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feat(AlgebraicGeometry/Morphisms): affine + ring hom property (#18356)
We add a constructor for properties of affine morphisms of schemes satisfying a given property of ring homomorphisms on sections.
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‎Mathlib.lean

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@@ -893,6 +893,7 @@ import Mathlib.AlgebraicGeometry.Modules.Presheaf
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import Mathlib.AlgebraicGeometry.Modules.Sheaf
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import Mathlib.AlgebraicGeometry.Modules.Tilde
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import Mathlib.AlgebraicGeometry.Morphisms.Affine
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import Mathlib.AlgebraicGeometry.Morphisms.AffineAnd
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import Mathlib.AlgebraicGeometry.Morphisms.Basic
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import Mathlib.AlgebraicGeometry.Morphisms.ClosedImmersion
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import Mathlib.AlgebraicGeometry.Morphisms.Constructors

‎Mathlib/AlgebraicGeometry/AffineScheme.lean

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@@ -574,6 +574,35 @@ lemma appLE_eq_away_map {X Y : Scheme.{u}} (f : X ⟶ Y) {U : Y.Opens} (hU : IsA
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RingHom.algebraMap_toAlgebra, RingHom.algebraMap_toAlgebra, ← CommRingCat.comp_eq_ring_hom_comp,
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Scheme.Hom.appLE_map, Scheme.Hom.map_appLE]
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lemma app_basicOpen_eq_away_map {X Y : Scheme.{u}} (f : X ⟶ Y) {U : Y.Opens}
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(hU : IsAffineOpen U) (h : IsAffineOpen (f ⁻¹ᵁ U)) (r : Γ(Y, U)) :
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haveI := hU.isLocalization_basicOpen r
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haveI := h.isLocalization_basicOpen (f.app U r)
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f.app (Y.basicOpen r) =
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IsLocalization.Away.map Γ(Y, Y.basicOpen r) Γ(X, X.basicOpen (f.app U r)) (f.app U) r ≫
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X.presheaf.map (eqToHom (by simp)).op := by
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haveI := hU.isLocalization_basicOpen r
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haveI := h.isLocalization_basicOpen (f.app U r)
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apply IsLocalization.ringHom_ext (.powers r)
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rw [← CommRingCat.comp_eq_ring_hom_comp, ← CommRingCat.comp_eq_ring_hom_comp,
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IsLocalization.Away.map, ← Category.assoc]
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nth_rw 3 [CommRingCat.comp_eq_ring_hom_comp]
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rw [IsLocalization.map_comp, RingHom.algebraMap_toAlgebra, ← CommRingCat.comp_eq_ring_hom_comp,
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RingHom.algebraMap_toAlgebra, Category.assoc, ← X.presheaf.map_comp]
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simp
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/-- `f.app (Y.basicOpen r)` is isomorphic to map induced on localizations
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`Γ(Y, Y.basicOpen r) ⟶ Γ(X, X.basicOpen (f.app U r))` -/
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def appBasicOpenIsoAwayMap {X Y : Scheme.{u}} (f : X ⟶ Y) {U : Y.Opens}
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(hU : IsAffineOpen U) (h : IsAffineOpen (f ⁻¹ᵁ U)) (r : Γ(Y, U)) :
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haveI := hU.isLocalization_basicOpen r
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haveI := h.isLocalization_basicOpen (f.app U r)
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Arrow.mk (f.app (Y.basicOpen r)) ≅
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Arrow.mk (IsLocalization.Away.map Γ(Y, Y.basicOpen r)
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Γ(X, X.basicOpen (f.app U r)) (f.app U) r) :=
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Arrow.isoMk (Iso.refl _) (X.presheaf.mapIso (eqToIso (by simp)).op) <| by
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simp [hU.app_basicOpen_eq_away_map f h]
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include hU in
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theorem isLocalization_of_eq_basicOpen {V : X.Opens} (i : V ⟶ U) (e : V = X.basicOpen f) :
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@IsLocalization.Away _ _ f Γ(X, V) _ (X.presheaf.map i.op).toAlgebra := by

