The module of kaehler differentials

Main results

• KaehlerDifferential: The module of kaehler differentials. For an R-algebra S, we provide the notation Ω[S⁄R] for KaehlerDifferential R S. Note that the slash is \textfractionsolidus.
• KaehlerDifferential.D: The derivation into the module of kaehler differentials.
• KaehlerDifferential.span_range_derivation: The image of D spans Ω[S⁄R] as an S-module.
• KaehlerDifferential.linearMapEquivDerivation: The isomorphism Hom_R(Ω[S⁄R], M) ≃ₗ[S] Der_R(S, M).
• KaehlerDifferential.quotKerTotalEquiv: An alternative description of Ω[S⁄R] as S copies of S with kernel (KaehlerDifferential.kerTotal) generated by the relations:
  1. dx + dy = d(x + y)
  2. x dy + y dx = d(x * y)
  3. dr = 0 for r ∈ R
• KaehlerDifferential.map: Given a map between the arrows R →+* A and S →+* B, we have an A-linear map Ω[A⁄R] → Ω[B⁄S].
• KaehlerDifferential.map_surjective: The sequence Ω[B⁄R] → Ω[B⁄A] → 0 is exact.
• KaehlerDifferential.exact_mapBaseChange_map: The sequence B ⊗[A] Ω[A⁄R] → Ω[B⁄R] → Ω[B⁄A] is exact.
• KaehlerDifferential.exact_kerCotangentToTensor_mapBaseChange: If A → B is surjective with kernel I, then the sequence I/I² → B ⊗[A] Ω[A⁄R] → Ω[B⁄R] is exact.
• KaehlerDifferential.mapBaseChange_surjective: If A → B is surjective, then the sequence B ⊗[A] Ω[A⁄R] → Ω[B⁄R] → 0 is exact.

Future project

• Define the IsKaehlerDifferential predicate.

@[reducible, inline]

noncomputable abbrev KaehlerDifferential.ideal (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : Ideal (TensorProduct R S S)

The kernel of the multiplication map S ⊗[R] S →ₐ[R] S.

theorem KaehlerDifferential.one_smul_sub_smul_one_mem_ideal (R : Type u) {S : Type v} [CommRing R] [CommRing S] [Algebra R S] (a : S) : 1 ⊗ₜ[R] a - a ⊗ₜ[R] 1 ∈ ideal R S

noncomputable def Derivation.tensorProductTo {R : Type u} {S : Type v} [CommRing R] [CommRing S] [Algebra R S] {M : Type u_1} [AddCommGroup M] [Module R M] [Module S M] [IsScalarTower R S M] (D : Derivation R S M) : TensorProduct R S S →ₗ[S] M

For a R-derivation S → M, this is the map S ⊗[R] S →ₗ[S] M sending s ⊗ₜ t ↦ s • D t.

theorem Derivation.tensorProductTo_tmul {R : Type u} {S : Type v} [CommRing R] [CommRing S] [Algebra R S] {M : Type u_1} [AddCommGroup M] [Module R M] [Module S M] [IsScalarTower R S M] (D : Derivation R S M) (s t : S) : D.tensorProductTo (s ⊗ₜ[R] t) = s • D t

theorem Derivation.tensorProductTo_mul {R : Type u} {S : Type v} [CommRing R] [CommRing S] [Algebra R S] {M : Type u_1} [AddCommGroup M] [Module R M] [Module S M] [IsScalarTower R S M] (D : Derivation R S M) (x y : TensorProduct R S S) : D.tensorProductTo (x * y) = (Algebra.TensorProduct.lmul' R) x • D.tensorProductTo y + (Algebra.TensorProduct.lmul' R) y • D.tensorProductTo x

theorem KaehlerDifferential.submodule_span_range_eq_ideal (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : Submodule.span S (Set.range fun (s : S) => 1 ⊗ₜ[R] s - s ⊗ₜ[R] 1) = Submodule.restrictScalars S (ideal R S)

