Documentation

Mathlib.AlgebraicGeometry.IdealSheaf

Ideal sheaves on schemes #

We define ideal sheaves of schemes and provide various constructors for it.

Main definition #

Main results #

Implementation detail #

Ideal sheaves are not yet defined in this file as actual subsheaves of 𝒪ₓ. Instead, for the ease of development and application, we define the structure IdealSheafData containing all necessary data to uniquely define an ideal sheaf. This should be refactored as a constructor for ideal sheaves once they are introduced into mathlib.

A structure that contains the data to uniquely define an ideal sheaf, consisting of

  1. an ideal I(U) ≤ Γ(X, U) for every affine open U
  2. a proof that I(D(f)) = I(U)_f for every affine open U and every section f : Γ(X, U)
  3. a subset of X equal to the support.

Also see Scheme.IdealSheafData.mkOfMemSupportIff for a constructor with the condition on the support being (usually) easier to prove.

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    The largest ideal sheaf contained in a family of ideals.

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      The galois coinsertion between ideal sheaves and arbitrary families of ideals.

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        @[simp]
        theorem AlgebraicGeometry.Scheme.IdealSheafData.ideal_iSup {X : Scheme} {ι : Type u_1} {I : ιX.IdealSheafData} :
        (iSup I).ideal = ⨆ (i : ι), (I i).ideal
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        theorem AlgebraicGeometry.Scheme.IdealSheafData.ideal_biInf {X : Scheme} {ι : Type u_1} (I : ιX.IdealSheafData) {s : Set ι} (hs : s.Finite) :
        (⨅ is, I i).ideal = is, (I i).ideal
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        theorem AlgebraicGeometry.Scheme.IdealSheafData.ideal_iInf {X : Scheme} {ι : Type u_1} (I : ιX.IdealSheafData) [Finite ι] :
        (⨅ (i : ι), I i).ideal = ⨅ (i : ι), (I i).ideal

        A form of map_ideal that is easier to rewrite with.

        The support of an ideal sheaf. Also see IdealSheafData.mem_support_iff_of_mem.

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          def AlgebraicGeometry.Scheme.IdealSheafData.mkOfMemSupportIff {X : Scheme} (ideal : (U : X.affineOpens) → Ideal (X.presheaf.obj (Opposite.op U))) (map_ideal_basicOpen : ∀ (U : X.affineOpens) (f : (X.presheaf.obj (Opposite.op U))), Ideal.map (CommRingCat.Hom.hom (X.presheaf.map (CategoryTheory.homOfLE ).op)) (ideal U) = ideal (X.affineBasicOpen f)) (supportSet : Set X.toPresheafedSpace) (supportSet_inter : ∀ (U : X.affineOpens), xU, x supportSet x X.zeroLocus (ideal U)) :

          A useful constructor of IdealSheafData with the condition on supportSet being easier to check.

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            @[simp]
            theorem AlgebraicGeometry.Scheme.IdealSheafData.coe_support_mkOfMemSupportIff {X : Scheme} (ideal : (U : X.affineOpens) → Ideal (X.presheaf.obj (Opposite.op U))) (map_ideal_basicOpen : ∀ (U : X.affineOpens) (f : (X.presheaf.obj (Opposite.op U))), Ideal.map (CommRingCat.Hom.hom (X.presheaf.map (CategoryTheory.homOfLE ).op)) (ideal U) = ideal (X.affineBasicOpen f)) (supportSet : Set X.toPresheafedSpace) (supportSet_inter : ∀ (U : X.affineOpens), xU, x supportSet x X.zeroLocus (ideal U)) :
            (mkOfMemSupportIff ideal map_ideal_basicOpen supportSet supportSet_inter).support = supportSet
            @[simp]
            theorem AlgebraicGeometry.Scheme.IdealSheafData.mkOfMemSupportIff_ideal {X : Scheme} (ideal : (U : X.affineOpens) → Ideal (X.presheaf.obj (Opposite.op U))) (map_ideal_basicOpen : ∀ (U : X.affineOpens) (f : (X.presheaf.obj (Opposite.op U))), Ideal.map (CommRingCat.Hom.hom (X.presheaf.map (CategoryTheory.homOfLE ).op)) (ideal U) = ideal (X.affineBasicOpen f)) (supportSet : Set X.toPresheafedSpace) (supportSet_inter : ∀ (U : X.affineOpens), xU, x supportSet x X.zeroLocus (ideal U)) (U : X.affineOpens) :
            (mkOfMemSupportIff ideal map_ideal_basicOpen supportSet supportSet_inter).ideal U = ideal U

            The ideal sheaf induced by an ideal of the global sections.

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              Over affine schemes, ideal sheaves are in bijection with ideals of the global sections.

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                The nilradical of a scheme.

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                  The vanishing ideal sheaf of a closed set, which is the largest ideal sheaf whose support is equal to it. The reduced induced scheme structure on the closed set is the quotient of this ideal.

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                    support and vanishingIdeal forms a galois connection. This is the global version of PrimeSpectrum.gc.

                    The kernel of a morphism, defined as the largest (quasi-coherent) ideal sheaf contained in the component-wise kernel. This is usually only well-behaved when f is quasi-compact.

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                      The functor taking a morphism into Y to its kernel as an ideal sheaf on Y.

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                        Spec (𝒪ₓ(U)/I(U)), the object to be glued into the closed subscheme.

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                          Spec (𝒪ₓ(U)/I(U)) ⟶ Spec (𝒪ₓ(U)) = U, the closed immersion into U.

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                            The underlying space of Spec (𝒪ₓ(U)/I(U)) is homeomorphic to its image in X.

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                              The open immersion Spec Γ(𝒪ₓ/I, U) ⟶ Spec Γ(𝒪ₓ/I, V) if U ≤ V.

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                                The subscheme associated to an ideal sheaf.

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                                  The inclusion from the subscheme associated to an ideal sheaf.

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                                    See AlgebraicGeometry.Morphisms.ClosedImmersion for the closed immersion version.

                                    The subscheme associated to an ideal sheaf I is covered by Spec(Γ(X, U)/I(U)).

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                                      Γ(𝒪ₓ/I, U) ≅ 𝒪ₓ(U)/I(U).

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                                        Given I ≤ J, this is the map Spec(Γ(X, U)/J(U)) ⟶ Spec(Γ(X, U)/I(U)).

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                                          The inclusion of ideal sheaf induces an inclusion of subschemes

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