On the monopole Lefschetz number of finite order diffeomorphisms

Let $K$ be a knot in an integral homology 3-sphere $Y$, and $\mathrm{\Sigma }$ the corresponding $n$-fold cyclic branched cover. Assuming that $\mathrm{\Sigma }$ is a rational homology sphere (which is always the case when $n$ is a prime power), we give a formula for the Lefschetz number of the action that the covering translation induces on the reduced monopole homology of $\mathrm{\Sigma }$. The proof relies on a careful analysis of the Seiberg–Witten equations on 3-orbifolds and of various $\eta$-invariants. We give several applications of our formula: (1) we calculate the Seiberg–Witten and Furuta–Ohta invariants for the mapping tori of all semi-free actions of $Z\mathrm{/}n$ on integral homology 3-spheres; (2) we give a novel obstruction (in terms of the Jones polynomial) for the branched cover of a knot in $S^3$ being an $L$-space; (3) we give a new set of knot concordance invariants in terms of the monopole Lefschetz numbers of covering translations on the branched covers.