Classifications of special double-coverings associated to a non-orientable surface
Résumé
This paper investigates some actions "á 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