-- Mathlib/RingTheory/Unramified/Field.lean
theorem bijective_of_isAlgClosed_of_isLocalRing
    [IsAlgClosed K] [IsLocalRing A] :
    Function.Bijective (algebraMap K A)

Proof state:
case e_f.a
K : Type u_1
A : Type u_2
inst✝⁶ : Field K
inst✝⁵ : CommRing A
inst✝⁴ : Algebra K A
inst✝³ : FormallyUnramified K A
inst✝² : EssFiniteType K A
inst✝¹ : IsAlgClosed K
inst✝ : IsLocalRing A
this✝¹ : Module.Finite K A
this✝ : IsArtinianRing A
hA : IsNilpotent (IsLocalRing.maximalIdeal A)
this : Function.Bijective ⇑(ofId K (A ⧸ IsLocalRing.maximalIdeal A))
e : K ≃ₐ[K] A ⧸ IsLocalRing.maximalIdeal A :=
  let __spread.0 := ofId K (A ⧸ IsLocalRing.maximalIdeal A);
  let __spread.1 := Equiv.ofBijective (⇑(ofId K (A ⧸ IsLocalRing.maximalIdeal A))) this;
  { toFun := (↑↑__spread.0.toRingHom).toFun, invFun := __spread.1.invFun, left_inv := ⋯, right_inv := ⋯, map_mul' := ⋯,
    map_add' := ⋯, commutes' := ⋯ }
e' : A ⊗[K] (A ⧸ IsLocalRing.maximalIdeal A) ≃ₐ[A] A :=
  (TensorProduct.congr AlgEquiv.refl e.symm).trans (TensorProduct.rid K A A)
f : A ⧸ IsLocalRing.maximalIdeal A →ₗ[A] A := e'.toLinearMap ∘ₗ sec K A (A ⧸ IsLocalRing.maximalIdeal A)
⊢ ↑K ((ofId A (A ⧸ IsLocalRing.maximalIdeal A)).toLinearMap ∘ₗ e'.toLinearMap) =
    ↑K (TensorProduct.AlgebraTensorModule.lift (↑K (lsmul A A (A ⧸ IsLocalRing.maximalIdeal A)).toLinearMap.flip).flip)

Tactic:
apply _root_.TensorProduct.ext'

Result:
case e_f.a.H
K : Type u_1
A : Type u_2
inst✝⁶ : Field K
inst✝⁵ : CommRing A
inst✝⁴ : Algebra K A
inst✝³ : FormallyUnramified K A
inst✝² : EssFiniteType K A
inst✝¹ : IsAlgClosed K
inst✝ : IsLocalRing A
this✝¹ : Module.Finite K A
this✝ : IsArtinianRing A
hA : IsNilpotent (IsLocalRing.maximalIdeal A)
this : Function.Bijective ⇑(ofId K (A ⧸ IsLocalRing.maximalIdeal A))
e : K ≃ₐ[K] A ⧸ IsLocalRing.maximalIdeal A :=
  let __spread.0 := ofId K (A ⧸ IsLocalRing.maximalIdeal A);
  let __spread.1 := Equiv.ofBijective (⇑(ofId K (A ⧸ IsLocalRing.maximalIdeal A))) this;
  { toFun := (↑↑__spread.0.toRingHom).toFun, invFun := __spread.1.invFun, left_inv := ⋯, right_inv := ⋯, map_mul' := ⋯,
    map_add' := ⋯, commutes' := ⋯ }
e' : A ⊗[K] (A ⧸ IsLocalRing.maximalIdeal A) ≃ₐ[A] A :=
  (TensorProduct.congr AlgEquiv.refl e.symm).trans (TensorProduct.rid K A A)
f : A ⧸ IsLocalRing.maximalIdeal A →ₗ[A] A := e'.toLinearMap ∘ₗ sec K A (A ⧸ IsLocalRing.maximalIdeal A)
⊢ ∀ (x : A) (y : A ⧸ IsLocalRing.maximalIdeal A),
    (↑K ((ofId A (A ⧸ IsLocalRing.maximalIdeal A)).toLinearMap ∘ₗ e'.toLinearMap)) (x ⊗ₜ[K] y) =
      (↑K
          (TensorProduct.AlgebraTensorModule.lift
            (↑K (lsmul A A (A ⧸ IsLocalRing.maximalIdeal A)).toLinearMap.flip).flip))
        (x ⊗ₜ[K] y)

