A Gap Theorem for Half-Conformally Flat Manifolds

We show that any compact half-conformally flat manifold of negative type, with bounded $L^2$ energy, sufficiently small scalar curvature, and a non-collapsing assumption, has all betti numbers bounded. We show that this result is optimal from an analytic perspective by demonstrating singularity models that are 2-ended, and are asymptotically K\"ahler on both ends. We show that bounded self-dual solutions of $d\omega=0$ on ALE manifold ends are either asymptotically K\"ahler, or they have a decay rate of $O(r^{-4})$ or better.