The kernel of S ⊗[R] S →ₐ[R] S is generated by 1 ⊗ s - s ⊗ 1 as a S-module.

theorem KaehlerDifferential.span_range_eq_ideal (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : Ideal.span (Set.range fun (s : S) => 1 ⊗ₜ[R] s - s ⊗ₜ[R] 1) = ideal R S

noncomputable def KaehlerDifferential (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : Type v

The module of Kähler differentials (Kahler differentials, Kaehler differentials). This is implemented as I / I ^ 2 with I the kernel of the multiplication map S ⊗[R] S →ₐ[R] S. To view elements as a linear combination of the form s • D s', use KaehlerDifferential.tensorProductTo_surjective and Derivation.tensorProductTo_tmul.

We also provide the notation Ω[S⁄R] for KaehlerDifferential R S. Note that the slash is \textfractionsolidus.

noncomputable instance instAddCommGroupKaehlerDifferential (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : AddCommGroup Ω[S⁄R]

noncomputable instance KaehlerDifferential.module (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : Module (TensorProduct R S S) Ω[S⁄R]

def «termΩ[_⁄_]» : Lean.ParserDescr

The module of Kähler differentials (Kahler differentials, Kaehler differentials). This is implemented as I / I ^ 2 with I the kernel of the multiplication map S ⊗[R] S →ₐ[R] S. To view elements as a linear combination of the form s • D s', use KaehlerDifferential.tensorProductTo_surjective and Derivation.tensorProductTo_tmul.

We also provide the notation Ω[S⁄R] for KaehlerDifferential R S. Note that the slash is \textfractionsolidus.

instance instNonemptyKaehlerDifferential (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : Nonempty Ω[S⁄R]

noncomputable instance KaehlerDifferential.module' (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] {R' : Type u_2} [CommRing R'] [Algebra R' S] [SMulCommClass R R' S] : Module R' Ω[S⁄R]

instance instIsScalarTowerTensorProductKaehlerDifferential (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : IsScalarTower S (TensorProduct R S S) Ω[S⁄R]

instance KaehlerDifferential.isScalarTower_of_tower (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] {R₁ : Type u_2} {R₂ : Type u_3} [CommRing R₁] [CommRing R₂] [Algebra R₁ S] [Algebra R₂ S] [SMul R₁ R₂] [SMulCommClass R R₁ S] [SMulCommClass R R₂ S] [IsScalarTower R₁ R₂ S] : IsScalarTower R₁ R₂ Ω[S⁄R]

instance KaehlerDifferential.isScalarTower' (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : IsScalarTower R (TensorProduct R S S) Ω[S⁄R]

noncomputable def KaehlerDifferential.fromIdeal (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : ↥(ideal R S) →ₗ[TensorProduct R S S] Ω[S⁄R]

The quotient map I → Ω[S⁄R] with I being the kernel of S ⊗[R] S → S.

noncomputable def KaehlerDifferential.DLinearMap (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : S →ₗ[R] Ω[S⁄R]

(Implementation) The underlying linear map of the derivation into Ω[S⁄R].

theorem KaehlerDifferential.DLinearMap_apply (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] (s : S) : (DLinearMap R S) s = (ideal R S).toCotangent ⟨1 ⊗ₜ[R] s - s ⊗ₜ[R] 1, ⋯⟩

noncomputable def KaehlerDifferential.D (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : Derivation R S Ω[S⁄R]

The universal derivation into Ω[S⁄R].

theorem KaehlerDifferential.D_apply (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] (s : S) : (D R S) s = (ideal R S).toCotangent ⟨1 ⊗ₜ[R] s - s ⊗ₜ[R] 1, ⋯⟩

theorem KaehlerDifferential.span_range_derivation (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : Submodule.span S (Set.range ⇑(D R S)) = ⊤

theorem KaehlerDifferential.subsingleton_of_surjective (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] (h : Function.Surjective ⇑(algebraMap R S)) : Subsingleton Ω[S⁄R]