Proof state:
K : Type u_1
A : Type u_2
inst✝⁶ : Field K
inst✝⁵ : CommRing A
inst✝⁴ : Algebra K A
inst✝³ : FormallyUnramified K A
inst✝² : EssFiniteType K A
inst✝¹ : IsAlgClosed K
inst✝ : IsLocalRing A
this✝² : Module.Finite K A
this✝¹ : IsArtinianRing A
hA : IsNilpotent (IsLocalRing.maximalIdeal A)
this✝ : Function.Bijective ⇑(ofId K (A ⧸ IsLocalRing.maximalIdeal A))
e : K ≃ₐ[K] A ⧸ IsLocalRing.maximalIdeal A :=
  let __spread.0 := ofId K (A ⧸ IsLocalRing.maximalIdeal A);
  let __spread.1 := Equiv.ofBijective (⇑(ofId K (A ⧸ IsLocalRing.maximalIdeal A))) this✝;
  { toFun := (↑↑__spread.0.toRingHom).toFun, invFun := __spread.1.invFun, left_inv := ⋯, right_inv := ⋯, map_mul' := ⋯,
    map_add' := ⋯, commutes' := ⋯ }
e' : A ⊗[K] (A ⧸ IsLocalRing.maximalIdeal A) ≃ₐ[A] A :=
  (TensorProduct.congr AlgEquiv.refl e.symm).trans (TensorProduct.rid K A A)
f : A ⧸ IsLocalRing.maximalIdeal A →ₗ[A] A := e'.toLinearMap ∘ₗ sec K A (A ⧸ IsLocalRing.maximalIdeal A)
hf : (ofId A (A ⧸ IsLocalRing.maximalIdeal A)).toLinearMap ∘ₗ f = LinearMap.id
hf₁ : f 1 • 1 = 1
hf₂ : 1 - f 1 ∈ IsLocalRing.maximalIdeal A
hf₃ : IsIdempotentElem (1 - f 1)
hf₄ : f 1 = 1
x : A
this : ⇑(algebraMap K A) = ⇑f ∘ ⇑e
⊢ (⇑f ∘ ⇑e) ((⇑e.symm ∘ ⇑(algebraMap A (A ⧸ IsLocalRing.maximalIdeal A))) x) = x

Tactic:
simp only [Function.comp_apply, AlgEquiv.apply_symm_apply, algebraMap_eq_smul_one,
  map_smul, hf₄, smul_eq_mul, mul_one]

Result:
no goals

Proof state:
K : Type u_1
A : Type u_2
inst✝⁶ : Field K
inst✝⁵ : CommRing A
inst✝⁴ : Algebra K A
inst✝³ : FormallyUnramified K A
inst✝² : EssFiniteType K A
inst✝¹ : IsAlgClosed K
inst✝ : IsLocalRing A
this✝¹ : Module.Finite K A
this✝ : IsArtinianRing A
hA : IsNilpotent (IsLocalRing.maximalIdeal A)
this : Function.Bijective ⇑(ofId K (A ⧸ IsLocalRing.maximalIdeal A))
e : K ≃ₐ[K] A ⧸ IsLocalRing.maximalIdeal A :=
  let __spread.0 := ofId K (A ⧸ IsLocalRing.maximalIdeal A);
  let __spread.1 := Equiv.ofBijective (⇑(ofId K (A ⧸ IsLocalRing.maximalIdeal A))) this;
  { toFun := (↑↑__spread.0.toRingHom).toFun, invFun := __spread.1.invFun, left_inv := ⋯, right_inv := ⋯, map_mul' := ⋯,
    map_add' := ⋯, commutes' := ⋯ }
e' : A ⊗[K] (A ⧸ IsLocalRing.maximalIdeal A) ≃ₐ[A] A :=
  (TensorProduct.congr AlgEquiv.refl e.symm).trans (TensorProduct.rid K A A)
f : A ⧸ IsLocalRing.maximalIdeal A →ₗ[A] A := e'.toLinearMap ∘ₗ sec K A (A ⧸ IsLocalRing.maximalIdeal A)
hf : (ofId A (A ⧸ IsLocalRing.maximalIdeal A)).toLinearMap ∘ₗ f = LinearMap.id
hf₁ : f 1 • 1 = 1
hf₂ : 1 - f 1 ∈ IsLocalRing.maximalIdeal A
⊢ IsIdempotentElem (1 - f 1)

