Ideal sheaves on schemes #
We define ideal sheaves of schemes and provide various constructors for it.
Main definition #
AlgebraicGeometry.Scheme.IdealSheafData
: A structure that contains the data to uniquely define an ideal sheaf, consisting of- an ideal
I(U) ≤ Γ(X, U)
for every affine openU
- a proof that
I(D(f)) = I(U)_f
for every affine openU
and every sectionf : Γ(X, U)
.
- an ideal
AlgebraicGeometry.Scheme.IdealSheafData.ofIdeals
: The largest ideal sheaf contained in a family of ideals.AlgebraicGeometry.Scheme.IdealSheafData.equivOfIsAffine
: Over affine schemes, ideal sheaves are in bijection with ideals of the global sections.AlgebraicGeometry.Scheme.IdealSheafData.support
: The support of an ideal sheaf.AlgebraicGeometry.Scheme.IdealSheafData.vanishingIdeal
: The vanishing ideal of a set.AlgebraicGeometry.Scheme.Hom.ker
: The kernel of a morphism.AlgebraicGeometry.Scheme.IdealSheafData.subscheme
: The subscheme associated to an ideal sheaf.AlgebraicGeometry.Scheme.IdealSheafData.subschemeι
: The inclusion from the subscheme.
Main results #
AlgebraicGeometry.Scheme.IdealSheafData.gc
:support
andvanishingIdeal
forms a galois connection.AlgebraicGeometry.Scheme.Hom.support_ker
: The support of a kernel of a quasi-compact morphism is the closure of the range.
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
- an ideal
I(U) ≤ Γ(X, U)
for every affine openU
- a proof that
I(D(f)) = I(U)_f
for every affine openU
and every sectionf : Γ(X, U)
- 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.
- ideal (U : ↑X.affineOpens) : Ideal ↑(X.presheaf.obj (Opposite.op ↑U))
The component of an ideal sheaf at an affine open.
- map_ideal_basicOpen (U : ↑X.affineOpens) (f : ↑(X.presheaf.obj (Opposite.op ↑U))) : Ideal.map (CommRingCat.Hom.hom (X.presheaf.map (CategoryTheory.homOfLE ⋯).op)) (self.ideal U) = self.ideal (X.affineBasicOpen f)
- supportSet : Set ↑↑X.toPresheafedSpace
The support of an ideal sheaf. Use
IdealSheafData.support
instead for most occasions. - supportSet_eq_iInter_zeroLocus : self.supportSet = ⋂ (U : ↑X.affineOpens), X.zeroLocus ↑(self.ideal U)
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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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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
.
Equations
- I.support = { carrier := I.supportSet, isClosed' := ⋯ }
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Custom simps projection for IdealSheafData
.
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A useful constructor of IdealSheafData
with the condition on supportSet
being easier to check.
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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 radical of a ideal sheaf.
Equations
- I.radical = AlgebraicGeometry.Scheme.IdealSheafData.mkOfMemSupportIff (fun (U : ↑X.affineOpens) => (I.ideal U).radical) ⋯ I.supportSet ⋯
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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.
Equations
- f.ker = AlgebraicGeometry.Scheme.IdealSheafData.ofIdeals fun (U : ↑Y.affineOpens) => RingHom.ker (CommRingCat.Hom.hom (f.app ↑U))
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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.
Equations
- I.glueDataObj U = AlgebraicGeometry.Spec (CommRingCat.of (↑(X.presheaf.obj (Opposite.op ↑U)) ⧸ I.ideal U))
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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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- I.glueDataObjMap h = AlgebraicGeometry.Spec.map (CommRingCat.ofHom (Ideal.quotientMap (I.ideal U) (CommRingCat.Hom.hom (X.presheaf.map (CategoryTheory.homOfLE h).op)) ⋯))
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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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