The coherent cohomological dimension conjecture for Shimura varieties

Let (G,X)(G,X) be a Shimura datum with GadG^{ad} simple, and let ShK(G,X)Sh_K(G,X) be the associated Shimura variety of level KK. Its dimension is denoted by dim(ShK(G,X))\dim(Sh_K(G,X)), and rankQGad\operatorname{rank}_{\mathbb{Q}}G^{ad} denotes the Q\mathbb{Q}-rank of the adjoint group. The coherent cohomological dimension of a scheme XX is

ccd(X)=max{i0FCoh(X), Hi(X,F)0}.\operatorname{ccd}(X)=\max\left\{i\geq 0\mid \exists\mathcal{F}\in\operatorname{Coh}(X),\ H^i(X,\mathcal{F})\neq 0\right\}.

The coherent cohomological dimension conjecture. One conjectures that

ccd(ShK(G,X))dim(ShK(G,X))rankQGad.\operatorname{ccd}(Sh_K(G,X))\leq \dim(Sh_K(G,X))-\operatorname{rank}_{\mathbb{Q}}G^{ad}.

This is presented as a general conjecture extending the preceding prediction for Siegel modular varieties. The paper does not establish the inequality in this generality.

Sources & referencesView supporting material

Primary source

Haocheng Fan, “On the cohomological dimension of Siegel modular varieties and the modularity of formal Siegel modular forms”, arXiv:2511.00799 (2025).

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