Properties of morphisms between Schemes #
We provide the basic framework for talking about properties of morphisms between Schemes.
A MorphismProperty Scheme is a predicate on morphisms between schemes. For properties local at
the target, its behaviour is entirely determined by its definition on morphisms into affine schemes,
which we call an AffineTargetMorphismProperty. In this file, we provide API lemmas for properties
local at the target, and special support for those properties whose AffineTargetMorphismProperty
takes on a more simple form. We also provide API lemmas for properties local at the target.
The main interfaces of the API are the typeclasses IsLocalAtTarget, IsLocalAtSource and
HasAffineProperty, which we describle in detail below.
IsLocalAtTarget #
AlgebraicGeometry.IsLocalAtTarget: We say thatIsLocalAtTarget PforP : MorphismProperty Schemeif
Prespects isomorphisms.Pholds forf ∣_ Ufor an open coverUofYif and only ifPholds forf.
For a morphism property P local at the target and f : X ⟶ Y, we provide these API lemmas:
AlgebraicGeometry.IsLocalAtTarget.of_isPullback:Pis preserved under pullback along open immersions.AlgebraicGeometry.IsLocalAtTarget.restrict:P f → P (f ∣_ U)for an openUofY.AlgebraicGeometry.IsLocalAtTarget.iff_of_iSup_eq_top:P f ↔ ∀ i, P (f ∣_ U i)for a familyU iof open sets coveringY.AlgebraicGeometry.IsLocalAtTarget.iff_of_openCover:P f ↔ ∀ i, P (𝒰.pullbackHom f i)for𝒰 : Y.openCover.
IsLocalAtSource #
AlgebraicGeometry.IsLocalAtSource: We say thatIsLocalAtSource PforP : MorphismProperty Schemeif
Prespects isomorphisms.Pholds for𝒰.map i ≫ ffor an open cover𝒰ofXiffPholds forf : X ⟶ Y.
For a morphism property P local at the source and f : X ⟶ Y, we provide these API lemmas:
AlgebraicGeometry.IsLocalAtTarget.comp:Pis preserved under composition with open immersions at the source.AlgebraicGeometry.IsLocalAtTarget.iff_of_iSup_eq_top:P f ↔ ∀ i, P (U.ι ≫ f)for a familyU iof open sets coveringX.AlgebraicGeometry.IsLocalAtTarget.iff_of_openCover:P f ↔ ∀ i, P (𝒰.map i ≫ f)for𝒰 : X.openCover.AlgebraicGeometry.IsLocalAtTarget.of_isOpenImmersion: IfPcontains identities thenPholds for open immersions.
AffineTargetMorphismProperty #
AlgebraicGeometry.AffineTargetMorphismProperty: The type of predicates onf : X ⟶ YwithYaffine.AlgebraicGeometry.AffineTargetMorphismProperty.IsLocal: We say thatP.IsLocalifPsatisfies the assumptions of the affine communication lemma (AlgebraicGeometry.of_affine_open_cover). That is,Prespects isomorphisms.- If
Pholds forf : X ⟶ Y, thenPholds forf ∣_ Y.basicOpen rfor any global sectionr. - If
Pholds forf ∣_ Y.basicOpen rfor allrin a spanning set of the global sections, thenPholds forf.
HasAffineProperty #
AlgebraicGeometry.HasAffineProperty:HasAffineProperty P Qis a type class asserting thatPis local at the target, and over affine schemes, it is equivalent toQ : AffineTargetMorphismProperty.
For HasAffineProperty P Q and f : X ⟶ Y, we provide these API lemmas:
AlgebraicGeometry.HasAffineProperty.of_isPullback:Pis preserved under pullback along open immersions from affine schemes.AlgebraicGeometry.HasAffineProperty.restrict:P f → Q (f ∣_ U)for affineUofY.AlgebraicGeometry.HasAffineProperty.iff_of_iSup_eq_top:P f ↔ ∀ i, Q (f ∣_ U i)for a familyU iof affine open sets coveringY.AlgebraicGeometry.HasAffineProperty.iff_of_openCover:P f ↔ ∀ i, P (𝒰.pullbackHom f i)for affine open covers𝒰ofY.AlgebraicGeometry.HasAffineProperty.isStableUnderBaseChange_mk: IfQis stable under affine base change, thenPis stable under arbitrary base change.
We say that P : MorphismProperty Scheme is local at the target if
Prespects isomorphisms.Pholds forf ∣_ Ufor an open coverUofYif and only ifPholds forf. Also seeIsLocalAtTarget.mk'for a convenient constructor.