‎Mathlib/AlgebraicGeometry/Morphisms/AffineAnd.lean

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/-
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Copyright (c) 2024 Christian Merten. All rights reserved.
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Released under Apache 2.0 license as described in the file LICENSE.
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Authors: Christian Merten
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-/
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import Mathlib.AlgebraicGeometry.Morphisms.Affine
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import Mathlib.AlgebraicGeometry.Morphisms.RingHomProperties
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/-!
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# Affine morphisms with additional ring hom property
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In this file we define a constructor `affineAnd Q` for affine target morphism properties of schemes
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from a property of ring homomorphisms `Q`: A morphism `f : X ⟶ Y` with affine target satisfies
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`affineAnd Q` if it is an affine morphim (i.e. `X` is affine) and the induced ring map on global
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sections satisfies `Q`.
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`affineAnd Q` inherits most stability properties of `Q` and is local at the target if `Q` is local
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at the (algebraic) source.
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Typical examples of this are affine morphisms (where `Q` is trivial), finite morphisms
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(where `Q` is module finite) or closed immersions (where `Q` is being surjective).
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-/
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universe v u
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open CategoryTheory TopologicalSpace Opposite MorphismProperty
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namespace AlgebraicGeometry
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section
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variable (Q : ∀ {R S : Type u} [CommRing R] [CommRing S], (R →+* S) → Prop)
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/-- This is the affine target morphism property where the source is affine and
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the induced map of rings on global sections satisfies `P`. -/
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def affineAnd : AffineTargetMorphismProperty :=
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fun X _ f ↦ IsAffine X ∧ Q (f.app ⊤)
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@[simp]
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lemma affineAnd_apply {X Y : Scheme.{u}} (f : X ⟶ Y) [IsAffine Y] :
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affineAnd Q f ↔ IsAffine X ∧ Q (f.app ⊤) :=
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Iff.rfl
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attribute [local simp] AffineTargetMorphismProperty.toProperty_apply
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variable {Q}
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/-- If `P` respects isos, also `affineAnd P` respects isomorphisms. -/
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lemma affineAnd_respectsIso (hP : RingHom.RespectsIso Q) :
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(affineAnd Q).toProperty.RespectsIso := by
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refine RespectsIso.mk _ ?_ ?_
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· intro X Y Z e f ⟨hZ, ⟨hY, hf⟩⟩
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simpa [hP.cancel_right_isIso, isAffine_of_isIso e.hom]
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· intro X Y Z e f ⟨hZ, hf⟩
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simpa [AffineTargetMorphismProperty.toProperty, isAffine_of_isIso e.inv, hP.cancel_left_isIso]
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/-- `affineAnd P` is local if `P` is local on the (algebraic) source. -/
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lemma affineAnd_isLocal (hPi : RingHom.RespectsIso Q) (hQl : RingHom.LocalizationPreserves Q)
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(hQs : RingHom.OfLocalizationSpan Q) : (affineAnd Q).IsLocal where
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respectsIso := affineAnd_respectsIso hPi
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to_basicOpen {X Y _} f r := fun ⟨hX, hf⟩ ↦ by
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simp only [Opens.map_top] at hf
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constructor
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· simp only [Scheme.preimage_basicOpen, Opens.map_top]
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exact (isAffineOpen_top X).basicOpen _
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· dsimp only [Opens.map_top]
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rw [morphismRestrict_app, hPi.cancel_right_isIso, Scheme.Opens.ι_image_top]
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rw [(isAffineOpen_top Y).app_basicOpen_eq_away_map f (isAffineOpen_top X),
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hPi.cancel_right_isIso]
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haveI := (isAffineOpen_top X).isLocalization_basicOpen (f.app ⊤ r)
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apply hQl
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exact hf
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of_basicOpenCover {X Y _} f s hs hf := by
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dsimp [affineAnd] at hf
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haveI : IsAffine X := by
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apply isAffine_of_isAffineOpen_basicOpen (f.app ⊤ '' s)
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· apply_fun Ideal.map (f.app ⊤) at hs
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rwa [Ideal.map_span, Ideal.map_top] at hs
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· rintro - ⟨r, hr, rfl⟩
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simp_rw [Scheme.preimage_basicOpen] at hf
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exact (hf ⟨r, hr⟩).left
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refine ⟨inferInstance, hQs.ofIsLocalization' hPi (f.app ⊤) s hs fun a ↦ ?_⟩
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refine ⟨Γ(Y, Y.basicOpen a.val), Γ(X, X.basicOpen (f.app ⊤ a.val)), inferInstance,
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inferInstance, inferInstance, inferInstance, inferInstance, ?_, ?_⟩
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· exact (isAffineOpen_top X).isLocalization_basicOpen (f.app ⊤ a.val)
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· obtain ⟨_, hf⟩ := hf a
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rw [morphismRestrict_app, hPi.cancel_right_isIso, Scheme.Opens.ι_image_top] at hf
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rw [(isAffineOpen_top Y).app_basicOpen_eq_away_map _ (isAffineOpen_top X)] at hf
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rwa [hPi.cancel_right_isIso] at hf
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/-- If `P` is stable under base change, so is `affineAnd P`. -/
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lemma affineAnd_stableUnderBaseChange (hQi : RingHom.RespectsIso Q)
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(hQb : RingHom.StableUnderBaseChange Q) :
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(affineAnd Q).StableUnderBaseChange := by
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haveI : (affineAnd Q).toProperty.RespectsIso := affineAnd_respectsIso hQi