Ω[S⁄R] is trivial if R → S is surjective. Also see Algebra.FormallyUnramified.iff_subsingleton_kaehlerDifferential.

noncomputable def Derivation.liftKaehlerDifferential {R : Type u} {S : Type v} [CommRing R] [CommRing S] [Algebra R S] {M : Type u_1} [AddCommGroup M] [Module R M] [Module S M] [IsScalarTower R S M] (D : Derivation R S M) : Ω[S⁄R] →ₗ[S] M

The linear map from Ω[S⁄R], associated with a derivation.

theorem Derivation.liftKaehlerDifferential_apply {R : Type u} {S : Type v} [CommRing R] [CommRing S] [Algebra R S] {M : Type u_1} [AddCommGroup M] [Module R M] [Module S M] [IsScalarTower R S M] (D : Derivation R S M) (x : ↥(KaehlerDifferential.ideal R S)) : D.liftKaehlerDifferential ((KaehlerDifferential.ideal R S).toCotangent x) = D.tensorProductTo ↑x

theorem Derivation.liftKaehlerDifferential_comp {R : Type u} {S : Type v} [CommRing R] [CommRing S] [Algebra R S] {M : Type u_1} [AddCommGroup M] [Module R M] [Module S M] [IsScalarTower R S M] (D : Derivation R S M) : D.liftKaehlerDifferential.compDer (KaehlerDifferential.D R S) = D

@[simp]

theorem Derivation.liftKaehlerDifferential_comp_D {R : Type u} {S : Type v} [CommRing R] [CommRing S] [Algebra R S] {M : Type u_1} [AddCommGroup M] [Module R M] [Module S M] [IsScalarTower R S M] (D' : Derivation R S M) (x : S) : D'.liftKaehlerDifferential ((KaehlerDifferential.D R S) x) = D' x

theorem Derivation.liftKaehlerDifferential_unique {R : Type u} {S : Type v} [CommRing R] [CommRing S] [Algebra R S] {M : Type u_1} [AddCommGroup M] [Module R M] [Module S M] [IsScalarTower R S M] (f f' : Ω[S⁄R] →ₗ[S] M) (hf : f.compDer (KaehlerDifferential.D R S) = f'.compDer (KaehlerDifferential.D R S)) : f = f'

theorem Derivation.liftKaehlerDifferential_unique_iff {R : Type u} {S : Type v} [CommRing R] [CommRing S] [Algebra R S] {M : Type u_1} [AddCommGroup M] [Module R M] [Module S M] [IsScalarTower R S M] {f f' : Ω[S⁄R] →ₗ[S] M} : f = f' ↔ f.compDer (KaehlerDifferential.D R S) = f'.compDer (KaehlerDifferential.D R S)

theorem Derivation.liftKaehlerDifferential_D (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : (KaehlerDifferential.D R S).liftKaehlerDifferential = LinearMap.id

theorem KaehlerDifferential.D_tensorProductTo {R : Type u} {S : Type v} [CommRing R] [CommRing S] [Algebra R S] (x : ↥(ideal R S)) : (D R S).tensorProductTo ↑x = (ideal R S).toCotangent x

theorem KaehlerDifferential.tensorProductTo_surjective (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : Function.Surjective ⇑(D R S).tensorProductTo

noncomputable def KaehlerDifferential.linearMapEquivDerivation (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] {M : Type u_1} [AddCommGroup M] [Module R M] [Module S M] [IsScalarTower R S M] : (Ω[S⁄R] →ₗ[S] M) ≃ₗ[S] Derivation R S M

The S-linear maps from Ω[S⁄R] to M are (S-linearly) equivalent to R-derivations from S to M.

