Documentation

Mathlib.Geometry.RingedSpace.OpenImmersion

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 #

Main results #

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.

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    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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            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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              (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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                      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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                        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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                            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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                                  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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                                  The functor Opens X ⥤ Opens Y associated with an open immersion f : X ⟶ Y.

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                                    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 eqToHom instead of homOfLe.

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                                      theorem AlgebraicGeometry.SheafedSpace.IsOpenImmersion.ofRestrict_invApp_apply {C : Type u_1} [CategoryTheory.Category.{u_2, u_1} C] (X : AlgebraicGeometry.SheafedSpace C) {Y : TopCat} {f : Y TopCat.of X.toPresheafedSpace} (h : Topology.IsOpenEmbedding f) (U : TopologicalSpace.Opens (X.restrict h).toPresheafedSpace) [inst : CategoryTheory.ConcreteCategory C] (x : (CategoryTheory.forget C).obj ((X.restrict h).presheaf.obj (Opposite.op U))) :

                                      An open immersion is an iso if the underlying continuous map is epi.

                                      theorem AlgebraicGeometry.SheafedSpace.IsOpenImmersion.image_preimage_is_empty {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Limits.HasLimits C] {ι : Type v} (F : CategoryTheory.Functor (CategoryTheory.Discrete ι) (AlgebraicGeometry.SheafedSpace C)) [CategoryTheory.Limits.HasColimit F] (i j : CategoryTheory.Discrete ι) (h : i j) (U : TopologicalSpace.Opens (F.obj i).toPresheafedSpace) :
                                      (TopologicalSpace.Opens.map (CategoryTheory.Limits.colimit.ι (F.comp AlgebraicGeometry.SheafedSpace.forgetToPresheafedSpace) j).base).obj ((TopologicalSpace.Opens.map (CategoryTheory.preservesColimitIso AlgebraicGeometry.SheafedSpace.forgetToPresheafedSpace F).inv.base).obj (.functor.obj U)) =

                                      An explicit pullback cone over cospan f g if f is an open immersion.

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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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                                            The functor Opens X ⥤ Opens Y associated with an open immersion f : X ⟶ Y.

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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.

                                              theorem AlgebraicGeometry.LocallyRingedSpace.IsOpenImmersion.ofRestrict_invApp_apply (X : AlgebraicGeometry.LocallyRingedSpace) {Y : TopCat} {f : Y TopCat.of X.toPresheafedSpace} (h : Topology.IsOpenEmbedding f) (U : TopologicalSpace.Opens (X.restrict h).toPresheafedSpace) (x : (CategoryTheory.forget CommRingCat).obj ((X.restrict h).presheaf.obj (Opposite.op U))) :