The orbit-inequality conjecture for automorphic forms on Hodge-type Shimura varieties

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Let SKS_K be the special fiber of a Hodge-type Shimura variety at a prime pp of good reduction which splits in the reflex field E\mathbf E. Let GG be the attached reductive group over Fp\mathbb F_p, assumed split over Fp2\mathbb F_{p^2}, and let L\mathbf L be the Levi subgroup determined by the Hodge cocharacter. Write WLW_{\mathbf L} for its Weyl group, and let Φ+\Phi^+ and ΦL+\Phi^+_{\mathbf L} denote the positive roots of GG and L\mathbf L. For a weight λ∈X∗(T)\lambda\in X^*(\mathbf T), a nonzero form f∈H0(SK,VI(λ))f\in H^0(S_K,\mathcal V_I(\lambda)) has weight λ\lambda. The orbit-inequality conjecture. For every WL⋊Gal⁡(Fp2/Fp)W_{\mathbf L}\rtimes\operatorname{Gal}(\mathbb F_{p^2}/\mathbb F_p)-orbit O⊂Φ+∖ΦL+\mathcal O\subset\Phi^+\setminus\Phi^+_{\mathbf L} and every subset S⊂OS\subset\mathcal O, one has

∑α∈O∖S⟨λ,α∨⟩+1p∑α∈S⟨λ,α∨⟩≤0.\sum_{\alpha\in\mathcal O\setminus S}\langle\lambda,\alpha^\vee\rangle+\frac{1}{p}\sum_{\alpha\in S}\langle\lambda,\alpha^\vee\rangle\leq 0.

Equivalently, the space H0(SK,VI(λ))H^0(S_K,\mathcal V_I(\lambda)) vanishes outside the locus defined by these inequalities. This is an explicit consequence predicted by the zip-cone conjecture and gives an upper bound on possible weights; the paper presents it as conjectural because the underlying equality of saturated cones is not known in general.

References

Primary source

Jean-Stefan Koskivirta, “A vanishing theorem for vector-valued Siegel automorphic forms in characteristic p”, arXiv:2308.06870 (2024).

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