Modularity of formal Fourier--Jacobi series from a cohomological point of view
We investigate the modularity of formal Fourier--Jacobi series by establishing cohomological vanishing results for line bundles defined on compactifications of $\mathcal{A}_g$. Working over $\mathbb{C}$, we show that the minimal compactification of $\mathcal{A}_2$ has only rational singularities, which allows us to characterize, for sufficiently large weights, the modularity of formal Fourier--Jacobi series of genus~$2$ via cohomological vanishing. Working over $\mathbb{Z}$, we introduce a level $n\geq 3$ structure and, via the the resolution morphism from a toroidal compactification of $\mathcal{A}_{g,n}$ to its minimal compactification, we characterize the modularity of arithmetic formal Fourier--Jacobi series with a level $n\geq 3$ structure and sufficiently large weight via cohomological vanishing.