Open immersions of structured spaces #
We say that a morphism of presheafed spaces f : X ⟶ Y
is an open immersion if
the underlying map of spaces is an open embedding f : X ⟶ U ⊆ Y
,
and the sheaf map Y(V) ⟶ f _* X(V)
is an iso for each V ⊆ U
.
Abbreviations are also provided for SheafedSpace
, LocallyRingedSpace
and Scheme
.
Main definitions #
AlgebraicGeometry.PresheafedSpace.IsOpenImmersion
: theProp
-valued typeclass asserting that a PresheafedSpace homf
is an open_immersion.AlgebraicGeometry.IsOpenImmersion
: theProp
-valued typeclass asserting that a Scheme morphismf
is an open_immersion.AlgebraicGeometry.PresheafedSpace.IsOpenImmersion.isoRestrict
: The source of an open immersion is isomorphic to the restriction of the target onto the image.AlgebraicGeometry.PresheafedSpace.IsOpenImmersion.lift
: Any morphism whose range is contained in an open immersion factors though the open immersion.AlgebraicGeometry.PresheafedSpace.IsOpenImmersion.toSheafedSpace
: Iff : X ⟶ Y
is an open immersion of presheafed spaces, andY
is a sheafed space, thenX
is also a sheafed space. The morphism as morphisms of sheafed spaces is given byto_SheafedSpace_hom
.AlgebraicGeometry.PresheafedSpace.IsOpenImmersion.toLocallyRingedSpace
: Iff : X ⟶ Y
is an open immersion of presheafed spaces, andY
is a locally ringed space, thenX
is also a locally ringed space. The morphism as morphisms of locally ringed spaces is given byto_LocallyRingedSpace_hom
.
Main results #
AlgebraicGeometry.PresheafedSpace.IsOpenImmersion.comp
: The composition of two open immersions is an open immersion.AlgebraicGeometry.PresheafedSpace.IsOpenImmersion.ofIso
: An iso is an open immersion.AlgebraicGeometry.PresheafedSpace.IsOpenImmersion.to_iso
: A surjective open immersion is an isomorphism.AlgebraicGeometry.PresheafedSpace.IsOpenImmersion.stalk_iso
: An open immersion induces an isomorphism on stalks.AlgebraicGeometry.PresheafedSpace.IsOpenImmersion.hasPullback_of_left
: Iff
is an open immersion, then the pullback(f, g)
exists (and the forgetful functor toTopCat
preserves it).AlgebraicGeometry.PresheafedSpace.IsOpenImmersion.pullbackSndOfLeft
: Open immersions are stable under pullbacks.AlgebraicGeometry.SheafedSpace.IsOpenImmersion.of_stalk_iso
An (topological) open embedding between two sheafed spaces is an open immersion if all the stalk maps are isomorphisms.
An open immersion of PresheafedSpaces is an open embedding f : X ⟶ U ⊆ Y
of the underlying
spaces, such that the sheaf map Y(V) ⟶ f _* X(V)
is an iso for each V ⊆ U
.
- base_open : OpenEmbedding ⇑f.base
the underlying continuous map of underlying spaces from the source to an open subset of the target.
- c_iso : ∀ (U : TopologicalSpace.Opens ↑↑X), CategoryTheory.IsIso (f.c.app (Opposite.op ((IsOpenMap.functor (_ : IsOpenMap ⇑f.base)).toPrefunctor.obj U)))
the underlying sheaf morphism is an isomorphism on each open subset
Instances
A morphism of SheafedSpaces is an open immersion if it is an open immersion as a morphism of PresheafedSpaces
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A morphism of LocallyRingedSpaces is an open immersion if it is an open immersion as a morphism of SheafedSpaces
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The functor opens X ⥤ opens Y
associated with an open immersion f : X ⟶ Y
.
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An open immersion f : X ⟶ Y
induces an isomorphism X ≅ Y|_{f(X)}
.
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- (_ : CategoryTheory.Mono f) = (_ : CategoryTheory.Mono f)
The composition of two open immersions is an open immersion.
For an open immersion f : X ⟶ Y
and an open set U ⊆ X
, we have the map X(U) ⟶ Y(U)
.
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A variant of app_inv_app
that gives an eq_to_hom
instead of hom_of_le
.
An isomorphism is an open immersion.
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- (_ : AlgebraicGeometry.PresheafedSpace.IsOpenImmersion H.hom) = (_ : AlgebraicGeometry.PresheafedSpace.IsOpenImmersion H.hom)
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An open immersion is an iso if the underlying continuous map is epi.
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(Implementation.) The projection map when constructing the pullback along an open immersion.
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We construct the pullback along an open immersion via restricting along the pullback of the maps of underlying spaces (which is also an open embedding).
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(Implementation.) Any cone over cospan f g
indeed factors through the constructed cone.
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The constructed pullback cone is indeed the pullback.
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- (_ : CategoryTheory.Limits.HasPullback f g) = (_ : CategoryTheory.Limits.HasLimit (CategoryTheory.Limits.cospan f g))
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- (_ : CategoryTheory.Limits.HasPullback g f) = (_ : CategoryTheory.Limits.HasPullback g f)
Open immersions are stable under base-change.
