-- Mathlib/RingTheory/Jacobson/Ring.lean
theorem quotient_mk_comp_C_isIntegral_of_jacobson' [Nontrivial R] (hR : IsJacobsonRing R)
    (hP' : ∀ x : R, C x ∈ P → x = 0) :
    ((Ideal.Quotient.mk P).comp C : R →+* R[X] ⧸ P).IsIntegral

Proof state:
R : Type u_1
inst✝¹ : CommRing R
P : Ideal R[X]
hP : P.IsMaximal
inst✝ : Nontrivial R
hR : IsJacobsonRing R
hP' : ∀ (x : R), C x ∈ P → x = 0
P' : Ideal R := comap C P
pX : R[X]
hpX : pX ∈ P
hp0 : map (Ideal.Quotient.mk (comap C P)) pX ≠ 0
a : R ⧸ P' := (map (Ideal.Quotient.mk P') pX).leadingCoeff
M : Submonoid (R ⧸ P') := Submonoid.powers a
φ : R ⧸ P' →+* R[X] ⧸ P := quotientMap P C ⋯
hP'_prime : P'.IsPrime
hM : 0 ∉ M
M' : Submonoid (R[X] ⧸ P) := Submonoid.map φ M
this :
  (algebraMap (R[X] ⧸ P) (Localization M')).comp φ =
    (IsLocalization.map (Localization M') φ ⋯).comp (algebraMap (R ⧸ P') (Localization M))
⊢ ((IsLocalization.map (Localization M') φ ⋯).comp (algebraMap (R ⧸ P') (Localization M))).IsIntegral

Tactic:
refine RingHom.IsIntegral.trans (algebraMap (R ⧸ P') (Localization M))
  (IsLocalization.map (Localization M') φ M.le_comap_map) ?_ ?_

Result:
case refine_1
R : Type u_1
inst✝¹ : CommRing R
P : Ideal R[X]
hP : P.IsMaximal
inst✝ : Nontrivial R
hR : IsJacobsonRing R
hP' : ∀ (x : R), C x ∈ P → x = 0
P' : Ideal R := comap C P
pX : R[X]
hpX : pX ∈ P
hp0 : map (Ideal.Quotient.mk (comap C P)) pX ≠ 0
a : R ⧸ P' := (map (Ideal.Quotient.mk P') pX).leadingCoeff
M : Submonoid (R ⧸ P') := Submonoid.powers a
φ : R ⧸ P' →+* R[X] ⧸ P := quotientMap P C ⋯
hP'_prime : P'.IsPrime
hM : 0 ∉ M
M' : Submonoid (R[X] ⧸ P) := Submonoid.map φ M
this :
  (algebraMap (R[X] ⧸ P) (Localization M')).comp φ =
    (IsLocalization.map (Localization M') φ ⋯).comp (algebraMap (R ⧸ P') (Localization M))
⊢ (algebraMap (R ⧸ P') (Localization M)).IsIntegral

case refine_2
R : Type u_1
inst✝¹ : CommRing R
P : Ideal R[X]
hP : P.IsMaximal
inst✝ : Nontrivial R
hR : IsJacobsonRing R
hP' : ∀ (x : R), C x ∈ P → x = 0
P' : Ideal R := comap C P
pX : R[X]
hpX : pX ∈ P
hp0 : map (Ideal.Quotient.mk (comap C P)) pX ≠ 0
a : R ⧸ P' := (map (Ideal.Quotient.mk P') pX).leadingCoeff
M : Submonoid (R ⧸ P') := Submonoid.powers a
φ : R ⧸ P' →+* R[X] ⧸ P := quotientMap P C ⋯
hP'_prime : P'.IsPrime
hM : 0 ∉ M
M' : Submonoid (R[X] ⧸ P) := Submonoid.map φ M
this :
  (algebraMap (R[X] ⧸ P) (Localization M')).comp φ =
    (IsLocalization.map (Localization M') φ ⋯).comp (algebraMap (R ⧸ P') (Localization M))
⊢ (IsLocalization.map (Localization M') φ ⋯).IsIntegral

