Classifications of special double-coverings associated to a non-orientable surface

TL;DR

This paper classifies special double-coverings related to non-orientable surfaces using group actions, analyzing orbit structures and comparing different classification methods.

Contribution

It introduces two new group action approaches to classify special double-coverings over non-orientable surfaces and compares these with existing weak-equivalence classifications.

Findings

Determines the number of orbits under group actions

Identifies the size of each orbit in the classification

Shows independence of results from certain choices in most cases

Abstract

This paper investigates some actions "\`a la Johnson" on the set, denoted by ${\cal E}$, of Spin-structures which are interpreted as special double-coverings of a trivial $S^1-$fibration over a non-orientable surface $N_{g+1}$. The group acting is first a group of orthogonal isomorphisms assoiciated to $N_{g+1}$. A second approach is to consider the subspace of ${\cal E}$ (with $2^{g}$ elements) coming from special double-coverings of $S^1\times F_g$, where $F_g$ is the orientation covering of $N_{g+1}$. The group acting now is a subgroup of the group of symplectic isomorphisms associated to $F_{g}$. In both situations, we obtain results on the number of orbits and the number of elements in each orbit. Except in one case, these results do not depend on any necessary choices. We compare both previous classifications to a third one: weak-equivalence of coverings