Tactic:
apply IsIdempotentElem.one_sub

Result:
case h
K : Type u_1
A : Type u_2
inst✝⁶ : Field K
inst✝⁵ : CommRing A
inst✝⁴ : Algebra K A
inst✝³ : FormallyUnramified K A
inst✝² : EssFiniteType K A
inst✝¹ : IsAlgClosed K
inst✝ : IsLocalRing A
this✝¹ : Module.Finite K A
this✝ : IsArtinianRing A
hA : IsNilpotent (IsLocalRing.maximalIdeal A)
this : Function.Bijective ⇑(ofId K (A ⧸ IsLocalRing.maximalIdeal A))
e : K ≃ₐ[K] A ⧸ IsLocalRing.maximalIdeal A :=
  let __spread.0 := ofId K (A ⧸ IsLocalRing.maximalIdeal A);
  let __spread.1 := Equiv.ofBijective (⇑(ofId K (A ⧸ IsLocalRing.maximalIdeal A))) this;
  { toFun := (↑↑__spread.0.toRingHom).toFun, invFun := __spread.1.invFun, left_inv := ⋯, right_inv := ⋯, map_mul' := ⋯,
    map_add' := ⋯, commutes' := ⋯ }
e' : A ⊗[K] (A ⧸ IsLocalRing.maximalIdeal A) ≃ₐ[A] A :=
  (TensorProduct.congr AlgEquiv.refl e.symm).trans (TensorProduct.rid K A A)
f : A ⧸ IsLocalRing.maximalIdeal A →ₗ[A] A := e'.toLinearMap ∘ₗ sec K A (A ⧸ IsLocalRing.maximalIdeal A)
hf : (ofId A (A ⧸ IsLocalRing.maximalIdeal A)).toLinearMap ∘ₗ f = LinearMap.id
hf₁ : f 1 • 1 = 1
hf₂ : 1 - f 1 ∈ IsLocalRing.maximalIdeal A
⊢ IsIdempotentElem (f 1)

Proof state:
K : Type u_1
A : Type u_2
inst✝⁶ : Field K
inst✝⁵ : CommRing A
inst✝⁴ : Algebra K A
inst✝³ : FormallyUnramified K A
inst✝² : EssFiniteType K A
inst✝¹ : IsAlgClosed K
inst✝ : IsLocalRing A
this✝¹ : Module.Finite K A
this✝ : IsArtinianRing A
hA : IsNilpotent (IsLocalRing.maximalIdeal A)
this : Function.Bijective ⇑(ofId K (A ⧸ IsLocalRing.maximalIdeal A))
e : K ≃ₐ[K] A ⧸ IsLocalRing.maximalIdeal A :=
  let __spread.0 := ofId K (A ⧸ IsLocalRing.maximalIdeal A);
  let __spread.1 := Equiv.ofBijective (⇑(ofId K (A ⧸ IsLocalRing.maximalIdeal A))) this;
  { toFun := (↑↑__spread.0.toRingHom).toFun, invFun := __spread.1.invFun, left_inv := ⋯, right_inv := ⋯, map_mul' := ⋯,
    map_add' := ⋯, commutes' := ⋯ }
e' : A ⊗[K] (A ⧸ IsLocalRing.maximalIdeal A) ≃ₐ[A] A :=
  (TensorProduct.congr AlgEquiv.refl e.symm).trans (TensorProduct.rid K A A)
f : A ⧸ IsLocalRing.maximalIdeal A →ₗ[A] A := e'.toLinearMap ∘ₗ sec K A (A ⧸ IsLocalRing.maximalIdeal A)
hf : (ofId A (A ⧸ IsLocalRing.maximalIdeal A)).toLinearMap ∘ₗ f = LinearMap.id
⊢ (algebraMap A (A ⧸ IsLocalRing.maximalIdeal A)) (f 1) = 1

Tactic:
exact LinearMap.congr_fun hf 1

Result:
no goals