-- Mathlib/RingTheory/Spectrum/Prime/Topology.lean
lemma isIntegral_of_isClosedMap_comap_mapRingHom (h : IsClosedMap (comap (mapRingHom f))) :
    f.IsIntegral

Proof state:
R : Type u_1
S : Type u_2
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
h : IsClosedMap ⇑(comap (mapRingHom f))
algInst✝ : Algebra R S := f.toAlgebra
a✝¹ : Nontrivial R
a✝ : Nontrivial S
r : S
p : S[X] := C r * X - 1
H : zeroLocus ↑(Ideal.comap (mapRingHom f) (Ideal.span {p})) = ⇑(comap (mapRingHom f)) '' zeroLocus ↑(Ideal.span {p})
⊢ 1 ∈ Ideal.span {X} ⊔ Ideal.comap (mapRingHom f) (Ideal.span {p})

Tactic:
rw [← Ideal.eq_top_iff_one, sup_comm, ← zeroLocus_empty_iff_eq_top, zeroLocus_sup, H]

Result:
R : Type u_1
S : Type u_2
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
h : IsClosedMap ⇑(comap (mapRingHom f))
algInst✝ : Algebra R S := f.toAlgebra
a✝¹ : Nontrivial R
a✝ : Nontrivial S
r : S
p : S[X] := C r * X - 1
H : zeroLocus ↑(Ideal.comap (mapRingHom f) (Ideal.span {p})) = ⇑(comap (mapRingHom f)) '' zeroLocus ↑(Ideal.span {p})
⊢ ⇑(comap (mapRingHom f)) '' zeroLocus ↑(Ideal.span {p}) ∩ zeroLocus ↑(Ideal.span {X}) = ∅

Proof state:
R : Type u_1
S : Type u_2
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
h : IsClosedMap ⇑(comap (mapRingHom f))
algInst✝ : Algebra R S := f.toAlgebra
a✝ : Nontrivial R
⊢ Algebra.IsIntegral R S

Tactic:
nontriviality S

Result:
R : Type u_1
S : Type u_2
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
h : IsClosedMap ⇑(comap (mapRingHom f))
algInst✝ : Algebra R S := f.toAlgebra
a✝¹ : Nontrivial R
a✝ : Nontrivial S
⊢ Algebra.IsIntegral R S

Proof state:
case isIntegral.intro.intro.intro.intro.refine_2
R : Type u_1
S : Type u_2
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
h : IsClosedMap ⇑(comap (mapRingHom f))
algInst✝ : Algebra R S := f.toAlgebra
a✝¹ : Nontrivial R
a✝ : Nontrivial S
r : S
p : S[X] := C r * X - 1
this✝ : 1 ∈ Ideal.span {X} ⊔ Ideal.comap (mapRingHom f) (Ideal.span {p})
a b : R[X]
hb : b ∈ Ideal.comap (mapRingHom f) (Ideal.span {p})
e : a * X + b = 1
c : S[X]
hc : map (algebraMap R S) b = p * c
this : p.natDegree ≤ 1
⊢ eval₂ (algebraMap R S) r (b.reverse * X ^ (1 + c.natDegree)) = 0

Tactic:
rw [eval₂_eq_eval_map, reverse, Polynomial.map_mul, ← reflect_map, Polynomial.map_pow,
  map_X, ← revAt_zero (1 + _), ← reflect_monomial,
  ← reflect_mul _ _ natDegree_map_le (by simp), pow_zero, mul_one, hc,
  ← add_assoc, reflect_mul _ _ (this.trans (by simp)) le_rfl,
  eval_mul, reflect_sub, reflect_mul _ _ (by simp) (by simp)]

Result:
case isIntegral.intro.intro.intro.intro.refine_2
R : Type u_1
S : Type u_2
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
h : IsClosedMap ⇑(comap (mapRingHom f))
algInst✝ : Algebra R S := f.toAlgebra
a✝¹ : Nontrivial R
a✝ : Nontrivial S
r : S
p : S[X] := C r * X - 1
this✝ : 1 ∈ Ideal.span {X} ⊔ Ideal.comap (mapRingHom f) (Ideal.span {p})
a b : R[X]
hb : b ∈ Ideal.comap (mapRingHom f) (Ideal.span {p})
e : a * X + b = 1
c : S[X]
hc : map (algebraMap R S) b = p * c
this : p.natDegree ≤ 1
⊢ eval r (reflect b.natDegree (C r) * reflect 1 X - reflect (b.natDegree + 1) 1) * eval r (reflect c.natDegree c) = 0

Proof state:
R : Type u_1
S : Type u_2
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
h : IsClosedMap ⇑(comap (mapRingHom f))
algInst✝ : Algebra R S := f.toAlgebra
this : Algebra.IsIntegral R S
⊢ f.IsIntegral

Tactic:
rwa [Algebra.isIntegral_def] at this

Result:
no goals

Proof state:
R : Type u_1
S : Type u_2
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
h : IsClosedMap ⇑(comap (mapRingHom f))
algInst✝ : Algebra R S := f.toAlgebra
a✝¹ : Nontrivial R
a✝ : Nontrivial S
⊢ Algebra.IsIntegral R S

Tactic:
constructor

Result:
case isIntegral
R : Type u_1
S : Type u_2
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
h : IsClosedMap ⇑(comap (mapRingHom f))
algInst✝ : Algebra R S := f.toAlgebra
a✝¹ : Nontrivial R
a✝ : Nontrivial S
⊢ ∀ (x : S), IsIntegral R x

