Extending free actions of finite groups on unoriented surfaces

Abstract: We present the unoriented version of the Schur and Bogomolov multiplier associated with a finite group $G$. We show that the unoriented Schur multiplier is isomorphic to the second cohomology group $H^2(G;\mathbb{Z}_2)$. We define the unoriented Bogomolov multiplier as the quotient of the unoriented Schur multiplier by the subgroup generated by classes over the disjoint union of tori, Klein bottles, and projective spaces. Disregarding the subgroup generated by the trivial $G$-bundle over the projective space, we show that the unoriented Bogomolov multiplier is a complete obstruction. This means that a free surface action over an unoriented surface equivariantly bounds if and only if the class in the unoriented Bogomolov multiplier is trivial. We show that the unoriented Bogomolov multiplier is trivial for abelian, dihedral, and symmetric groups. Because the group cohomology $H^2(G;\mathbb{Z}_2)$ is trivial for a group of odd order, there are plenty of examples where the usual Bogomolov multiplier is not trivial while the unoriented Bogomolov multiplier is trivial. However, we show that there is a group of order $64$ where the unoriented Bogomolov multiplier is non-trivial.