Lian's quasimodularity conjecture for elliptic Hurwitz loci

From papers

Fix an integer g2g\ge2. Let Mg\overline{\mathcal{M}}_g be the moduli space of stable curves of genus gg, and for each d1d\ge1 let ϕg/1,d\phi_{g/1,d} be the morphism from the compactified Hurwitz space of simply branched degree-dd covers from genus-gg curves to genus-11 curves, remembering the source curve and its ramification points. Write (ϕg/1,d)(1)(\phi_{g/1,d})_*(1) for the resulting cycle class, and let Ag1(Mg)A^{g-1}(\overline{\mathcal{M}}_g) denote the codimension-g1g-1 Chow group. Let Qmod\operatorname{Qmod} be the ring of quasimodular forms. Lian's quasimodularity conjecture. The generating series of these Hurwitz loci satisfies

d1[(ϕg/1,d)(1)]qdAg1(Mg)Qmod.\sum_{d\ge1}[(\phi_{g/1,d})_*(1)]q^d\in A^{g-1}(\overline{\mathcal{M}}_g)\otimes\operatorname{Qmod}.

The conjecture predicts that the elliptic Hurwitz loci exhibit the same quasimodular-form structure familiar from generating series of curve-counting invariants. Its status is not established by the supplied source context.

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Sources & referencesView supporting material

Primary source

Carl Lian, “The H-tautological ring”, arXiv:2011.11565 (2021).

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