A Seifert Dichotomy for Z2-Harmonic One-Forms

Abstract: We establish a dichotomy for Z/2-harmonic one-forms on Seifert fibered rational homology three-spheres. For an oriented base with at most three exceptional fibers, we prove nonexistence along any fixed connection-metric family when the fibers are sufficiently collapsed. With at least four exceptional fibers, existing results give existence for every Riemannian metric. We also complete the corresponding classification for nonorientable bases.