-- Mathlib/RingTheory/Etale/Kaehler.lean
lemma tensorCotangentInvFun_smul_mk
    [alg : Algebra P.Ring Q.Ring] (halg : algebraMap P.Ring Q.Ring = f.toRingHom)
    (H : Function.Bijective ((f.mapKer halg).liftBaseChange Q.Ring)) (x : Q.Ring) (y : P.ker) :
    tensorCotangentInvFun f halg H (x • .mk ⟨f.toRingHom y, (f.mapKer halg y).2⟩) =
      x • 1 ⊗ₜ .mk y

Proof state:
R S T : Type u
inst✝⁵ : CommRing R
inst✝⁴ : CommRing S
inst✝³ : CommRing T
inst✝² : Algebra R S
inst✝¹ : Algebra R T
inst✝ : Algebra S T
P : Extension R S
Q : Extension R T
f : P.Hom Q
alg : Algebra P.Ring Q.Ring
halg : algebraMap P.Ring Q.Ring = f.toRingHom
H : Function.Bijective ⇑(LinearMap.liftBaseChange Q.Ring (f.mapKer halg))
x : Q.Ring
y : ↥P.ker
this✝¹ : Algebra P.Ring T := ((algebraMap S T).comp (algebraMap P.Ring S)).toAlgebra
this✝ : IsScalarTower P.Ring S T
this : IsScalarTower P.Ring Q.Ring T
e : Q.Ring ⊗[P.Ring] ↥P.ker ≃ₗ[Q.Ring] ↥Q.ker :=
  LinearEquiv.ofBijective (LinearMap.liftBaseChange Q.Ring (f.mapKer halg)) H
⊢ (tensorCotangentInvFun f halg H) (Cotangent.mk ((LinearMap.liftBaseChange Q.Ring (f.mapKer halg)) (x ⊗ₜ[P.Ring] y))) =
    x • 1 ⊗ₜ[S] Cotangent.mk y

Tactic:
show ((TensorProduct.mk _ _ _ 1).restrictScalars _ ∘ₗ Cotangent.mk).liftBaseChange _
  (e.symm (e (x ⊗ₜ y))) = _

Result:
R S T : Type u
inst✝⁵ : CommRing R
inst✝⁴ : CommRing S
inst✝³ : CommRing T
inst✝² : Algebra R S
inst✝¹ : Algebra R T
inst✝ : Algebra S T
P : Extension R S
Q : Extension R T
f : P.Hom Q
alg : Algebra P.Ring Q.Ring
halg : algebraMap P.Ring Q.Ring = f.toRingHom
H : Function.Bijective ⇑(LinearMap.liftBaseChange Q.Ring (f.mapKer halg))
x : Q.Ring
y : ↥P.ker
this✝¹ : Algebra P.Ring T := ((algebraMap S T).comp (algebraMap P.Ring S)).toAlgebra
this✝ : IsScalarTower P.Ring S T
this : IsScalarTower P.Ring Q.Ring T
e : Q.Ring ⊗[P.Ring] ↥P.ker ≃ₗ[Q.Ring] ↥Q.ker :=
  LinearEquiv.ofBijective (LinearMap.liftBaseChange Q.Ring (f.mapKer halg)) H
⊢ (LinearMap.liftBaseChange Q.Ring (↑P.Ring ((TensorProduct.mk S T P.Cotangent) 1) ∘ₗ Cotangent.mk))
      (e.symm (e (x ⊗ₜ[P.Ring] y))) =
    x • 1 ⊗ₜ[S] Cotangent.mk y

Proof state:
R S T : Type u
inst✝⁵ : CommRing R
inst✝⁴ : CommRing S
inst✝³ : CommRing T
inst✝² : Algebra R S
inst✝¹ : Algebra R T
inst✝ : Algebra S T
P : Extension R S
Q : Extension R T
f : P.Hom Q
alg : Algebra P.Ring Q.Ring
halg : algebraMap P.Ring Q.Ring = f.toRingHom
H : Function.Bijective ⇑(LinearMap.liftBaseChange Q.Ring (f.mapKer halg))
x : Q.Ring
y : ↥P.ker
this✝¹ : Algebra P.Ring T := ((algebraMap S T).comp (algebraMap P.Ring S)).toAlgebra
this✝ : IsScalarTower P.Ring S T
this : IsScalarTower P.Ring Q.Ring T
e : Q.Ring ⊗[P.Ring] ↥P.ker ≃ₗ[Q.Ring] ↥Q.ker :=
  LinearEquiv.ofBijective (LinearMap.liftBaseChange Q.Ring (f.mapKer halg)) H
⊢ (tensorCotangentInvFun f halg H) (x • Cotangent.mk ⟨f.toRingHom ↑y, ⋯⟩) =
    (tensorCotangentInvFun f halg H) (x • Cotangent.mk ((f.mapKer halg) y))

