-- Mathlib/RingTheory/Jacobson/Ring.lean
theorem isIntegral_isLocalization_polynomial_quotient
    (P : Ideal R[X]) (pX : R[X]) (hpX : pX ∈ P) [Algebra (R ⧸ P.comap (C : R →+* R[X])) Rₘ]
    [IsLocalization.Away (pX.map (Ideal.Quotient.mk (P.comap (C : R →+* R[X])))).leadingCoeff Rₘ]
    [Algebra (R[X] ⧸ P) Sₘ] [IsLocalization ((Submonoid.powers (pX.map (Ideal.Quotient.mk (P.comap
      (C : R →+* R[X])))).leadingCoeff).map (quotientMap P C le_rfl) : Submonoid (R[X] ⧸ P)) Sₘ] :
    (IsLocalization.map Sₘ (quotientMap P C le_rfl) (Submonoid.powers (pX.map (Ideal.Quotient.mk
      (P.comap (C : R →+* R[X])))).leadingCoeff).le_comap_map : Rₘ →+* Sₘ).IsIntegral

Proof state:
case intro.mk.mk
R : Type u_1
inst✝⁶ : CommRing R
Rₘ : Type u_3
Sₘ : Type u_4
inst✝⁵ : CommRing Rₘ
inst✝⁴ : CommRing Sₘ
P : Ideal R[X]
pX : R[X]
hpX : pX ∈ P
inst✝³ : Algebra (R ⧸ comap C P) Rₘ
inst✝² : IsLocalization.Away (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff Rₘ
inst✝¹ : Algebra (R[X] ⧸ P) Sₘ
inst✝ :
  IsLocalization
    (Submonoid.map (quotientMap P C ⋯) (Submonoid.powers (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff)) Sₘ
P' : Ideal R := comap C P
M : Submonoid (R ⧸ P') := Submonoid.powers (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff
M' : Submonoid (R[X] ⧸ P) :=
  Submonoid.map (quotientMap P C ⋯) (Submonoid.powers (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff)
φ : R ⧸ P' →+* R[X] ⧸ P := quotientMap P C ⋯
φ' : Rₘ →+* Sₘ := IsLocalization.map Sₘ φ ⋯
hφ' : φ.comp (Ideal.Quotient.mk P') = (Ideal.Quotient.mk P).comp C
p : Sₘ
p' q : R[X] ⧸ P
hq : q ∈ M'
hp : p * (algebraMap (R[X] ⧸ P) Sₘ) ↑(p', ⟨q, hq⟩).2 = (algebraMap (R[X] ⧸ P) Sₘ) (p', ⟨q, hq⟩).1
⊢ φ'.IsIntegralElem ((algebraMap (R[X] ⧸ P) Sₘ) p')

Tactic:
dsimp at hp

Result:
case intro.mk.mk
R : Type u_1
inst✝⁶ : CommRing R
Rₘ : Type u_3
Sₘ : Type u_4
inst✝⁵ : CommRing Rₘ
inst✝⁴ : CommRing Sₘ
P : Ideal R[X]
pX : R[X]
hpX : pX ∈ P
inst✝³ : Algebra (R ⧸ comap C P) Rₘ
inst✝² : IsLocalization.Away (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff Rₘ
inst✝¹ : Algebra (R[X] ⧸ P) Sₘ
inst✝ :
  IsLocalization
    (Submonoid.map (quotientMap P C ⋯) (Submonoid.powers (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff)) Sₘ
P' : Ideal R := comap C P
M : Submonoid (R ⧸ P') := Submonoid.powers (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff
M' : Submonoid (R[X] ⧸ P) :=
  Submonoid.map (quotientMap P C ⋯) (Submonoid.powers (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff)
φ : R ⧸ P' →+* R[X] ⧸ P := quotientMap P C ⋯
φ' : Rₘ →+* Sₘ := IsLocalization.map Sₘ φ ⋯
hφ' : φ.comp (Ideal.Quotient.mk P') = (Ideal.Quotient.mk P).comp C
p : Sₘ
p' q : R[X] ⧸ P
hq : q ∈ M'
hp : p * (algebraMap (R[X] ⧸ P) Sₘ) q = (algebraMap (R[X] ⧸ P) Sₘ) p'
⊢ φ'.IsIntegralElem ((algebraMap (R[X] ⧸ P) Sₘ) p')

