-- Mathlib/FieldTheory/PrimitiveElement.lean
theorem primitive_element_inf_aux [Algebra.IsSeparable F E] : ∃ γ : E, F⟮α, β⟯ = F⟮γ⟯

Proof state:
case intro
F : Type u_1
inst✝⁴ : Field F
inst✝³ : Infinite F
E : Type u_2
inst✝² : Field E
α β : E
inst✝¹ : Algebra F E
inst✝ : Algebra.IsSeparable F E
hα : IsIntegral F α
hβ : IsIntegral F β
f : F[X] := minpoly F α
g : F[X] := minpoly F β
ιFE : F →+* E := algebraMap F E
ιEE' : E →+* (Polynomial.map ιFE g).SplittingField := algebraMap E (Polynomial.map ιFE g).SplittingField
c : F
hc :
  ∀ α' ∈ (Polynomial.map (ιEE'.comp ιFE) f).roots,
    ∀ β' ∈ (Polynomial.map (ιEE'.comp ιFE) g).roots, -(α' - ιEE' α) / (β' - ιEE' β) ≠ (ιEE'.comp ιFE) c
γ : E := α + c • β
p : (↥F⟮γ⟯)[X] :=
  EuclideanDomain.gcd
    ((Polynomial.map (algebraMap F ↥F⟮γ⟯) f).comp (C (AdjoinSimple.gen F γ) - C ⟨(algebraMap F E) c, ⋯⟩ * X))
    (Polynomial.map (algebraMap F ↥F⟮γ⟯) g)
h : E[X] := EuclideanDomain.gcd ((Polynomial.map ιFE f).comp (C γ - C (ιFE c) * X)) (Polynomial.map ιFE g)
map_g_ne_zero : Polynomial.map ιFE g ≠ 0
h_ne_zero : h ≠ 0
h_sep : h.Separable
h_root : eval β h = 0
h_splits : Splits ιEE' h
h_roots : ∀ x ∈ (Polynomial.map ιEE' h).roots, x = ιEE' β
⊢ Polynomial.map (algebraMap (↥F⟮γ⟯) E) p = h

Tactic:
trans EuclideanDomain.gcd (?_ : E[X]) (?_ : E[X])

Result:
F : Type u_1
inst✝⁴ : Field F
inst✝³ : Infinite F
E : Type u_2
inst✝² : Field E
α β : E
inst✝¹ : Algebra F E
inst✝ : Algebra.IsSeparable F E
hα : IsIntegral F α
hβ : IsIntegral F β
f : F[X] := minpoly F α
g : F[X] := minpoly F β
ιFE : F →+* E := algebraMap F E
ιEE' : E →+* (Polynomial.map ιFE g).SplittingField := algebraMap E (Polynomial.map ιFE g).SplittingField
c : F
hc :
  ∀ α' ∈ (Polynomial.map (ιEE'.comp ιFE) f).roots,
    ∀ β' ∈ (Polynomial.map (ιEE'.comp ιFE) g).roots, -(α' - ιEE' α) / (β' - ιEE' β) ≠ (ιEE'.comp ιFE) c
γ : E := α + c • β
p : (↥F⟮γ⟯)[X] :=
  EuclideanDomain.gcd
    ((Polynomial.map (algebraMap F ↥F⟮γ⟯) f).comp (C (AdjoinSimple.gen F γ) - C ⟨(algebraMap F E) c, ⋯⟩ * X))
    (Polynomial.map (algebraMap F ↥F⟮γ⟯) g)
h : E[X] := EuclideanDomain.gcd ((Polynomial.map ιFE f).comp (C γ - C (ιFE c) * X)) (Polynomial.map ιFE g)
map_g_ne_zero : Polynomial.map ιFE g ≠ 0
h_ne_zero : h ≠ 0
h_sep : h.Separable
h_root : eval β h = 0
h_splits : Splits ιEE' h
h_roots : ∀ x ∈ (Polynomial.map ιEE' h).roots, x = ιEE' β
⊢ Polynomial.map (algebraMap (↥F⟮γ⟯) E) p = EuclideanDomain.gcd ?intro.intro_1 ?intro.intro_2