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apply AffineTargetMorphismProperty.StableUnderBaseChange.mk
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intro X Y S _ _ f g ⟨hY, hg⟩
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exact ⟨inferInstance, hQb.pullback_fst_app_top _ hQi f _ hg⟩
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lemma targetAffineLocally_affineAnd_iff (hQi : RingHom.RespectsIso Q)
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{X Y : Scheme.{u}} (f : X ⟶ Y) :
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targetAffineLocally (affineAnd Q) f ↔ ∀ U : Y.Opens, IsAffineOpen U →
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IsAffineOpen (f ⁻¹ᵁ U) ∧ Q (f.app U) := by
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simp only [targetAffineLocally, affineAnd_apply, morphismRestrict_app, hQi.cancel_right_isIso]
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refine ⟨fun hf U hU ↦ ?_, fun h U ↦ ?_⟩
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· obtain ⟨hfU, hf⟩ := hf ⟨U, hU⟩
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exact ⟨hfU, by rwa [Scheme.Opens.ι_image_top] at hf⟩
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· refine ⟨(h U U.2).1, ?_⟩
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rw [Scheme.Opens.ι_image_top]
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exact (h U U.2).2
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end
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section
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variable {Q : ∀ {R S : Type u} [CommRing R] [CommRing S], (R →+* S) → Prop}
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/-- If `P` is a morphism property affine locally defined by `affineAnd Q`, `P` is stable under
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composition if `Q` is. -/
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lemma HasAffineProperty.affineAnd_isStableUnderComposition {P : MorphismProperty Scheme.{u}}
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(hA : HasAffineProperty P (affineAnd Q)) (hQ : RingHom.StableUnderComposition Q) :
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P.IsStableUnderComposition where
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comp_mem {X Y Z} f g hf hg := by
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haveI := hA
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wlog hZ : IsAffine Z
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· rw [IsLocalAtTarget.iff_of_iSup_eq_top (P := P) _ (iSup_affineOpens_eq_top _)]
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intro U
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rw [morphismRestrict_comp]
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exact this hA hQ _ _ (IsLocalAtTarget.restrict hf _) (IsLocalAtTarget.restrict hg _) hA U.2
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rw [HasAffineProperty.iff_of_isAffine (P := P) (Q := (affineAnd Q))] at hg
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obtain ⟨hY, hg⟩ := hg
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rw [HasAffineProperty.iff_of_isAffine (P := P) (Q := (affineAnd Q))] at hf
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obtain ⟨hX, hf⟩ := hf
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rw [HasAffineProperty.iff_of_isAffine (P := P) (Q := (affineAnd Q))]
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exact ⟨hX, hQ _ _ hg hf⟩
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/-- If `P` is a morphism property affine locally defined by `affineAnd Q`, `P` is stable under
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base change if `Q` is. -/
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lemma HasAffineProperty.affineAnd_stableUnderBaseChange {P : MorphismProperty Scheme.{u}}
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(_ : HasAffineProperty P (affineAnd Q)) (hQi : RingHom.RespectsIso Q)
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(hQb : RingHom.StableUnderBaseChange Q) :
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P.StableUnderBaseChange :=
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HasAffineProperty.stableUnderBaseChange
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(AlgebraicGeometry.affineAnd_stableUnderBaseChange hQi hQb)
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/-- If `Q` contains identities and respects isomorphisms (i.e. is satisfied by isomorphisms),
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and `P` is affine locally defined by `affineAnd Q`, then `P` contains identities. -/
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lemma HasAffineProperty.affineAnd_containsIdentities {P : MorphismProperty Scheme.{u}}
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(hA : HasAffineProperty P (affineAnd Q)) (hQi : RingHom.RespectsIso Q)
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(hQ : RingHom.ContainsIdentities Q) :
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P.ContainsIdentities where
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id_mem X := by
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rw [eq_targetAffineLocally P, targetAffineLocally_affineAnd_iff hQi]
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intro U hU
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exact ⟨hU, hQ _⟩
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/-- A convenience constructor for `HasAffineProperty P (affineAnd Q)`. The `IsAffineHom` is bundled,
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since this goes well with defining morphism properties via `extends IsAffineHom`. -/
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lemma HasAffineProperty.affineAnd_iff (P : MorphismProperty Scheme.{u})
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(hQi : RingHom.RespectsIso Q) (hQl : RingHom.LocalizationPreserves Q)
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(hQs : RingHom.OfLocalizationSpan Q) :
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HasAffineProperty P (affineAnd Q) ↔
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∀ {X Y : Scheme.{u}} (f : X ⟶ Y), P f ↔
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(IsAffineHom f ∧ ∀ U : Y.Opens, IsAffineOpen U → Q (f.app U)) := by
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simp_rw [isAffineHom_iff]
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refine ⟨fun h X Y f ↦ ?_, fun h ↦ ⟨affineAnd_isLocal hQi hQl hQs, ?_⟩⟩
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· rw [eq_targetAffineLocally P, targetAffineLocally_affineAnd_iff hQi]
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aesop
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· ext X Y f
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rw [targetAffineLocally_affineAnd_iff hQi, h f]
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aesop
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lemma HasAffineProperty.affineAnd_le_isAffineHom (P : MorphismProperty Scheme.{u})
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(hA : HasAffineProperty P (affineAnd Q)) : P ≤ @IsAffineHom := by
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intro X Y f hf
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wlog hY : IsAffine Y
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· rw [IsLocalAtTarget.iff_of_iSup_eq_top (P := @IsAffineHom) _ (iSup_affineOpens_eq_top _)]
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intro U
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exact this P hA _ (IsLocalAtTarget.restrict hf _) U.2
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rw [HasAffineProperty.iff_of_isAffine (P := P) (Q := (affineAnd Q))] at hf
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rw [HasAffineProperty.iff_of_isAffine (P := @IsAffineHom)]
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exact hf.1
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end
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end AlgebraicGeometry