- respectsIso : P.RespectsIso
Prespects isomorphisms. - iff_of_openCover' {X Y : Scheme} (f : X ⟶ Y) (𝒰 : Y.OpenCover) : P f ↔ ∀ (i : 𝒰.1), P (Scheme.Cover.pullbackHom 𝒰 f i)
Pholds forf ∣_ Ufor an open coverUofYif and only ifPholds forf.
Instances
P is local at the target if
Prespects isomorphisms.- If
Pholds forf : X ⟶ Y, thenPholds forf ∣_ Ufor anyU. - If
Pholds forf ∣_ Ufor an open coverUofY, thenPholds forf.
The intersection of two morphism properties that are local at the target is again local at the target.
We say that P : MorphismProperty Scheme is local at the source if
Prespects isomorphisms.Pholds for𝒰.map i ≫ ffor an open cover𝒰ofXiffPholds forf : X ⟶ Y. Also seeIsLocalAtSource.mk'for a convenient constructor.
- respectsIso : P.RespectsIso
Prespects isomorphisms. - iff_of_openCover' {X Y : Scheme} (f : X ⟶ Y) (𝒰 : X.OpenCover) : P f ↔ ∀ (i : 𝒰.J), P (CategoryTheory.CategoryStruct.comp (𝒰.map i) f)
Pholds forf ∣_ Ufor an open coverUofYif and only ifPholds forf.
Instances
P is local at the source if
Prespects isomorphisms.- If
Pholds forf : X ⟶ Y, thenPholds forU.ι ≫ ffor anyU. - If
Pholds forU.ι ≫ ffor an open coverUofX, thenPholds forf.
The intersection of two morphism properties that are local at the target is again local at the target.
If P is local at the source, then it respects composition on the left with open immersions.
If P is local at the source and the target, then restriction on both source and target
preserves P.
If P is local at the source, local at the target and is stable under post-composition with
open immersions, then P can be checked locally around points.
An AffineTargetMorphismProperty is a class of morphisms from an arbitrary scheme into an
affine scheme.
Equations
- AlgebraicGeometry.AffineTargetMorphismProperty = (⦃X Y : AlgebraicGeometry.Scheme⦄ → (X ⟶ Y) → [AlgebraicGeometry.IsAffine Y] → Prop)
Instances For
The restriction of a MorphismProperty Scheme to morphisms with affine target.
Equations
Instances For
An AffineTargetMorphismProperty can be extended to a MorphismProperty such that it
never holds when the target is not affine
Equations
- P.toProperty f = ∃ (h : AlgebraicGeometry.IsAffine x✝), P f
Instances For
We say that P : AffineTargetMorphismProperty is a local property if
Prespects isomorphisms.- If
Pholds forf : X ⟶ Y, thenPholds forf ∣_ Y.basicOpen rfor any global sectionr. - If
Pholds forf ∣_ Y.basicOpen rfor allrin a spanning set of the global sections, thenPholds forf.
- respectsIso : P.toProperty.RespectsIso
Pas a morphism property respects isomorphisms - to_basicOpen {X Y : Scheme} [IsAffine Y] (f : X ⟶ Y) (r : ↑(Y.presheaf.obj (Opposite.op ⊤))) : P f → P (f ∣_ Y.basicOpen r)
Pis stable under restriction to basic open set of global sections. - of_basicOpenCover {X Y : Scheme} [IsAffine Y] (f : X ⟶ Y) (s : Finset ↑(Y.presheaf.obj (Opposite.op ⊤))) : Ideal.span ↑s = ⊤ → (∀ (r : { x : ↑(Y.presheaf.obj (Opposite.op ⊤)) // x ∈ s }), P (f ∣_ Y.basicOpen ↑r)) → P f
PforfifPholds forfrestricted to basic sets of a spanning set of the global sections
Instances
A P : AffineTargetMorphismProperty is stable under base change if P holds for Y ⟶ S
implies that P holds for X ×ₛ Y ⟶ X with X and S affine schemes.
Equations
- One or more equations did not get rendered due to their size.
Instances For
For a P : AffineTargetMorphismProperty, targetAffineLocally P holds for
f : X ⟶ Y whenever P holds for the restriction of f on every affine open subset of Y.
Equations
- AlgebraicGeometry.targetAffineLocally P f = ∀ (U : ↑Y.affineOpens), P (f ∣_ ↑U)
Instances For
HasAffineProperty P Q is a type class asserting that P is local at the target, and over affine
schemes, it is equivalent to Q : AffineTargetMorphismProperty.
To make the proofs easier, we state it instead as
Qis local at the targetP fif and only if∀ U, Q (f ∣_ U)ranging over all affine opens ofU. SeeHasAffineProperty.iff.
- isLocal_affineProperty : AffineTargetMorphismProperty.IsLocal Q
Instances
Every property local at the target can be associated with an affine target property. This is not an instance as the associated property can often take on simpler forms.