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- (_ : AlgebraicGeometry.PresheafedSpace.IsOpenImmersion CategoryTheory.Limits.pullback.snd) = (_ : AlgebraicGeometry.PresheafedSpace.IsOpenImmersion CategoryTheory.Limits.pullback.snd)
Open immersions are stable under base-change.
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- (_ : AlgebraicGeometry.PresheafedSpace.IsOpenImmersion CategoryTheory.Limits.pullback.fst) = (_ : AlgebraicGeometry.PresheafedSpace.IsOpenImmersion CategoryTheory.Limits.pullback.fst)
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The universal property of open immersions:
For an open immersion f : X ⟶ Z
, given any morphism of schemes g : Y ⟶ Z
whose topological
image is contained in the image of f
, we can lift this morphism to a unique Y ⟶ X
that
commutes with these maps.
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- AlgebraicGeometry.PresheafedSpace.IsOpenImmersion.lift f g H = CategoryTheory.CategoryStruct.comp (CategoryTheory.inv CategoryTheory.Limits.pullback.snd) CategoryTheory.Limits.pullback.fst
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Two open immersions with equal range is isomorphic.
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If X ⟶ Y
is an open immersion, and Y
is a SheafedSpace, then so is X
.
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- AlgebraicGeometry.PresheafedSpace.IsOpenImmersion.toSheafedSpace Y f = { toPresheafedSpace := X, IsSheaf := (_ : TopCat.Presheaf.IsSheaf X.presheaf) }
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If X ⟶ Y
is an open immersion of PresheafedSpaces, and Y
is a SheafedSpace, we can
upgrade it into a morphism of SheafedSpaces.
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If X ⟶ Y
is an open immersion, and Y
is a LocallyRingedSpace, then so is X
.
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If X ⟶ Y
is an open immersion of PresheafedSpaces, and Y
is a LocallyRingedSpace, we can
upgrade it into a morphism of LocallyRingedSpace.
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- (_ : CategoryTheory.Mono f) = (_ : CategoryTheory.Mono f)
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- (_ : CategoryTheory.Limits.HasPullback f g) = (_ : CategoryTheory.Limits.HasLimit (CategoryTheory.Limits.cospan f g))
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- (_ : CategoryTheory.Limits.HasPullback g f) = (_ : CategoryTheory.Limits.HasLimit (CategoryTheory.Limits.cospan g f))
Open immersions are stable under base-change.
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- (_ : AlgebraicGeometry.SheafedSpace.IsOpenImmersion CategoryTheory.Limits.pullback.snd) = (_ : AlgebraicGeometry.SheafedSpace.IsOpenImmersion CategoryTheory.Limits.pullback.snd)
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- (_ : AlgebraicGeometry.SheafedSpace.IsOpenImmersion CategoryTheory.Limits.pullback.fst) = (_ : AlgebraicGeometry.SheafedSpace.IsOpenImmersion CategoryTheory.Limits.pullback.fst)
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Suppose X Y : SheafedSpace C
, where C
is a concrete category,
whose forgetful functor reflects isomorphisms, preserves limits and filtered colimits.
Then a morphism X ⟶ Y
that is a topological open embedding
is an open immersion iff every stalk map is an iso.
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- (_ : CategoryTheory.Mono f) = (_ : CategoryTheory.Mono f)
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- (_ : AlgebraicGeometry.SheafedSpace.IsOpenImmersion (AlgebraicGeometry.LocallyRingedSpace.forgetToSheafedSpace.toPrefunctor.map f)) = H
An explicit pullback cone over cospan f g
if f
is an open immersion.
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The constructed pullbackConeOfLeft
is indeed limiting.
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- (_ : CategoryTheory.Limits.HasPullback f g) = (_ : CategoryTheory.Limits.HasLimit (CategoryTheory.Limits.cospan f g))
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- (_ : CategoryTheory.Limits.HasPullback g f) = (_ : CategoryTheory.Limits.HasPullback g f)
Open immersions are stable under base-change.
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- (_ : AlgebraicGeometry.LocallyRingedSpace.IsOpenImmersion CategoryTheory.Limits.pullback.snd) = (_ : AlgebraicGeometry.LocallyRingedSpace.IsOpenImmersion CategoryTheory.Limits.pullback.snd)
Open immersions are stable under base-change.
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- (_ : AlgebraicGeometry.LocallyRingedSpace.IsOpenImmersion CategoryTheory.Limits.pullback.fst) = (_ : AlgebraicGeometry.LocallyRingedSpace.IsOpenImmersion CategoryTheory.Limits.pullback.fst)
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The universal property of open immersions:
For an open immersion f : X ⟶ Z
, given any morphism of schemes g : Y ⟶ Z
whose topological
image is contained in the image of f
, we can lift this morphism to a unique Y ⟶ X
that
commutes with these maps.
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An open immersion is isomorphic to the induced open subscheme on its image.
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