Documentation

Mathlib.AlgebraicGeometry.Cover.Open

Open covers of schemes #

This file provides the basic API for open covers of schemes.

Main definition #

@[reducible, inline]
abbrev AlgebraicGeometry.Scheme.OpenCover (X : Scheme) :
Type (max (v + 1) (u + 1))

An open cover of a scheme X is a cover where all component maps are open immersions.

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    @[deprecated AlgebraicGeometry.Scheme.Cover.map_prop (since := "2024-11-06")]
    theorem AlgebraicGeometry.Scheme.OpenCover.IsOpen {P : CategoryTheory.MorphismProperty Scheme} {X : Scheme} (self : Cover P X) (j : self.J) :
    P (self.map j)

    Alias of AlgebraicGeometry.Scheme.Cover.map_prop.


    the component maps satisfy P

    The affine cover of a scheme.

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      Every open cover of a quasi-compact scheme can be refined into a finite subcover.

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        @[simp]
        theorem AlgebraicGeometry.Scheme.OpenCover.finiteSubcover_map {X : Scheme} (𝒰 : X.OpenCover) [H : CompactSpace X.toPresheafedSpace] (x : { x : X.toPresheafedSpace // x .choose }) :
        𝒰.finiteSubcover.map x = 𝒰.map (𝒰.f x)
        @[simp]
        theorem AlgebraicGeometry.Scheme.OpenCover.finiteSubcover_obj {X : Scheme} (𝒰 : X.OpenCover) [H : CompactSpace X.toPresheafedSpace] (x : { x : X.toPresheafedSpace // x .choose }) :
        𝒰.finiteSubcover.obj x = 𝒰.obj (𝒰.f x)
        @[reducible, inline]
        abbrev AlgebraicGeometry.Scheme.AffineOpenCover (X : Scheme) :
        Type (max (v + 1) (u + 1))

        An affine open cover of X consists of a family of open immersions into X from spectra of rings.

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          The open cover associated to an affine open cover.

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            @[simp]
            @[simp]

            A choice of an affine open cover of a scheme.

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              Given any open cover 𝓤, this is an affine open cover which refines it. The morphism in the category of open covers which proves that this is indeed a refinement, see AlgebraicGeometry.Scheme.OpenCover.fromAffineRefinement.

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                The pullback of the affine refinement is the pullback of the affine cover.

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                  noncomputable def AlgebraicGeometry.Scheme.affineOpenCoverOfSpanRangeEqTop {R : CommRingCat} {ι : Type u_1} (s : ιR) (hs : Ideal.span (Set.range s) = ) :

                  A family of elements spanning the unit ideal of R gives a affine open cover of Spec R.

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                    @[simp]
                    theorem AlgebraicGeometry.Scheme.affineOpenCoverOfSpanRangeEqTop_f {R : CommRingCat} {ι : Type u_1} (s : ιR) (hs : Ideal.span (Set.range s) = ) (x : (Spec R).toPresheafedSpace) :
                    (affineOpenCoverOfSpanRangeEqTop s hs).f x = let_fun this := ; this.choose

                    Given any open cover 𝓤, this is an affine open cover which refines it.

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                      theorem AlgebraicGeometry.Scheme.OpenCover.ext_elem {X : Scheme} {U : X.Opens} (f g : (X.presheaf.obj (Opposite.op U))) (𝒰 : X.OpenCover) (h : ∀ (i : 𝒰.J), (CategoryTheory.ConcreteCategory.hom (Hom.app (𝒰.map i) U)) f = (CategoryTheory.ConcreteCategory.hom (Hom.app (𝒰.map i) U)) g) :
                      f = g

                      If two global sections agree after restriction to each member of an open cover, then they agree globally.

                      theorem AlgebraicGeometry.Scheme.zero_of_zero_cover {X : Scheme} {U : X.Opens} (s : (X.presheaf.obj (Opposite.op U))) (𝒰 : X.OpenCover) (h : ∀ (i : 𝒰.J), (CategoryTheory.ConcreteCategory.hom (Hom.app (𝒰.map i) U)) s = 0) :
                      s = 0

                      If the restriction of a global section to each member of an open cover is zero, then it is globally zero.

                      If a global section is nilpotent on each member of a finite open cover, then f is nilpotent.

                      The basic open sets form an affine open cover of Spec R.

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                        We may bind the basic open sets of an open affine cover to form an affine cover that is also a basis.

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                          The coordinate ring of a component in the affine_basis_cover.

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