The cohomology of $C_2$-surfaces with $\underline{\mathbb{Z}}$-coefficients

TL;DR

This paper computes the $RO(C_2)$-graded cohomology of all $C_2$-surfaces with constant integral coefficients, revealing how the cohomology depends on surface genus, orientability, and fixed point data.

Contribution

It provides a complete calculation of the $RO(C_2)$-graded cohomology for $C_2$-surfaces, detailing how it varies with fixed point structures and surface properties.

Findings

Cohomology depends on genus, orientability, fixed points, and fixed circles.

Nonfree actions' cohomology determined by fixed point data.

Free actions' cohomology depends on genus, orientability, and action type.

Abstract

Let $C_2$ denote the cyclic group of order 2. We compute the $RO(C_2)$-graded cohomology of all $C_2$-surfaces with constant integral coefficients. We show when the action is nonfree, the answer depends only on the genus, the orientability of the underlying surface, the number of isolated fixed points, the number of fixed circles with trivial normal bundles, and the number of fixed circles with nontrivial normal bundles. When the action on the surface is free, we show the answer depends only on the genus, the orientability of the underlying surface, whether the action is orientation preserving versus reversing in the orientable case, and one other invariant.