Katz's intersection-cohomology Newton–Hodge inequality

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Let XX be a projective variety over a number field FF, let nn be an integer, and let FsF^s be a separable closure of FF. For every prime number ℓ\ell and maximal ideal p\mathfrak{p} of OF\mathscr{O}_F of residue characteristic different from ℓ\ell, consider the Newton polygon NP⁡(Frob⁡p∣IHn(X⊗FFs,Qℓ))\operatorname{NP}(\operatorname{Frob}_{\mathfrak{p}}|_{IH^n(X\otimes_FF^s,\mathbb{Q}_{\ell})}) and the Hodge–Tate polygon HTP⁡(IHn(X⊗FFs,Qℓ))\operatorname{HTP}(IH^n(X\otimes_FF^s,\mathbb{Q}_{\ell})).

Katz's intersection-cohomology Newton–Hodge inequality. There exists a finite set S=S(X,n)S=S(X,n) of maximal ideals of OF\mathscr{O}_F such that for every prime number ℓ\ell and every maximal ideal p\mathfrak{p} outside SS with residue characteristic different from ℓ\ell,

NP⁡(Frob⁡p∣IHn(X⊗FFs,Qℓ))≥HTP⁡(IHn(X⊗FFs,Qℓ)).\operatorname{NP}(\operatorname{Frob}_{\mathfrak{p}}|_{IH^n(X\otimes_FF^s,\mathbb{Q}_{\ell})})\ge \operatorname{HTP}(IH^n(X\otimes_FF^s,\mathbb{Q}_{\ell})).

The source notes that this is known when XX is smooth, by results of Katz, Messing, Mazur, and Faltings; the singular case is the conjectural part.

References

Primary source

Junecue Suh, “Ordinary primes in Hilbert modular varieties”, arXiv:2410.01182 (2024).

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