The cohomology of \(C_2\)-surfaces with \({\underline{{\mathbb {Z}}}}\)-coefficients

Let \(C_2\) denote the cyclic group of order 2. We compute the \(RO(C_2)\)-graded cohomology of all \(C_2\)-surfaces with constant integral coefficients. We show when the action is nonfree, the answer depends only on the genus, the orientability of the underlying surface, the number of isolated fixed points, the number of fixed circles with trivial normal bundles, and the number of fixed circles with nontrivial normal bundles. When the action on the surface is free, we show the answer depends only on the genus, the orientability of the underlying surface, whether or not the action preserves the orientation, and one other invariant.