@[simp]

theorem KaehlerDifferential.linearMapEquivDerivation_symm_apply (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] {M : Type u_1} [AddCommGroup M] [Module R M] [Module S M] [IsScalarTower R S M] (D : Derivation R S M) : (linearMapEquivDerivation R S).symm D = D.liftKaehlerDifferential

@[simp]

theorem KaehlerDifferential.linearMapEquivDerivation_apply_apply (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] {M : Type u_1} [AddCommGroup M] [Module R M] [Module S M] [IsScalarTower R S M] (m : Ω[S⁄R] →ₗ[S] M) (a✝ : S) : ((linearMapEquivDerivation R S) m) a✝ = m ((D R S) a✝)

noncomputable def KaehlerDifferential.quotientCotangentIdealRingEquiv (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : (TensorProduct R S S ⧸ ideal R S ^ 2) ⧸ (ideal R S).cotangentIdeal ≃+* S

The quotient ring of S ⊗ S ⧸ J ^ 2 by Ω[S⁄R] is isomorphic to S.

noncomputable def KaehlerDifferential.quotientCotangentIdeal (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : ((TensorProduct R S S ⧸ ideal R S ^ 2) ⧸ (ideal R S).cotangentIdeal) ≃ₐ[S] S

The quotient ring of S ⊗ S ⧸ J ^ 2 by Ω[S⁄R] is isomorphic to S as an S-algebra.

theorem KaehlerDifferential.End_equiv_aux (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] (f : S →ₐ[R] TensorProduct R S S ⧸ ideal R S ^ 2) : (Ideal.Quotient.mkₐ R (ideal R S).cotangentIdeal).comp f = IsScalarTower.toAlgHom R S ((TensorProduct R S S ⧸ ideal R S ^ 2) ⧸ (ideal R S).cotangentIdeal) ↔ (Algebra.TensorProduct.lmul' R).kerSquareLift.comp f = AlgHom.id R S

noncomputable def smul_SSmod_SSmod (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : SMul (TensorProduct R S S ⧸ KaehlerDifferential.ideal R S ^ 2) (TensorProduct R S S ⧸ KaehlerDifferential.ideal R S ^ 2)

A shortcut instance to prevent timing out. Hopefully to be removed in the future.

theorem isScalarTower_S_right (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : IsScalarTower S (TensorProduct R S S ⧸ KaehlerDifferential.ideal R S ^ 2) (TensorProduct R S S ⧸ KaehlerDifferential.ideal R S ^ 2)

A shortcut instance to prevent timing out. Hopefully to be removed in the future.

theorem isScalarTower_R_right (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : IsScalarTower R (TensorProduct R S S ⧸ KaehlerDifferential.ideal R S ^ 2) (TensorProduct R S S ⧸ KaehlerDifferential.ideal R S ^ 2)

A shortcut instance to prevent timing out. Hopefully to be removed in the future.

theorem isScalarTower_SS_right (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : IsScalarTower (TensorProduct R S S) (TensorProduct R S S ⧸ KaehlerDifferential.ideal R S ^ 2) (TensorProduct R S S ⧸ KaehlerDifferential.ideal R S ^ 2)

A shortcut instance to prevent timing out. Hopefully to be removed in the future.

noncomputable def instS (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : Module S ↥(KaehlerDifferential.ideal R S).cotangentIdeal

A shortcut instance to prevent timing out. Hopefully to be removed in the future.

noncomputable def instR (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : Module R ↥(KaehlerDifferential.ideal R S).cotangentIdeal

A shortcut instance to prevent timing out. Hopefully to be removed in the future.

noncomputable def instSS (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : Module (TensorProduct R S S) ↥(KaehlerDifferential.ideal R S).cotangentIdeal

A shortcut instance to prevent timing out. Hopefully to be removed in the future.

noncomputable def KaehlerDifferential.endEquivDerivation' (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : Derivation R S Ω[S⁄R] ≃ₗ[R] Derivation R S ↥(ideal R S).cotangentIdeal

Derivations into Ω[S⁄R] is equivalent to derivations into (KaehlerDifferential.ideal R S).cotangentIdeal.

