On finite symmetries of simply connected four-manifolds

For most positive integer pairs $(a,b)$, the topological space $#a{\mathbb C \mathbb P}^2#b{\bar{\mathbb C \mathbb P^2}}$ is shown to admit infinitely many inequivalent smooth structures which dissolve upon performing a single connected sum with $S^2\times S^2$. This is then used to construct infinitely many non-equivalent smooth free actions of suitable finite groups on the connected sum #a{\mathbb C \mathbb P}^2#b{\bar{\mathbb C \mathbb P^2}}. We then investigate the behavior of the sign of the Yamabe invariant for the resulting finite covers, and observe that these constructions provide many new counter-examples to the $4$-dimensional Rosenberg Conjecture.