‎Mathlib/AlgebraicGeometry/Morphisms/RingHomProperties.lean

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@@ -16,15 +16,15 @@ of ring homs. For `P` a property of ring homs, we have two ways of defining a pr
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morphisms:
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Let `f : X ⟶ Y`,
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- `targetAffineLocally (affine_and P)`: the preimage of an affine open `U = Spec A` is affine
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(`= Spec B`) and `A ⟶ B` satisfies `P`. (TODO)
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- `targetAffineLocally (affineAnd P)`: the preimage of an affine open `U = Spec A` is affine
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(`= Spec B`) and `A ⟶ B` satisfies `P`. (in `Mathlib/AlgebraicGeometry/Morphisms/AffineAnd.lean`)
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- `affineLocally P`: For each pair of affine open `U = Spec A ⊆ X` and `V = Spec B ⊆ f ⁻¹' U`,
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the ring hom `A ⟶ B` satisfies `P`.
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For these notions to be well defined, we require `P` be a sufficient local property. For the former,
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`P` should be local on the source (`RingHom.RespectsIso P`, `RingHom.LocalizationPreserves P`,
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`RingHom.OfLocalizationSpan`), and `targetAffineLocally (affine_and P)` will be local on
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the target. (TODO)
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the target.
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For the latter `P` should be local on the target (`RingHom.PropertyIsLocal P`), and
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`affineLocally P` will be local on both the source and the target.