noncomputable def KaehlerDifferential.endEquivAuxEquiv (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : { f : S →ₐ[R] TensorProduct R S S ⧸ ideal R S ^ 2 // (Ideal.Quotient.mkₐ R (ideal R S).cotangentIdeal).comp f = IsScalarTower.toAlgHom R S ((TensorProduct R S S ⧸ ideal R S ^ 2) ⧸ (ideal R S).cotangentIdeal) } ≃ { f : S →ₐ[R] TensorProduct R S S ⧸ RingHom.ker (Algebra.TensorProduct.lmul' R).toRingHom ^ 2 // (Algebra.TensorProduct.lmul' R).kerSquareLift.comp f = AlgHom.id R S }

(Implementation) An Equiv version of KaehlerDifferential.End_equiv_aux. Used in KaehlerDifferential.endEquiv.

noncomputable def KaehlerDifferential.endEquiv (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : Module.End S Ω[S⁄R] ≃ { f : S →ₐ[R] TensorProduct R S S ⧸ RingHom.ker (Algebra.TensorProduct.lmul' R).toRingHom ^ 2 // (Algebra.TensorProduct.lmul' R).kerSquareLift.comp f = AlgHom.id R S }

The endomorphisms of Ω[S⁄R] corresponds to sections of the surjection S ⊗[R] S ⧸ J ^ 2 →ₐ[R] S, with J being the kernel of the multiplication map S ⊗[R] S →ₐ[R] S.

theorem KaehlerDifferential.ideal_fg (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] [Algebra.EssFiniteType R S] : (ideal R S).FG

instance KaehlerDifferential.finite (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] [Algebra.EssFiniteType R S] : Module.Finite S Ω[S⁄R]

noncomputable def KaehlerDifferential.kerTotal (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : Submodule S (S →₀ S)

The S-submodule of S →₀ S (the direct sum of copies of S indexed by S) generated by the relations:

1. dx + dy = d(x + y)
2. x dy + y dx = d(x * y)
3. dr = 0 for r ∈ R where db is the unit in the copy of S with index b.

This is the kernel of the surjection Finsupp.linearCombination S Ω[S⁄R] S (KaehlerDifferential.D R S). See KaehlerDifferential.kerTotal_eq and KaehlerDifferential.linearCombination_surjective.

theorem KaehlerDifferential.kerTotal_mkQ_single_add (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] (x y z : S) : (kerTotal R S).mkQ (Finsupp.single (x + y) z) = (kerTotal R S).mkQ (Finsupp.single x z) + (kerTotal R S).mkQ (Finsupp.single y z)

theorem KaehlerDifferential.kerTotal_mkQ_single_mul (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] (x y z : S) : (kerTotal R S).mkQ (Finsupp.single (x * y) z) = (kerTotal R S).mkQ (Finsupp.single y (z * x)) + (kerTotal R S).mkQ (Finsupp.single x (z * y))

theorem KaehlerDifferential.kerTotal_mkQ_single_algebraMap (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] (x : R) (y : S) : (kerTotal R S).mkQ (Finsupp.single ((algebraMap R S) x) y) = 0

theorem KaehlerDifferential.kerTotal_mkQ_single_algebraMap_one (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] (x : S) : (kerTotal R S).mkQ (Finsupp.single 1 x) = 0

theorem KaehlerDifferential.kerTotal_mkQ_single_smul (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] (r : R) (x y : S) : (kerTotal R S).mkQ (Finsupp.single (r • x) y) = r • (kerTotal R S).mkQ (Finsupp.single x y)

noncomputable def KaehlerDifferential.derivationQuotKerTotal (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : Derivation R S ((S →₀ S) ⧸ kerTotal R S)

The (universal) derivation into (S →₀ S) ⧸ KaehlerDifferential.kerTotal R S.