Proof state:
R : Type u_1
inst✝¹ : CommRing R
P : Ideal R[X]
hP : P.IsMaximal
inst✝ : Nontrivial R
hR : IsJacobsonRing R
hP' : ∀ (x : R), C x ∈ P → x = 0
⊢ ((Ideal.Quotient.mk P).comp C).IsIntegral

Tactic:
refine (isIntegral_quotientMap_iff _).mp ?_

Result:
R : Type u_1
inst✝¹ : CommRing R
P : Ideal R[X]
hP : P.IsMaximal
inst✝ : Nontrivial R
hR : IsJacobsonRing R
hP' : ∀ (x : R), C x ∈ P → x = 0
⊢ (quotientMap P C ⋯).IsIntegral

Proof state:
R : Type u_1
inst✝¹ : CommRing R
P : Ideal R[X]
hP : P.IsMaximal
inst✝ : Nontrivial R
hR : IsJacobsonRing R
hP' : ∀ (x : R), C x ∈ P → x = 0
P' : Ideal R := comap C P
pX : R[X]
hpX : pX ∈ P
hp0 : map (Ideal.Quotient.mk (comap C P)) pX ≠ 0
a : R ⧸ P' := (map (Ideal.Quotient.mk P') pX).leadingCoeff
M : Submonoid (R ⧸ P') := Submonoid.powers a
φ : R ⧸ P' →+* R[X] ⧸ P := quotientMap P C ⋯
hP'_prime : P'.IsPrime
hM : 0 ∉ M
M' : Submonoid (R[X] ⧸ P) := Submonoid.map φ M
this :
  (algebraMap (R[X] ⧸ P) (Localization M')).comp φ =
    (IsLocalization.map (Localization M') φ ⋯).comp (algebraMap (R ⧸ P') (Localization M))
⊢ ((algebraMap (R[X] ⧸ P) (Localization M')).comp φ).IsIntegral

Tactic:
rw [this]

Result:
R : Type u_1
inst✝¹ : CommRing R
P : Ideal R[X]
hP : P.IsMaximal
inst✝ : Nontrivial R
hR : IsJacobsonRing R
hP' : ∀ (x : R), C x ∈ P → x = 0
P' : Ideal R := comap C P
pX : R[X]
hpX : pX ∈ P
hp0 : map (Ideal.Quotient.mk (comap C P)) pX ≠ 0
a : R ⧸ P' := (map (Ideal.Quotient.mk P') pX).leadingCoeff
M : Submonoid (R ⧸ P') := Submonoid.powers a
φ : R ⧸ P' →+* R[X] ⧸ P := quotientMap P C ⋯
hP'_prime : P'.IsPrime
hM : 0 ∉ M
M' : Submonoid (R[X] ⧸ P) := Submonoid.map φ M
this :
  (algebraMap (R[X] ⧸ P) (Localization M')).comp φ =
    (IsLocalization.map (Localization M') φ ⋯).comp (algebraMap (R ⧸ P') (Localization M))
⊢ ((IsLocalization.map (Localization M') φ ⋯).comp (algebraMap (R ⧸ P') (Localization M))).IsIntegral

Proof state:
case intro.intro
R : Type u_1
inst✝¹ : CommRing R
P : Ideal R[X]
hP : P.IsMaximal
inst✝ : Nontrivial R
hR : IsJacobsonRing R
hP' : ∀ (x : R), C x ∈ P → x = 0
P' : Ideal R := comap C P
pX : R[X]
hpX : pX ∈ P
hp0 : map (Ideal.Quotient.mk (comap C P)) pX ≠ 0
⊢ (quotientMap P C ⋯).IsIntegral

Tactic:
let a : R ⧸ P' := (pX.map (Ideal.Quotient.mk P')).leadingCoeff

Result:
case intro.intro
R : Type u_1
inst✝¹ : CommRing R
P : Ideal R[X]
hP : P.IsMaximal
inst✝ : Nontrivial R
hR : IsJacobsonRing R
hP' : ∀ (x : R), C x ∈ P → x = 0
P' : Ideal R := comap C P
pX : R[X]
hpX : pX ∈ P
hp0 : map (Ideal.Quotient.mk (comap C P)) pX ≠ 0
a : R ⧸ P' := (map (Ideal.Quotient.mk P') pX).leadingCoeff
⊢ (quotientMap P C ⋯).IsIntegral