Cohomology of fixed point sets of anti-symplectic involutions in the Hilbert scheme of points on a surface

Abstract

Let $S$ be a smooth, quasi-projective complex surface with complex symplectic form $ω\in H^0(S, K_S)$. This determines a symplectic form $ω_n$ on the Hilbert scheme of points $S^{[n]}$ for $n \geq 1$. Let $τ$ be an anti-symplectic involution of $(S,ω)$: an order two automorphism of $S$ such that $ τ^*ω=-ω$. Then $τ$ induces an anti-symplectic involution on $(S^{[n]},ω_n)$ and the fixed point set $(S^{[n]})^τ$ is a smooth Lagrangian subvariety of $S^{[n]}$. In this paper, we calculate the mixed Hodge structure of $H^*( (S^{[n]})^τ; \mathbb{Q})$ in terms of the mixed Hodge structures of $H^*( S^τ;\mathbb{Q})$ and of $H^*( S / τ; \mathbb{Q})$. We also classify the connected components of $(S^{[n]})^τ$ and determine their mixed Hodge structures. Our results apply more generally whenever $S$ is a smooth quasi-projective surface, and $τ$ is an involution of $S$ for which $S^τ$ is a curve.