theorem KaehlerDifferential.derivationQuotKerTotal_apply (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] (x : S) : (derivationQuotKerTotal R S) x = (kerTotal R S).mkQ (Finsupp.single x 1)

theorem KaehlerDifferential.derivationQuotKerTotal_lift_comp_linearCombination (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : (derivationQuotKerTotal R S).liftKaehlerDifferential ∘ₗ Finsupp.linearCombination S ⇑(D R S) = (kerTotal R S).mkQ

theorem KaehlerDifferential.kerTotal_eq (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : LinearMap.ker (Finsupp.linearCombination S ⇑(D R S)) = kerTotal R S

theorem KaehlerDifferential.linearCombination_surjective (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : Function.Surjective ⇑(Finsupp.linearCombination S ⇑(D R S))

noncomputable def KaehlerDifferential.quotKerTotalEquiv (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : ((S →₀ S) ⧸ kerTotal R S) ≃ₗ[S] Ω[S⁄R]

Ω[S⁄R] is isomorphic to S copies of S with kernel KaehlerDifferential.kerTotal.

@[simp]

theorem KaehlerDifferential.quotKerTotalEquiv_apply (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] (a✝ : (S →₀ S) ⧸ (kerTotal R S).toAddSubgroup) : (quotKerTotalEquiv R S) a✝ = (QuotientAddGroup.lift (kerTotal R S).toAddSubgroup (Finsupp.linearCombination S ⇑(D R S)).toAddMonoidHom ⋯) a✝

@[simp]

theorem KaehlerDifferential.quotKerTotalEquiv_symm_apply (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] (a : Ω[S⁄R]) : (quotKerTotalEquiv R S).symm a = (derivationQuotKerTotal R S).liftKaehlerDifferential a

theorem KaehlerDifferential.quotKerTotalEquiv_symm_comp_D (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] : (↑(quotKerTotalEquiv R S).symm).compDer (D R S) = derivationQuotKerTotal R S

theorem KaehlerDifferential.kerTotal_map (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] (A : Type u_2) (B : Type u_3) [CommRing A] [CommRing B] [Algebra R A] [Algebra A B] [Algebra S B] [Algebra R B] [IsScalarTower R A B] [IsScalarTower R S B] (h : Function.Surjective ⇑(algebraMap A B)) : Submodule.map (Finsupp.mapRange.linearMap (Algebra.linearMap A B) ∘ₗ Finsupp.lmapDomain A A ⇑(algebraMap A B)) (kerTotal R A) ⊔ Submodule.span A (Set.range fun (x : S) => Finsupp.single ((algebraMap S B) x) 1) = Submodule.restrictScalars A (kerTotal S B)

Given the commutative diagram

A --→ B
↑     ↑
|     |
R --→ S

The kernel of the presentation ⊕ₓ B dx ↠ Ω_{B/S} is spanned by the image of the kernel of ⊕ₓ A dx ↠ Ω_{A/R} and all ds with s : S. See kerTotal_map' for the special case where R = S.

theorem KaehlerDifferential.kerTotal_map' (R : Type u) [CommRing R] (A : Type u_2) (B : Type u_3) [CommRing A] [CommRing B] [Algebra R A] [Algebra A B] [Algebra R B] [IsScalarTower R A B] (h : Function.Surjective ⇑(algebraMap A B)) : Submodule.map (Finsupp.mapRange.linearMap (Algebra.linearMap A B) ∘ₗ Finsupp.lmapDomain A A ⇑(algebraMap A B)) (kerTotal R A ⊔ Submodule.span A (Set.range fun (x : R) => Finsupp.single ((algebraMap R A) x) 1)) = Submodule.restrictScalars A (kerTotal R B)

This is a special case of kerTotal_map where R = S. The kernel of the presentation ⊕ₓ B dx ↠ Ω_{B/R} is spanned by the image of the kernel of ⊕ₓ A dx ↠ Ω_{A/R} and all da with a : A.

noncomputable def KaehlerDifferential.map (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] (A : Type u_2) (B : Type u_3) [CommRing A] [CommRing B] [Algebra R A] [Algebra A B] [Algebra S B] [Algebra R B] [IsScalarTower R A B] [IsScalarTower R S B] [SMulCommClass S A B] : Ω[A⁄R] →ₗ[A] Ω[B⁄S]