Proof state:
case isIntegral.intro.intro.intro
R : Type u_1
S : Type u_2
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
h : IsClosedMap ⇑(comap (mapRingHom f))
algInst✝ : Algebra R S := f.toAlgebra
a✝¹ : Nontrivial R
a✝ : Nontrivial S
r : S
p : S[X] := C r * X - 1
this : 1 ∈ Ideal.span {X} ⊔ Ideal.comap (mapRingHom f) (Ideal.span {p})
a b : R[X]
hb : b ∈ Ideal.comap (mapRingHom f) (Ideal.span {p})
e : a * X + b = 1
⊢ IsIntegral R r

Tactic:
obtain ⟨c, hc : b.map (algebraMap R S) = _⟩ := Ideal.mem_span_singleton.mp hb

Result:
case isIntegral.intro.intro.intro.intro
R : Type u_1
S : Type u_2
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
h : IsClosedMap ⇑(comap (mapRingHom f))
algInst✝ : Algebra R S := f.toAlgebra
a✝¹ : Nontrivial R
a✝ : Nontrivial S
r : S
p : S[X] := C r * X - 1
this : 1 ∈ Ideal.span {X} ⊔ Ideal.comap (mapRingHom f) (Ideal.span {p})
a b : R[X]
hb : b ∈ Ideal.comap (mapRingHom f) (Ideal.span {p})
e : a * X + b = 1
c : S[X]
hc : map (algebraMap R S) b = p * c
⊢ IsIntegral R r

Proof state:
R : Type u_1
S : Type u_2
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
h : IsClosedMap ⇑(comap (mapRingHom f))
algInst✝ : Algebra R S := f.toAlgebra
a✝¹ : Nontrivial R
a✝ : Nontrivial S
r : S
p : S[X] := C r * X - 1
H : zeroLocus ↑(Ideal.comap (mapRingHom f) (Ideal.span {p})) = ⇑(comap (mapRingHom f)) '' zeroLocus ↑(Ideal.span {p})
⊢ ⇑(comap (mapRingHom f)) '' zeroLocus ↑(Ideal.span {p}) ∩ zeroLocus ↑(Ideal.span {X}) = ∅

Tactic:
suffices ∀ (a : PrimeSpectrum S[X]), p ∈ a.asIdeal → X ∉ a.asIdeal by
  simpa [Set.eq_empty_iff_forall_not_mem]

Result:
R : Type u_1
S : Type u_2
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
h : IsClosedMap ⇑(comap (mapRingHom f))
algInst✝ : Algebra R S := f.toAlgebra
a✝¹ : Nontrivial R
a✝ : Nontrivial S
r : S
p : S[X] := C r * X - 1
H : zeroLocus ↑(Ideal.comap (mapRingHom f) (Ideal.span {p})) = ⇑(comap (mapRingHom f)) '' zeroLocus ↑(Ideal.span {p})
⊢ ∀ (a : PrimeSpectrum S[X]), p ∈ a.asIdeal → X ∉ a.asIdeal

Proof state:
case isIntegral
R : Type u_1
S : Type u_2
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
h : IsClosedMap ⇑(comap (mapRingHom f))
algInst✝ : Algebra R S := f.toAlgebra
a✝¹ : Nontrivial R
a✝ : Nontrivial S
r : S
p : S[X] := C r * X - 1
this : 1 ∈ Ideal.span {X} ⊔ Ideal.comap (mapRingHom f) (Ideal.span {p})
⊢ IsIntegral R r

Tactic:
obtain ⟨a, b, hb, e⟩ := Ideal.mem_span_singleton_sup.mp this

Result:
case isIntegral.intro.intro.intro
R : Type u_1
S : Type u_2
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
h : IsClosedMap ⇑(comap (mapRingHom f))
algInst✝ : Algebra R S := f.toAlgebra
a✝¹ : Nontrivial R
a✝ : Nontrivial S
r : S
p : S[X] := C r * X - 1
this : 1 ∈ Ideal.span {X} ⊔ Ideal.comap (mapRingHom f) (Ideal.span {p})
a b : R[X]
hb : b ∈ Ideal.comap (mapRingHom f) (Ideal.span {p})
e : a * X + b = 1
⊢ IsIntegral R r

Proof state:
R : Type u_1
S : Type u_2
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
h : IsClosedMap ⇑(comap (mapRingHom f))
algInst✝ : Algebra R S := f.toAlgebra
a✝¹ : Nontrivial R
a✝ : Nontrivial S
r : S
p : S[X] := C r * X - 1
this✝ : 1 ∈ Ideal.span {X} ⊔ Ideal.comap (mapRingHom f) (Ideal.span {p})
a b : R[X]
hb : b ∈ Ideal.comap (mapRingHom f) (Ideal.span {p})
e : a * X + b = 1
c : S[X]
hc : map (algebraMap R S) b = p * c
this : p.natDegree ≤ 1
⊢ 1 ≤ b.natDegree + 1

Tactic:
simp

Result:
no goals

Proof state:
R : Type u_1
S : Type u_2
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
h : IsClosedMap ⇑(comap (mapRingHom f))
algInst✝ : Algebra R S := f.toAlgebra
a✝¹ : Nontrivial R
a✝ : Nontrivial S
r : S
p : S[X] := C r * X - 1
H : zeroLocus ↑(Ideal.comap (mapRingHom f) (Ideal.span {p})) = ⇑(comap (mapRingHom f)) '' zeroLocus ↑(Ideal.span {p})
q : PrimeSpectrum S[X]
hpq : p ∈ q.asIdeal
hXq : X ∈ q.asIdeal
⊢ 1 ∈ q.asIdeal

Tactic:
simpa [p] using (sub_mem (q.asIdeal.mul_mem_left (C r) hXq) hpq)

Result:
no goals