Proof state:
case intro.intro.intro
R : Type u_1
inst✝⁶ : CommRing R
Rₘ : Type u_3
Sₘ : Type u_4
inst✝⁵ : CommRing Rₘ
inst✝⁴ : CommRing Sₘ
P : Ideal R[X]
pX : R[X]
hpX : pX ∈ P
inst✝³ : Algebra (R ⧸ comap C P) Rₘ
inst✝² : IsLocalization.Away (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff Rₘ
inst✝¹ : Algebra (R[X] ⧸ P) Sₘ
inst✝ :
  IsLocalization
    (Submonoid.map (quotientMap P C ⋯) (Submonoid.powers (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff)) Sₘ
P' : Ideal R := comap C P
M : Submonoid (R ⧸ P') := Submonoid.powers (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff
M' : Submonoid (R[X] ⧸ P) :=
  Submonoid.map (quotientMap P C ⋯) (Submonoid.powers (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff)
φ : R ⧸ P' →+* R[X] ⧸ P := quotientMap P C ⋯
φ' : Rₘ →+* Sₘ := IsLocalization.map Sₘ φ ⋯
hφ' : φ.comp (Ideal.Quotient.mk P') = (Ideal.Quotient.mk P).comp C
p : Sₘ
p' : R[X] ⧸ P
this : φ'.IsIntegralElem ((algebraMap (R[X] ⧸ P) Sₘ) p')
q' : R ⧸ comap C P
hq' : q' ∈ ↑(Submonoid.powers (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff)
hp :
  p * (algebraMap (R[X] ⧸ P) Sₘ) ↑(p', ⟨(quotientMap P C ⋯) q', ⋯⟩).2 =
    (algebraMap (R[X] ⧸ P) Sₘ) (p', ⟨(quotientMap P C ⋯) q', ⋯⟩).1
q'' : Rₘ
hq'' : q'' * (algebraMap (R ⧸ P') Rₘ) ↑⟨q', hq'⟩ = 1
⊢ φ' q'' * (algebraMap (R[X] ⧸ P) Sₘ) (φ q') = φ' q'' * (φ'.comp (algebraMap (R ⧸ P') Rₘ)) ↑⟨q', hq'⟩

Tactic:
simp only [φ', IsLocalization.map_comp _]

Result:
case intro.intro.intro
R : Type u_1
inst✝⁶ : CommRing R
Rₘ : Type u_3
Sₘ : Type u_4
inst✝⁵ : CommRing Rₘ
inst✝⁴ : CommRing Sₘ
P : Ideal R[X]
pX : R[X]
hpX : pX ∈ P
inst✝³ : Algebra (R ⧸ comap C P) Rₘ
inst✝² : IsLocalization.Away (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff Rₘ
inst✝¹ : Algebra (R[X] ⧸ P) Sₘ
inst✝ :
  IsLocalization
    (Submonoid.map (quotientMap P C ⋯) (Submonoid.powers (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff)) Sₘ
P' : Ideal R := comap C P
M : Submonoid (R ⧸ P') := Submonoid.powers (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff
M' : Submonoid (R[X] ⧸ P) :=
  Submonoid.map (quotientMap P C ⋯) (Submonoid.powers (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff)
φ : R ⧸ P' →+* R[X] ⧸ P := quotientMap P C ⋯
φ' : Rₘ →+* Sₘ := IsLocalization.map Sₘ φ ⋯
hφ' : φ.comp (Ideal.Quotient.mk P') = (Ideal.Quotient.mk P).comp C
p : Sₘ
p' : R[X] ⧸ P
this : φ'.IsIntegralElem ((algebraMap (R[X] ⧸ P) Sₘ) p')
q' : R ⧸ comap C P
hq' : q' ∈ ↑(Submonoid.powers (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff)
hp :
  p * (algebraMap (R[X] ⧸ P) Sₘ) ↑(p', ⟨(quotientMap P C ⋯) q', ⋯⟩).2 =
    (algebraMap (R[X] ⧸ P) Sₘ) (p', ⟨(quotientMap P C ⋯) q', ⋯⟩).1
q'' : Rₘ
hq'' : q'' * (algebraMap (R ⧸ P') Rₘ) ↑⟨q', hq'⟩ = 1
⊢ (IsLocalization.map Sₘ φ ⋯) q'' * (algebraMap (R[X] ⧸ P) Sₘ) (φ q') =
    (IsLocalization.map Sₘ φ ⋯) q'' * ((algebraMap (R[X] ⧸ P) Sₘ).comp φ) q'