The map Ω[A⁄R] →ₗ[A] Ω[B⁄S] given a square

A --→ B
↑     ↑
|     |
R --→ S

theorem KaehlerDifferential.map_compDer (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] (A : Type u_2) (B : Type u_3) [CommRing A] [CommRing B] [Algebra R A] [Algebra A B] [Algebra S B] [Algebra R B] [IsScalarTower R A B] [IsScalarTower R S B] [SMulCommClass S A B] : (map R S A B).compDer (D R A) = Derivation.compAlgebraMap A (Derivation.restrictScalars R (D S B))

@[simp]

theorem KaehlerDifferential.map_D (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] (A : Type u_2) (B : Type u_3) [CommRing A] [CommRing B] [Algebra R A] [Algebra A B] [Algebra S B] [Algebra R B] [IsScalarTower R A B] [IsScalarTower R S B] [SMulCommClass S A B] (x : A) : (map R S A B) ((D R A) x) = (D S B) ((algebraMap A B) x)

theorem KaehlerDifferential.ker_map (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] (A : Type u_2) (B : Type u_3) [CommRing A] [CommRing B] [Algebra R A] [Algebra A B] [Algebra S B] [Algebra R B] [IsScalarTower R A B] [IsScalarTower R S B] [SMulCommClass S A B] : LinearMap.ker (map R S A B) = Submodule.map (Finsupp.linearCombination A ⇑(D R A)) (Submodule.comap (Finsupp.mapRange.linearMap (Algebra.linearMap A B) ∘ₗ Finsupp.lmapDomain A A ⇑(algebraMap A B)) (Submodule.restrictScalars A (kerTotal S B)))

theorem KaehlerDifferential.ker_map_of_surjective (R : Type u) [CommRing R] (A : Type u_2) (B : Type u_3) [CommRing A] [CommRing B] [Algebra R A] [Algebra A B] [Algebra R B] [IsScalarTower R A B] (h : Function.Surjective ⇑(algebraMap A B)) : LinearMap.ker (map R R A B) = Submodule.map (Finsupp.linearCombination A ⇑(D R A)) (LinearMap.ker (Finsupp.mapRange.linearMap (Algebra.linearMap A B) ∘ₗ Finsupp.lmapDomain A A ⇑(algebraMap A B)))

theorem KaehlerDifferential.map_surjective_of_surjective (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] (A : Type u_2) (B : Type u_3) [CommRing A] [CommRing B] [Algebra R A] [Algebra A B] [Algebra S B] [Algebra R B] [IsScalarTower R A B] [IsScalarTower R S B] [SMulCommClass S A B] (h : Function.Surjective ⇑(algebraMap A B)) : Function.Surjective ⇑(map R S A B)

theorem KaehlerDifferential.map_surjective (R : Type u) (S : Type v) [CommRing R] [CommRing S] [Algebra R S] (B : Type u_3) [CommRing B] [Algebra S B] [Algebra R B] [IsScalarTower R S B] : Function.Surjective ⇑(map R S B B)

noncomputable def KaehlerDifferential.mapBaseChange (R : Type u) [CommRing R] (A : Type u_2) (B : Type u_3) [CommRing A] [CommRing B] [Algebra R A] [Algebra A B] [Algebra R B] [IsScalarTower R A B] : TensorProduct A B Ω[A⁄R] →ₗ[B] Ω[B⁄R]

The lift of the map Ω[A⁄R] →ₗ[A] Ω[B⁄R] to the base change along A → B. This is the first map in the exact sequence B ⊗[A] Ω[A⁄R] → Ω[B⁄R] → Ω[B⁄A] → 0.

