Curves on K3 surfaces in divisibility two

TL;DR

This paper proves a conjecture relating Gromov--Witten invariants of K3 surfaces for divisibility two classes to quasimodular forms, establishes the holomorphic anomaly equation, and introduces new techniques involving boundary induction and double ramification relations.

Contribution

It provides the first proof of the divisibility two case of the conjecture, linking Gromov--Witten invariants to quasimodular forms and establishing the holomorphic anomaly equation in all genus.

Findings

Gromov--Witten invariants expressed via quasimodular forms

Holomorphic anomaly equation established for divisibility two

New boundary induction and degeneration techniques introduced

Abstract

We prove a conjecture of Maulik, Pandharipande, and Thomas expressing the Gromov--Witten invariants of K3 surfaces for divisibility two curve classes in all genus in terms of weakly holomorphic quasimodular forms of level two. Then, we establish the holomorphic anomaly equation in divisibility two in all genus. Our approach involves a refined boundary induction, relying on the top tautological group of the moduli space of smooth curves, together with a degeneration formula for reduced virtual fundamental class with imprimitive curve classes. We use the double ramification relations with target variety as a new tool to prove the initial condition. The relationship between the holomorphic anomaly equation for higher divisibility and the conjectural multiple cover formula of Oberdieck and Pandharipande is discussed in detail and illustrated with several examples.