Surgery of real symplectic fourfolds and Welschinger invariants

TL;DR

This paper studies how surgeries along real Lagrangian spheres affect genus 0 Welschinger invariants of real symplectic 4-manifolds, leading to explicit computations for del Pezzo surfaces and conic bundles.

Contribution

It provides simplified formulas for Welschinger invariants after surgeries, completes their computation for key surfaces, and extends results to higher genus curve enumeration.

Findings

Explicit formulas for invariants after surgeries

Complete genus 0 Welschinger invariants for del Pezzo surfaces

Existence of new relative Welschinger invariants

Abstract

A surgery of a real symplectic manifold $X_{\mathbb R}$ along a real Lagrangian sphere $S$ is a modification of the symplectic and real structure on $X_{\mathbb R}$ in a neigborhood of $S$. Genus 0 Welschinger invariants of two real symplectic $4$-manifolds differing by such a surgery have been related in a previous work in collaboration with N. Puignau. In the present paper, we explore some particular situations where these general formulas greatly simplify. As an application, we complete the computation of genus 0 Welschinger invariants of all del~Pezzo surfaces, and of all $\mathbb R$-minimal real conic bundles. As a by-product, we establish the existence of some new relative Welschinger invariants. We also generalize our results to the enumeration of curves of higher genus, and give relations between hypothetical invariants defined in the same vein as a previous work by Shustin.