@[simp]

theorem KaehlerDifferential.mapBaseChange_tmul (R : Type u) [CommRing R] (A : Type u_2) (B : Type u_3) [CommRing A] [CommRing B] [Algebra R A] [Algebra A B] [Algebra R B] [IsScalarTower R A B] (x : B) (y : Ω[A⁄R]) : (mapBaseChange R A B) (x ⊗ₜ[A] y) = x • (map R R A B) y

theorem KaehlerDifferential.range_mapBaseChange (R : Type u) [CommRing R] (A : Type u_2) (B : Type u_3) [CommRing A] [CommRing B] [Algebra R A] [Algebra A B] [Algebra R B] [IsScalarTower R A B] : LinearMap.range (mapBaseChange R A B) = LinearMap.ker (map R A B B)

theorem KaehlerDifferential.exact_mapBaseChange_map (R : Type u) [CommRing R] (A : Type u_2) (B : Type u_3) [CommRing A] [CommRing B] [Algebra R A] [Algebra A B] [Algebra R B] [IsScalarTower R A B] : Function.Exact ⇑(mapBaseChange R A B) ⇑(map R A B B)

The sequence B ⊗[A] Ω[A⁄R] → Ω[B⁄R] → Ω[B⁄A] → 0 is exact. Also see KaehlerDifferential.map_surjective.

noncomputable def KaehlerDifferential.kerToTensor (R : Type u) [CommRing R] (A : Type u_2) (B : Type u_3) [CommRing A] [CommRing B] [Algebra R A] [Algebra A B] : ↥(RingHom.ker (algebraMap A B)) →ₗ[A] TensorProduct A B Ω[A⁄R]

The map I → B ⊗[A] Ω[A⁄R] where I = ker(A → B).

@[simp]

theorem KaehlerDifferential.kerToTensor_apply (R : Type u) [CommRing R] (A : Type u_2) (B : Type u_3) [CommRing A] [CommRing B] [Algebra R A] [Algebra A B] (x : ↥(RingHom.ker (algebraMap A B))) : (kerToTensor R A B) x = 1 ⊗ₜ[A] (D R A) ↑x

noncomputable def KaehlerDifferential.kerCotangentToTensor (R : Type u) [CommRing R] (A : Type u_2) (B : Type u_3) [CommRing A] [CommRing B] [Algebra R A] [Algebra A B] : (RingHom.ker (algebraMap A B)).Cotangent →ₗ[A] TensorProduct A B Ω[A⁄R]

The map I/I² → B ⊗[A] Ω[A⁄R] where I = ker(A → B).

@[simp]

theorem KaehlerDifferential.kerCotangentToTensor_toCotangent (R : Type u) [CommRing R] (A : Type u_2) (B : Type u_3) [CommRing A] [CommRing B] [Algebra R A] [Algebra A B] (x : ↥(RingHom.ker (algebraMap A B))) : (kerCotangentToTensor R A B) ((RingHom.ker (algebraMap A B)).toCotangent x) = 1 ⊗ₜ[A] (D R A) ↑x

theorem KaehlerDifferential.range_kerCotangentToTensor (R : Type u) [CommRing R] (A : Type u_2) (B : Type u_3) [CommRing A] [CommRing B] [Algebra R A] [Algebra A B] [Algebra R B] [IsScalarTower R A B] (h : Function.Surjective ⇑(algebraMap A B)) : LinearMap.range (kerCotangentToTensor R A B) = Submodule.restrictScalars A (LinearMap.ker (mapBaseChange R A B))

theorem KaehlerDifferential.exact_kerCotangentToTensor_mapBaseChange (R : Type u) [CommRing R] (A : Type u_2) (B : Type u_3) [CommRing A] [CommRing B] [Algebra R A] [Algebra A B] [Algebra R B] [IsScalarTower R A B] (h : Function.Surjective ⇑(algebraMap A B)) : Function.Exact ⇑(kerCotangentToTensor R A B) ⇑(mapBaseChange R A B)

theorem KaehlerDifferential.mapBaseChange_surjective (R : Type u) [CommRing R] (A : Type u_2) (B : Type u_3) [CommRing A] [CommRing B] [Algebra R A] [Algebra A B] [Algebra R B] [IsScalarTower R A B] (h : Function.Surjective ⇑(algebraMap A B)) : Function.Surjective ⇑(mapBaseChange R A B)