manifolds when $G$ is a compact Lie group and prove a local to global result. We show the local degree can be used to compute the equivariant Euler characteristic of a smooth, compact $G$-manifold and the Euler number of a relatively oriented $G$-equivariant vector bundle when $G$ is finite. As an application, we give an equivariantly enriched count of rational plane cubics through a $G$-invariant set of 8 general points in $\mathbb{C}\mathbb{P}^2$, valued in the representation ring and Burnside ring of a finite group. When $\mathbb{Z}/2$ acts by pointwise complex conjugation this recovers a signed count of real rational cubics.