Proof state:
case intro.mk.mk.refine_1.intro.intro.inl
R : Type u_1
inst✝⁶ : CommRing R
Rₘ : Type u_3
Sₘ : Type u_4
inst✝⁵ : CommRing Rₘ
inst✝⁴ : CommRing Sₘ
P : Ideal R[X]
pX : R[X]
hpX : pX ∈ P
inst✝³ : Algebra (R ⧸ comap C P) Rₘ
inst✝² : IsLocalization.Away (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff Rₘ
inst✝¹ : Algebra (R[X] ⧸ P) Sₘ
inst✝ :
  IsLocalization
    (Submonoid.map (quotientMap P C ⋯) (Submonoid.powers (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff)) Sₘ
P' : Ideal R := comap C P
M : Submonoid (R ⧸ P') := Submonoid.powers (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff
M' : Submonoid (R[X] ⧸ P) :=
  Submonoid.map (quotientMap P C ⋯) (Submonoid.powers (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff)
φ : R ⧸ P' →+* R[X] ⧸ P := quotientMap P C ⋯
φ' : Rₘ →+* Sₘ := IsLocalization.map Sₘ φ ⋯
hφ' : φ.comp (Ideal.Quotient.mk P') = (Ideal.Quotient.mk P).comp C
p✝ : Sₘ
p' q : R[X] ⧸ P
hq : q ∈ M'
hp : p✝ * (algebraMap (R[X] ⧸ P) Sₘ) q = (algebraMap (R[X] ⧸ P) Sₘ) p'
p : R[X]
hy : p = X
⊢ φ'.IsIntegralElem (((algebraMap (R[X] ⧸ P) Sₘ).comp (Ideal.Quotient.mk P)) p)

Tactic:
rw [hy]

Result:
case intro.mk.mk.refine_1.intro.intro.inl
R : Type u_1
inst✝⁶ : CommRing R
Rₘ : Type u_3
Sₘ : Type u_4
inst✝⁵ : CommRing Rₘ
inst✝⁴ : CommRing Sₘ
P : Ideal R[X]
pX : R[X]
hpX : pX ∈ P
inst✝³ : Algebra (R ⧸ comap C P) Rₘ
inst✝² : IsLocalization.Away (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff Rₘ
inst✝¹ : Algebra (R[X] ⧸ P) Sₘ
inst✝ :
  IsLocalization
    (Submonoid.map (quotientMap P C ⋯) (Submonoid.powers (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff)) Sₘ
P' : Ideal R := comap C P
M : Submonoid (R ⧸ P') := Submonoid.powers (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff
M' : Submonoid (R[X] ⧸ P) :=
  Submonoid.map (quotientMap P C ⋯) (Submonoid.powers (map (Ideal.Quotient.mk (comap C P)) pX).leadingCoeff)
φ : R ⧸ P' →+* R[X] ⧸ P := quotientMap P C ⋯
φ' : Rₘ →+* Sₘ := IsLocalization.map Sₘ φ ⋯
hφ' : φ.comp (Ideal.Quotient.mk P') = (Ideal.Quotient.mk P).comp C
p✝ : Sₘ
p' q : R[X] ⧸ P
hq : q ∈ M'
hp : p✝ * (algebraMap (R[X] ⧸ P) Sₘ) q = (algebraMap (R[X] ⧸ P) Sₘ) p'
p : R[X]
hy : p = X
⊢ φ'.IsIntegralElem (((algebraMap (R[X] ⧸ P) Sₘ).comp (Ideal.Quotient.mk P)) X)