Finite group actions and G-monopole classes on smooth 4-manifolds

Abstract

On a smooth closed oriented $4$-manifold $M$ with a smooth action by a compact Lie group $G$, we define a $G$-monopole class as an element of $H^2(M;\Bbb Z)$ which is the first Chern class of a $G$-equivariant Spin$^c$ structure which has a solution of the Seiberg-Witten equations for any $G$-invariant Riemannian metric on $M$. We find $\Bbb Z_k$-monopole classes on some $\Bbb Z_k$-manifolds such as the connected sum of $k$ copies of a 4-manifold with nontrivial mod 2 Seiberg-Witten invariant or Bauer-Furuta invariant, where the $\Bbb Z_k$-action is a cyclic permutation of $k$ summands. As an application, we produce infinitely many exotic non-free actions of $\Bbb Z_k\oplus H$ on some connected sums of finite number of $S^2\times S^2$, $\Bbb CP_2$, $\overline{\Bbb CP}_2$, and $K3$ surfaces, where $k\geq 2$, and $H$ is any nontrivial finite group acting freely on $S^3$.