Mod 2 cohomology of 2-configuration space of a closed surface and Stiefel--Whitney class

Abstract

In this paper, we compute the singular cohomology groups $H^*(C_2(M);\mathbb{F}_2)$ of the ordered 2-configuration space $C_2(M)$ as $\Sigma_2$-representations. Using the result, we determine the mod 2 cohomology of the unordered 2-configuration space $B_2(M)$ as a $H^*(\mathbb{R}P^{\infty};\mathbb{F}_2)$-module. As a corollary of our computation, we see that the Stiefel--Whitney height of $M$ is $2$ or $3$ when $M$ is orientable or not, respectively.