-- Mathlib/RingTheory/RingHom/Integral.lean
theorem isIntegral_ofLocalizationSpan :
    OfLocalizationSpan (RingHom.IsIntegral ·)

Proof state:
case H.mk.intro.mk.intro.intro.intro.intro.mk.intro
R S : Type u_1
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
s : Set R
hs : Ideal.span s = ⊤
H : ∀ (r : ↑s), (fun {R S} [CommRing R] [CommRing S] x => x.IsIntegral) (Localization.awayMap f ↑r)
r : S
this✝² : Algebra R S := f.toAlgebra
t : R
ht : t ∈ s
this✝¹ : Algebra (Localization.Away t) (Localization.Away (f t)) := (Localization.awayMap f t).toAlgebra
this✝ : IsScalarTower R (Localization.Away t) (Localization.Away (f t))
this : _root_.IsIntegral (Localization.Away t) ((algebraMap S (Localization.Away (f t))) r)
n : ℕ
p : R[X]
hp : p.Monic
m : ℕ
e : ↑⟨(algebraMap R S) t ^ m, ⋯⟩ * eval₂ (algebraMap R S) ((algebraMap R S) ↑⟨(fun x => t ^ x) n, ⋯⟩ * r) p = 0
⊢ ∃ n, ↑⟨t, ht⟩ ^ n • r ∈ Subalgebra.toSubmodule (integralClosure R S)

Tactic:
by_cases hp' : 1 ≤ p.natDegree

Result:
case pos
R S : Type u_1
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
s : Set R
hs : Ideal.span s = ⊤
H : ∀ (r : ↑s), (fun {R S} [CommRing R] [CommRing S] x => x.IsIntegral) (Localization.awayMap f ↑r)
r : S
this✝² : Algebra R S := f.toAlgebra
t : R
ht : t ∈ s
this✝¹ : Algebra (Localization.Away t) (Localization.Away (f t)) := (Localization.awayMap f t).toAlgebra
this✝ : IsScalarTower R (Localization.Away t) (Localization.Away (f t))
this : _root_.IsIntegral (Localization.Away t) ((algebraMap S (Localization.Away (f t))) r)
n : ℕ
p : R[X]
hp : p.Monic
m : ℕ
e : ↑⟨(algebraMap R S) t ^ m, ⋯⟩ * eval₂ (algebraMap R S) ((algebraMap R S) ↑⟨(fun x => t ^ x) n, ⋯⟩ * r) p = 0
hp' : 1 ≤ p.natDegree
⊢ ∃ n, ↑⟨t, ht⟩ ^ n • r ∈ Subalgebra.toSubmodule (integralClosure R S)

case neg
R S : Type u_1
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
s : Set R
hs : Ideal.span s = ⊤
H : ∀ (r : ↑s), (fun {R S} [CommRing R] [CommRing S] x => x.IsIntegral) (Localization.awayMap f ↑r)
r : S
this✝² : Algebra R S := f.toAlgebra
t : R
ht : t ∈ s
this✝¹ : Algebra (Localization.Away t) (Localization.Away (f t)) := (Localization.awayMap f t).toAlgebra
this✝ : IsScalarTower R (Localization.Away t) (Localization.Away (f t))
this : _root_.IsIntegral (Localization.Away t) ((algebraMap S (Localization.Away (f t))) r)
n : ℕ
p : R[X]
hp : p.Monic
m : ℕ
e : ↑⟨(algebraMap R S) t ^ m, ⋯⟩ * eval₂ (algebraMap R S) ((algebraMap R S) ↑⟨(fun x => t ^ x) n, ⋯⟩ * r) p = 0
hp' : ¬1 ≤ p.natDegree
⊢ ∃ n, ↑⟨t, ht⟩ ^ n • r ∈ Subalgebra.toSubmodule (integralClosure R S)