‎Mathlib/RingTheory/LocalProperties/Basic.lean

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· apply hP.StableUnderCompositionWithLocalizationAway.right _ r
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exact hs' ⟨r, hr⟩
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lemma RingHom.OfLocalizationSpan.ofIsLocalization
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(hP : RingHom.OfLocalizationSpan P) (hPi : RingHom.RespectsIso P)
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{R S : Type u} [CommRing R] [CommRing S] (f : R →+* S) (s : Set R) (hs : Ideal.span s = ⊤)
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(hT : ∀ r : s, ∃ (Rᵣ Sᵣ : Type u) (_ : CommRing Rᵣ) (_ : CommRing Sᵣ)
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(_ : Algebra R Rᵣ) (_ : Algebra S Sᵣ) (_ : IsLocalization.Away r.val Rᵣ)
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(_ : IsLocalization.Away (f r.val) Sᵣ) (fᵣ : Rᵣ →+* Sᵣ)
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(_ : fᵣ.comp (algebraMap R Rᵣ) = (algebraMap S Sᵣ).comp f),
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P fᵣ) : P f := by
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apply hP _ s hs
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intro r
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obtain ⟨Rᵣ, Sᵣ, _, _, _, _, _, _, fᵣ, hfᵣ, hf⟩ := hT r
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let e₁ := (Localization.algEquiv (.powers r.val) Rᵣ).toRingEquiv
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let e₂ := (IsLocalization.algEquiv (.powers (f r.val))
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(Localization (.powers (f r.val))) Sᵣ).symm.toRingEquiv
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have : Localization.awayMap f r.val =
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(e₂.toRingHom.comp fᵣ).comp e₁.toRingHom := by
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apply IsLocalization.ringHom_ext (.powers r.val)
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ext x
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have : fᵣ ((algebraMap R Rᵣ) x) = algebraMap S Sᵣ (f x) := by
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rw [← RingHom.comp_apply, hfᵣ, RingHom.comp_apply]
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simp [-AlgEquiv.symm_toRingEquiv, e₂, e₁, Localization.awayMap, IsLocalization.Away.map, this]
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rw [this]
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apply hPi.right
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apply hPi.left
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exact hf
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/-- Variant of `RingHom.OfLocalizationSpan.ofIsLocalization` where
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`fᵣ = IsLocalization.Away.map`. -/
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lemma RingHom.OfLocalizationSpan.ofIsLocalization'
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(hP : RingHom.OfLocalizationSpan P) (hPi : RingHom.RespectsIso P)
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{R S : Type u} [CommRing R] [CommRing S] (f : R →+* S) (s : Set R) (hs : Ideal.span s = ⊤)
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(hT : ∀ r : s, ∃ (Rᵣ Sᵣ : Type u) (_ : CommRing Rᵣ) (_ : CommRing Sᵣ)
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(_ : Algebra R Rᵣ) (_ : Algebra S Sᵣ) (_ : IsLocalization.Away r.val Rᵣ)
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(_ : IsLocalization.Away (f r.val) Sᵣ),
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P (IsLocalization.Away.map Rᵣ Sᵣ f r)) : P f := by
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apply hP.ofIsLocalization hPi _ s hs
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intro r
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obtain ⟨Rᵣ, Sᵣ, _, _, _, _, _, _, hf⟩ := hT r
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exact ⟨Rᵣ, Sᵣ, inferInstance, inferInstance, inferInstance, inferInstance,
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inferInstance, inferInstance, IsLocalization.Away.map Rᵣ Sᵣ f r, IsLocalization.map_comp _, hf⟩
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lemma RingHom.OfLocalizationSpanTarget.ofIsLocalization
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(hP : RingHom.OfLocalizationSpanTarget P) (hP' : RingHom.RespectsIso P)
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{R S : Type u} [CommRing R] [CommRing S] (f : R →+* S) (s : Set S) (hs : Ideal.span s = ⊤)

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