Module: Mathlib.RingTheory.Smooth.Pi

Goal state before:
pp: case intro.mpr
R : Type u_1
I : Type u_2
A : I → Type u_3
inst✝³ : CommRing R
inst✝² : (i : I) → CommRing (A i)
inst✝¹ : (i : I) → Algebra R (A i)
inst✝ : Finite I
val✝ : Fintype I
H : ∀ (i : I), FormallySmooth R (A i)
B : Type (max u_1 u_2 u_3)
x✝¹ : CommRing B
x✝ : Algebra R B
J : Ideal B
hJ : J ^ 2 = ⊥
g : ((i : I) → A i) →ₐ[R] B ⧸ J
hJ' : ∀ x ∈ RingHom.ker (Ideal.Quotient.mk J), IsNilpotent x
e : I → B
he : CompleteOrthogonalIdempotents e
he' : ∀ (i : I), (Ideal.Quotient.mk J) (e i) = g (Pi.single i 1)
iso : B ≃ₐ[R] (i : I) → B ⧸ Ideal.span {1 - e i} :=
  let __spread.0 := AlgHom.pi fun i ↦ Ideal.Quotient.mkₐ R (Ideal.span {1 - e i});
  let __spread.1 := Equiv.ofBijective ⇑(RingHom.pi fun i ↦ Ideal.Quotient.mk (Ideal.span {1 - e i})) ⋯;
  { toFun := (↑↑__spread.0.toRingHom).toFun, invFun := __spread.1.invFun, left_inv := ⋯, right_inv := ⋯, map_mul' := ⋯,
    map_add' := ⋯, commutes' := ⋯ }
J' : (i : I) → Ideal (B ⧸ Ideal.span {1 - e i}) := fun i ↦ Ideal.map (Ideal.Quotient.mk (Ideal.span {1 - e i})) J
ι : (i : I) → B ⧸ J →ₐ[R] (B ⧸ Ideal.span {1 - e i}) ⧸ J' i :=
  fun i ↦ Ideal.quotientMapₐ (J' i) (IsScalarTower.toAlgHom R B (B ⧸ Ideal.span {1 - e i})) ⋯
hι : ∀ (i : I) (x : B ⧸ J), (ι i) x = 0 → (Ideal.Quotient.mk J) (e i) * x = 0
⊢ ∃ a, (Ideal.Quotient.mkₐ R J).comp a = g

Tactic:
have : ∀ i : I, ∃ a : A i →ₐ[R] B ⧸ Ideal.span {1 - e i}, ∀ x,
        Ideal.Quotient.mk (J' i) (a x) = ι i (g (Pi.single i x)) := by
      intro i
      let g' : A i →ₐ[R] (B ⧸ _) ⧸ (J' i) := by
        apply AlgHom.ofLinearMap (((ι i).comp g).toLinearMap ∘ₗ LinearMap.single _ _ i)
        · suffices Ideal.Quotient.mk (Ideal.span {1 - e i}) (e i) = 1 by simp [ι, ← he', this]
          rw [← (Ideal.Quotient.mk _).map_one, eq_comm, Ideal.Quotient.mk_eq_mk_iff_sub_mem,
            Ideal.mem_span_singleton]
        · intro x y; simp [Pi.single_mul]
      obtain ⟨a, ha⟩ := FormallySmooth.comp_surjective _ _ (I := J' i)
        (by rw [← Ideal.map_pow, hJ, Ideal.map_bot]) g'
      exact ⟨a, AlgHom.congr_fun ha⟩

Goal state after:
case intro.mpr
R : Type u_1
I : Type u_2
A : I → Type u_3
inst✝³ : CommRing R
inst✝² : (i : I) → CommRing (A i)
inst✝¹ : (i : I) → Algebra R (A i)
inst✝ : Finite I
val✝ : Fintype I
H : ∀ (i : I), FormallySmooth R (A i)
B : Type (max u_1 u_2 u_3)
x✝¹ : CommRing B
x✝ : Algebra R B
J : Ideal B
hJ : J ^ 2 = ⊥
g : ((i : I) → A i) →ₐ[R] B ⧸ J
hJ' : ∀ x ∈ RingHom.ker (Ideal.Quotient.mk J), IsNilpotent x
e : I → B
he : CompleteOrthogonalIdempotents e
he' : ∀ (i : I), (Ideal.Quotient.mk J) (e i) = g (Pi.single i 1)
iso : B ≃ₐ[R] (i : I) → B ⧸ Ideal.span {1 - e i} :=
  let __spread.0 := AlgHom.pi fun i ↦ Ideal.Quotient.mkₐ R (Ideal.span {1 - e i});
  let __spread.1 := Equiv.ofBijective ⇑(RingHom.pi fun i ↦ Ideal.Quotient.mk (Ideal.span {1 - e i})) ⋯;
  { toFun := (↑↑__spread.0.toRingHom).toFun, invFun := __spread.1.invFun, left_inv := ⋯, right_inv := ⋯, map_mul' := ⋯,
    map_add' := ⋯, commutes' := ⋯ }
J' : (i : I) → Ideal (B ⧸ Ideal.span {1 - e i}) := fun i ↦ Ideal.map (Ideal.Quotient.mk (Ideal.span {1 - e i})) J
ι : (i : I) → B ⧸ J →ₐ[R] (B ⧸ Ideal.span {1 - e i}) ⧸ J' i :=
  fun i ↦ Ideal.quotientMapₐ (J' i) (IsScalarTower.toAlgHom R B (B ⧸ Ideal.span {1 - e i})) ⋯
hι : ∀ (i : I) (x : B ⧸ J), (ι i) x = 0 → (Ideal.Quotient.mk J) (e i) * x = 0
this : ∀ (i : I), ∃ a, ∀ (x : A i), (Ideal.Quotient.mk (J' i)) (a x) = (ι i) (g (Pi.single i x))
⊢ ∃ a, (Ideal.Quotient.mkₐ R J).comp a = g