Proof state:
case H.mk
R S : Type u_1
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
s : Set R
hs : Ideal.span s = ⊤
H : ∀ (r : ↑s), (fun {R S} [CommRing R] [CommRing S] x => x.IsIntegral) (Localization.awayMap f ↑r)
r : S
this✝² : Algebra R S := f.toAlgebra
t : R
ht : t ∈ s
this✝¹ : Algebra (Localization.Away t) (Localization.Away (f t)) := (Localization.awayMap f t).toAlgebra
this✝ : IsScalarTower R (Localization.Away t) (Localization.Away (f t))
this : _root_.IsIntegral (Localization.Away t) ((algebraMap S (Localization.Away (f t))) r)
⊢ ∃ n, ↑⟨t, ht⟩ ^ n • r ∈ Subalgebra.toSubmodule (integralClosure R S)

Tactic:
obtain ⟨⟨_, n, rfl⟩, p, hp, hp'⟩ := this.exists_multiple_integral_of_isLocalization (.powers t)

Result:
case H.mk.intro.mk.intro.intro.intro
R S : Type u_1
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
s : Set R
hs : Ideal.span s = ⊤
H : ∀ (r : ↑s), (fun {R S} [CommRing R] [CommRing S] x => x.IsIntegral) (Localization.awayMap f ↑r)
r : S
this✝² : Algebra R S := f.toAlgebra
t : R
ht : t ∈ s
this✝¹ : Algebra (Localization.Away t) (Localization.Away (f t)) := (Localization.awayMap f t).toAlgebra
this✝ : IsScalarTower R (Localization.Away t) (Localization.Away (f t))
this : _root_.IsIntegral (Localization.Away t) ((algebraMap S (Localization.Away (f t))) r)
n : ℕ
p : R[X]
hp : p.Monic
hp' :
  eval₂ (algebraMap R (Localization.Away (f t))) (⟨(fun x => t ^ x) n, ⋯⟩ • (algebraMap S (Localization.Away (f t))) r)
      p =
    0
⊢ ∃ n, ↑⟨t, ht⟩ ^ n • r ∈ Subalgebra.toSubmodule (integralClosure R S)

Proof state:
R S : Type u_1
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
s : Set R
hs : Ideal.span s = ⊤
H : ∀ (r : ↑s), (fun {R S} [CommRing R] [CommRing S] x => x.IsIntegral) (Localization.awayMap f ↑r)
r : S
this : Algebra R S := f.toAlgebra
⊢ r ∈ Subalgebra.toSubmodule (integralClosure R S)

Tactic:
apply Submodule.mem_of_span_eq_top_of_smul_pow_mem _ s hs

Result:
case H
R S : Type u_1
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
s : Set R
hs : Ideal.span s = ⊤
H : ∀ (r : ↑s), (fun {R S} [CommRing R] [CommRing S] x => x.IsIntegral) (Localization.awayMap f ↑r)
r : S
this : Algebra R S := f.toAlgebra
⊢ ∀ (r_1 : ↑s), ∃ n, ↑r_1 ^ n • r ∈ Subalgebra.toSubmodule (integralClosure R S)

Proof state:
case pos
R S : Type u_1
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
s : Set R
hs : Ideal.span s = ⊤
H : ∀ (r : ↑s), (fun {R S} [CommRing R] [CommRing S] x => x.IsIntegral) (Localization.awayMap f ↑r)
r : S
this✝² : Algebra R S := f.toAlgebra
t : R
ht : t ∈ s
this✝¹ : Algebra (Localization.Away t) (Localization.Away (f t)) := (Localization.awayMap f t).toAlgebra
this✝ : IsScalarTower R (Localization.Away t) (Localization.Away (f t))
this : _root_.IsIntegral (Localization.Away t) ((algebraMap S (Localization.Away (f t))) r)
n : ℕ
p : R[X]
hp : p.Monic
m : ℕ
e : ↑⟨(algebraMap R S) t ^ m, ⋯⟩ * eval₂ (algebraMap R S) ((algebraMap R S) ↑⟨(fun x => t ^ x) n, ⋯⟩ * r) p = 0
hp' : 1 ≤ p.natDegree
⊢ ∃ n, ↑⟨t, ht⟩ ^ n • r ∈ Subalgebra.toSubmodule (integralClosure R S)