Tactic:
rfl

Result:
no goals

Proof state:
R S T : Type u
inst✝⁵ : CommRing R
inst✝⁴ : CommRing S
inst✝³ : CommRing T
inst✝² : Algebra R S
inst✝¹ : Algebra R T
inst✝ : Algebra S T
P : Extension R S
Q : Extension R T
f : P.Hom Q
alg : Algebra P.Ring Q.Ring
halg : algebraMap P.Ring Q.Ring = f.toRingHom
H : Function.Bijective ⇑(LinearMap.liftBaseChange Q.Ring (f.mapKer halg))
x : Q.Ring
y : ↥P.ker
this✝¹ : Algebra P.Ring T := ((algebraMap S T).comp (algebraMap P.Ring S)).toAlgebra
this✝ : IsScalarTower P.Ring S T
this : IsScalarTower P.Ring Q.Ring T
e : Q.Ring ⊗[P.Ring] ↥P.ker ≃ₗ[Q.Ring] ↥Q.ker :=
  LinearEquiv.ofBijective (LinearMap.liftBaseChange Q.Ring (f.mapKer halg)) H
⊢ (tensorCotangentInvFun f halg H) (x • Cotangent.mk ⟨f.toRingHom ↑y, ⋯⟩) =
    (tensorCotangentInvFun f halg H) (Cotangent.mk ((LinearMap.liftBaseChange Q.Ring (f.mapKer halg)) (x ⊗ₜ[P.Ring] y)))

Tactic:
simp

Result:
R S T : Type u
inst✝⁵ : CommRing R
inst✝⁴ : CommRing S
inst✝³ : CommRing T
inst✝² : Algebra R S
inst✝¹ : Algebra R T
inst✝ : Algebra S T
P : Extension R S
Q : Extension R T
f : P.Hom Q
alg : Algebra P.Ring Q.Ring
halg : algebraMap P.Ring Q.Ring = f.toRingHom
H : Function.Bijective ⇑(LinearMap.liftBaseChange Q.Ring (f.mapKer halg))
x : Q.Ring
y : ↥P.ker
this✝¹ : Algebra P.Ring T := ((algebraMap S T).comp (algebraMap P.Ring S)).toAlgebra
this✝ : IsScalarTower P.Ring S T
this : IsScalarTower P.Ring Q.Ring T
e : Q.Ring ⊗[P.Ring] ↥P.ker ≃ₗ[Q.Ring] ↥Q.ker :=
  LinearEquiv.ofBijective (LinearMap.liftBaseChange Q.Ring (f.mapKer halg)) H
⊢ (tensorCotangentInvFun f halg H) (x • Cotangent.mk ⟨f.toRingHom ↑y, ⋯⟩) =
    (tensorCotangentInvFun f halg H) (x • Cotangent.mk ((f.mapKer halg) y))

Proof state:
R S T : Type u
inst✝⁵ : CommRing R
inst✝⁴ : CommRing S
inst✝³ : CommRing T
inst✝² : Algebra R S
inst✝¹ : Algebra R T
inst✝ : Algebra S T
P : Extension R S
Q : Extension R T
f : P.Hom Q
alg : Algebra P.Ring Q.Ring
halg : algebraMap P.Ring Q.Ring = f.toRingHom
H : Function.Bijective ⇑(LinearMap.liftBaseChange Q.Ring (f.mapKer halg))
x : Q.Ring
y : ↥P.ker
this✝ : Algebra P.Ring T := ((algebraMap S T).comp (algebraMap P.Ring S)).toAlgebra
this : IsScalarTower P.Ring S T
⊢ (tensorCotangentInvFun f halg H) (x • Cotangent.mk ⟨f.toRingHom ↑y, ⋯⟩) = x • 1 ⊗ₜ[S] Cotangent.mk y

Tactic:
haveI : IsScalarTower P.Ring Q.Ring T :=
  .of_algebraMap_eq fun r ↦ halg ▸ (f.algebraMap_toRingHom r).symm

Result:
R S T : Type u
inst✝⁵ : CommRing R
inst✝⁴ : CommRing S
inst✝³ : CommRing T
inst✝² : Algebra R S
inst✝¹ : Algebra R T
inst✝ : Algebra S T
P : Extension R S
Q : Extension R T
f : P.Hom Q
alg : Algebra P.Ring Q.Ring
halg : algebraMap P.Ring Q.Ring = f.toRingHom
H : Function.Bijective ⇑(LinearMap.liftBaseChange Q.Ring (f.mapKer halg))
x : Q.Ring
y : ↥P.ker
this✝¹ : Algebra P.Ring T := ((algebraMap S T).comp (algebraMap P.Ring S)).toAlgebra
this✝ : IsScalarTower P.Ring S T
this : IsScalarTower P.Ring Q.Ring T
⊢ (tensorCotangentInvFun f halg H) (x • Cotangent.mk ⟨f.toRingHom ↑y, ⋯⟩) = x • 1 ⊗ₜ[S] Cotangent.mk y