-- Mathlib/RingTheory/Polynomial/GaussLemma.lean
theorem integralClosure.mem_lifts_of_monic_of_dvd_map {f : R[X]} (hf : f.Monic) {g : K[X]}
    (hg : g.Monic) (hd : g ∣ f.map (algebraMap R K)) :
    g ∈ lifts (algebraMap (integralClosure R K) K)

Proof state:
case refine_2
R : Type u_1
inst✝² : CommRing R
K : Type u_2
inst✝¹ : Field K
inst✝ : Algebra R K
f : R[X]
hf : f.Monic
g : K[X]
hg : g.Monic
hd : g ∣ map (algebraMap R K) f
this :
  map (algebraMap K g.SplittingField) g ∈ lifts (algebraMap (↥(integralClosure R g.SplittingField)) g.SplittingField)
⊢ g ∈ lifts (algebraMap (↥(integralClosure R K)) K)

Tactic:
rw [lifts_iff_coeff_lifts, ← RingHom.coe_range, Subalgebra.range_algebraMap] at this

Result:
case refine_2
R : Type u_1
inst✝² : CommRing R
K : Type u_2
inst✝¹ : Field K
inst✝ : Algebra R K
f : R[X]
hf : f.Monic
g : K[X]
hg : g.Monic
hd : g ∣ map (algebraMap R K) f
this : ∀ (n : ℕ), (map (algebraMap K g.SplittingField) g).coeff n ∈ ↑(integralClosure R g.SplittingField).toSubring
⊢ g ∈ lifts (algebraMap (↥(integralClosure R K)) K)

Proof state:
case refine_2
R : Type u_1
inst✝² : CommRing R
K : Type u_2
inst✝¹ : Field K
inst✝ : Algebra R K
f : R[X]
hf : f.Monic
g : K[X]
hg : g.Monic
hd : g ∣ map (algebraMap R K) f
this : ∀ (n : ℕ), (map (algebraMap K g.SplittingField) g).coeff n ∈ ↑(integralClosure R g.SplittingField).toSubring
n : ℕ
⊢ g.coeff n ∈ Set.range ⇑(algebraMap (↥(integralClosure R K)) K)

Tactic:
rw [← RingHom.coe_range, Subalgebra.range_algebraMap]

Result:
case refine_2
R : Type u_1
inst✝² : CommRing R
K : Type u_2
inst✝¹ : Field K
inst✝ : Algebra R K
f : R[X]
hf : f.Monic
g : K[X]
hg : g.Monic
hd : g ∣ map (algebraMap R K) f
this : ∀ (n : ℕ), (map (algebraMap K g.SplittingField) g).coeff n ∈ ↑(integralClosure R g.SplittingField).toSubring
n : ℕ
⊢ g.coeff n ∈ ↑(integralClosure R K).toSubring

Proof state:
R : Type u_1
inst✝² : CommRing R
K : Type u_2
inst✝¹ : Field K
inst✝ : Algebra R K
f : R[X]
hf : f.Monic
g : K[X]
hg : g.Monic
hd : g ∣ map (algebraMap R K) f
this : ∀ (n : ℕ), (map (algebraMap K g.SplittingField) g).coeff n ∈ ↑(integralClosure R g.SplittingField).toSubring
n : ℕ
p : R[X]
hp : p.Monic
he : (algebraMap K g.SplittingField) (eval (g.coeff n) (map (algebraMap R K) p)) = 0
⊢ Function.Injective ⇑(algebraMap K g.SplittingField)

Tactic:
apply RingHom.injective

Result:
no goals

Proof state:
case right
R : Type u_1
inst✝² : CommRing R
K : Type u_2
inst✝¹ : Field K
inst✝ : Algebra R K
f : R[X]
hf : f.Monic
g : K[X]
hg : g.Monic
hd : g ∣ map (algebraMap R K) f
this : ∀ (n : ℕ), (map (algebraMap K g.SplittingField) g).coeff n ∈ ↑(integralClosure R g.SplittingField).toSubring
n : ℕ
p : R[X]
hp : p.Monic
he : eval₂ (algebraMap R g.SplittingField) ((map (algebraMap K g.SplittingField) g).coeff n) p = 0
⊢ eval₂ (algebraMap R K) (g.coeff n) p = 0

Tactic:
rw [IsScalarTower.algebraMap_eq R K, coeff_map, ← eval₂_map, eval₂_at_apply] at he

Result:
case right
R : Type u_1
inst✝² : CommRing R
K : Type u_2
inst✝¹ : Field K
inst✝ : Algebra R K
f : R[X]
hf : f.Monic
g : K[X]
hg : g.Monic
hd : g ∣ map (algebraMap R K) f
this : ∀ (n : ℕ), (map (algebraMap K g.SplittingField) g).coeff n ∈ ↑(integralClosure R g.SplittingField).toSubring
n : ℕ
p : R[X]
hp : p.Monic
he : (algebraMap K g.SplittingField) (eval (g.coeff n) (map (algebraMap R K) p)) = 0
⊢ eval₂ (algebraMap R K) (g.coeff n) p = 0