case neg
R S : Type u_1
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
s : Set R
hs : Ideal.span s = ⊤
H : ∀ (r : ↑s), (fun {R S} [CommRing R] [CommRing S] x => x.IsIntegral) (Localization.awayMap f ↑r)
r : S
this✝² : Algebra R S := f.toAlgebra
t : R
ht : t ∈ s
this✝¹ : Algebra (Localization.Away t) (Localization.Away (f t)) := (Localization.awayMap f t).toAlgebra
this✝ : IsScalarTower R (Localization.Away t) (Localization.Away (f t))
this : _root_.IsIntegral (Localization.Away t) ((algebraMap S (Localization.Away (f t))) r)
n : ℕ
p : R[X]
hp : p.Monic
m : ℕ
e : ↑⟨(algebraMap R S) t ^ m, ⋯⟩ * eval₂ (algebraMap R S) ((algebraMap R S) ↑⟨(fun x => t ^ x) n, ⋯⟩ * r) p = 0
hp' : ¬1 ≤ p.natDegree
⊢ ∃ n, ↑⟨t, ht⟩ ^ n • r ∈ Subalgebra.toSubmodule (integralClosure R S)

Tactic:
swap

Result:
case neg
R S : Type u_1
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
s : Set R
hs : Ideal.span s = ⊤
H : ∀ (r : ↑s), (fun {R S} [CommRing R] [CommRing S] x => x.IsIntegral) (Localization.awayMap f ↑r)
r : S
this✝² : Algebra R S := f.toAlgebra
t : R
ht : t ∈ s
this✝¹ : Algebra (Localization.Away t) (Localization.Away (f t)) := (Localization.awayMap f t).toAlgebra
this✝ : IsScalarTower R (Localization.Away t) (Localization.Away (f t))
this : _root_.IsIntegral (Localization.Away t) ((algebraMap S (Localization.Away (f t))) r)
n : ℕ
p : R[X]
hp : p.Monic
m : ℕ
e : ↑⟨(algebraMap R S) t ^ m, ⋯⟩ * eval₂ (algebraMap R S) ((algebraMap R S) ↑⟨(fun x => t ^ x) n, ⋯⟩ * r) p = 0
hp' : ¬1 ≤ p.natDegree
⊢ ∃ n, ↑⟨t, ht⟩ ^ n • r ∈ Subalgebra.toSubmodule (integralClosure R S)

case pos
R S : Type u_1
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
s : Set R
hs : Ideal.span s = ⊤
H : ∀ (r : ↑s), (fun {R S} [CommRing R] [CommRing S] x => x.IsIntegral) (Localization.awayMap f ↑r)
r : S
this✝² : Algebra R S := f.toAlgebra
t : R
ht : t ∈ s
this✝¹ : Algebra (Localization.Away t) (Localization.Away (f t)) := (Localization.awayMap f t).toAlgebra
this✝ : IsScalarTower R (Localization.Away t) (Localization.Away (f t))
this : _root_.IsIntegral (Localization.Away t) ((algebraMap S (Localization.Away (f t))) r)
n : ℕ
p : R[X]
hp : p.Monic
m : ℕ
e : ↑⟨(algebraMap R S) t ^ m, ⋯⟩ * eval₂ (algebraMap R S) ((algebraMap R S) ↑⟨(fun x => t ^ x) n, ⋯⟩ * r) p = 0
hp' : 1 ≤ p.natDegree
⊢ ∃ n, ↑⟨t, ht⟩ ^ n • r ∈ Subalgebra.toSubmodule (integralClosure R S)