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Section counts on the blow-up of a surface

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OAI.MaximalSeshadri.Geometry.Surface.exceptional_twist_sections_bounds

by wurtle · Nov 4, 2026 · Mathlib 0df444a (Lean v4.33.1)

maximal-seshadrioai-math

Let S,TS,TS,T be surfaces with LLL ample on SSS and AAA ample on TTT, ppp a configuration of rrr points on SSS, f:T→Sf:T\to Sf:T→S a blow-up of the centre ideal over C\mathbb CC, and J,ιJ,\iotaJ,ι presenting the pullback of the ideal. Then for a line bundle MMM on SSS and n∈Nn\in\mathbb Nn∈N, h0(T,Jn⊗f∗M)≤h0(S,M)h^0(T,J^n\otimes f^*M)\le h^0(S,M)h0(T,Jn⊗f∗M)≤h0(S,M) and 2h0(S,M)≤2h0(T,Jn⊗f∗M)+rn(n+1)2h^0(S,M)\le 2h^0(T,J^n\otimes f^*M)+rn(n+1)2h0(S,M)≤2h0(T,Jn⊗f∗M)+rn(n+1).

Notation. hnh^nhn is cohomologyDimension, the Module.finrank over C\mathbb CC of Ext⁡n(O,−)\operatorname{Ext}^n(\mathcal O,-)Extn(O,−), which is 0 for an infinite-dimensional space. PresentsPullbackIdeal I f J ι says that ι:J→OY\iota:J\to\mathcal O_Yι:J→OY​ is a monomorphism whose image on affine opens is the extension of III along fff. IsBlowup I π says that III pulls back to an invertible ideal along π\piπ and that π\piπ is universal with this property. IsAmple means that for each point xxx and open V∋xV\ni xV∋x, some positive power has a global section with nonvanishing open containing xxx, inside VVV, and affine. A Surface is an integral scheme smooth of relative dimension 2 over Spec⁡C\operatorname{Spec}\mathbb CSpecC with a closed immersion into some complex projective space over C\mathbb CC. Configuration S r is the set of rrr-tuples of complex points with distinct images, with the Zariski topology induced from the product.

Formalization Note The statement is verbatim from OpenAI's Lean development, with Definitions files of this development imported in place of the upstream modules; these are supporting code for the statement.

Preamble
import Mathlib
import Definitions.Def_MaximalSeshadriConstants
import Definitions.Def_SeshadriPrelim6

namespace OAI

section

namespace MaximalSeshadri.Geometry
noncomputable section
open AlgebraicGeometry CategoryTheory TopologicalSpace
open MaximalSeshadri.Frames MaximalSeshadri.ProjectiveBertini MaximalSeshadri.AlgebraicJets

Formal statement
theorem Surface.exceptional_twist_sections_bounds (S T : Surface)
    (L : LineBundle S.scheme) (hL : L.IsAmple)
    (A : LineBundle T.scheme) (hA : A.IsAmple)
    {r : ℕ} (p : Configuration S r) (f : T.scheme ⟶ S.scheme)
    (hf : f ≫ S.structureMap = T.structureMap)
    (hbl : IsBlowup (centreIdeal S r p) f)
    (J : LineBundle T.scheme) (ι : J.sheaf ⟶ O T.scheme)
    (hJ : PresentsPullbackIdeal (centreIdeal S r p) f J ι)
    (M : LineBundle S.scheme) (n : ℕ) :
    cohomologyDimension T.structureMap ((J.pow n).tensor (M.pullback f)).sheaf 0 ≤
      cohomologyDimension S.structureMap M.sheaf 0 ∧
    2*cohomologyDimension S.structureMap M.sheaf 0 ≤
      2*cohomologyDimension T.structureMap ((J.pow n).tensor (M.pullback f)).sheaf 0 + r*n*(n+1) := by
  sorry

end
end MaximalSeshadri.Geometry
end
end OAI
Source
OpenAI, Maximal Seshadri constants on arbitrary polarized surfaces, September 23, 2026, https://github.com/openai/math (paper: preprints/Maximal-Seshadri-Constants-on-Arbitrary-Polarized-Surfaces-September-23-2026/main.pdf; Lean: lean/OAI/AlgebraicGeometry/Seshadri, Apache License 2.0); Lean declaration `OAI.MaximalSeshadri.Geometry.Surface.exceptional_twist_sections_bounds`, https://github.com/openai/math/blob/adc7f1241b42e322a6451854ab7e4b4c146bf78a/lean/OAI/AlgebraicGeometry/Seshadri/Blowup/BlowupSectionDimensions.lean#L43

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