F : Type u_1
inst✝⁴ : Field F
inst✝³ : Infinite F
E : Type u_2
inst✝² : Field E
α β : E
inst✝¹ : Algebra F E
inst✝ : Algebra.IsSeparable F E
hα : IsIntegral F α
hβ : IsIntegral F β
f : F[X] := minpoly F α
g : F[X] := minpoly F β
ιFE : F →+* E := algebraMap F E
ιEE' : E →+* (Polynomial.map ιFE g).SplittingField := algebraMap E (Polynomial.map ιFE g).SplittingField
c : F
hc :
  ∀ α' ∈ (Polynomial.map (ιEE'.comp ιFE) f).roots,
    ∀ β' ∈ (Polynomial.map (ιEE'.comp ιFE) g).roots, -(α' - ιEE' α) / (β' - ιEE' β) ≠ (ιEE'.comp ιFE) c
γ : E := α + c • β
p : (↥F⟮γ⟯)[X] :=
  EuclideanDomain.gcd
    ((Polynomial.map (algebraMap F ↥F⟮γ⟯) f).comp (C (AdjoinSimple.gen F γ) - C ⟨(algebraMap F E) c, ⋯⟩ * X))
    (Polynomial.map (algebraMap F ↥F⟮γ⟯) g)
h : E[X] := EuclideanDomain.gcd ((Polynomial.map ιFE f).comp (C γ - C (ιFE c) * X)) (Polynomial.map ιFE g)
map_g_ne_zero : Polynomial.map ιFE g ≠ 0
h_ne_zero : h ≠ 0
h_sep : h.Separable
h_root : eval β h = 0
h_splits : Splits ιEE' h
h_roots : ∀ x ∈ (Polynomial.map ιEE' h).roots, x = ιEE' β
⊢ EuclideanDomain.gcd ?intro.intro_1 ?intro.intro_2 = h

case intro.intro_1
F : Type u_1
inst✝⁴ : Field F
inst✝³ : Infinite F
E : Type u_2
inst✝² : Field E
α β : E
inst✝¹ : Algebra F E
inst✝ : Algebra.IsSeparable F E
hα : IsIntegral F α
hβ : IsIntegral F β
f : F[X] := minpoly F α
g : F[X] := minpoly F β
ιFE : F →+* E := algebraMap F E
ιEE' : E →+* (Polynomial.map ιFE g).SplittingField := algebraMap E (Polynomial.map ιFE g).SplittingField
c : F
hc :
  ∀ α' ∈ (Polynomial.map (ιEE'.comp ιFE) f).roots,
    ∀ β' ∈ (Polynomial.map (ιEE'.comp ιFE) g).roots, -(α' - ιEE' α) / (β' - ιEE' β) ≠ (ιEE'.comp ιFE) c
γ : E := α + c • β
p : (↥F⟮γ⟯)[X] :=
  EuclideanDomain.gcd
    ((Polynomial.map (algebraMap F ↥F⟮γ⟯) f).comp (C (AdjoinSimple.gen F γ) - C ⟨(algebraMap F E) c, ⋯⟩ * X))
    (Polynomial.map (algebraMap F ↥F⟮γ⟯) g)
h : E[X] := EuclideanDomain.gcd ((Polynomial.map ιFE f).comp (C γ - C (ιFE c) * X)) (Polynomial.map ιFE g)
map_g_ne_zero : Polynomial.map ιFE g ≠ 0
h_ne_zero : h ≠ 0
h_sep : h.Separable
h_root : eval β h = 0
h_splits : Splits ιEE' h
h_roots : ∀ x ∈ (Polynomial.map ιEE' h).roots, x = ιEE' β
⊢ E[X]

case intro.intro_2
F : Type u_1
inst✝⁴ : Field F
inst✝³ : Infinite F
E : Type u_2
inst✝² : Field E
α β : E
inst✝¹ : Algebra F E
inst✝ : Algebra.IsSeparable F E
hα : IsIntegral F α
hβ : IsIntegral F β
f : F[X] := minpoly F α
g : F[X] := minpoly F β
ιFE : F →+* E := algebraMap F E
ιEE' : E →+* (Polynomial.map ιFE g).SplittingField := algebraMap E (Polynomial.map ιFE g).SplittingField
c : F
hc :
  ∀ α' ∈ (Polynomial.map (ιEE'.comp ιFE) f).roots,
    ∀ β' ∈ (Polynomial.map (ιEE'.comp ιFE) g).roots, -(α' - ιEE' α) / (β' - ιEE' β) ≠ (ιEE'.comp ιFE) c
γ : E := α + c • β
p : (↥F⟮γ⟯)[X] :=
  EuclideanDomain.gcd
    ((Polynomial.map (algebraMap F ↥F⟮γ⟯) f).comp (C (AdjoinSimple.gen F γ) - C ⟨(algebraMap F E) c, ⋯⟩ * X))
    (Polynomial.map (algebraMap F ↥F⟮γ⟯) g)
h : E[X] := EuclideanDomain.gcd ((Polynomial.map ιFE f).comp (C γ - C (ιFE c) * X)) (Polynomial.map ιFE g)
map_g_ne_zero : Polynomial.map ιFE g ≠ 0
h_ne_zero : h ≠ 0
h_sep : h.Separable
h_root : eval β h = 0
h_splits : Splits ιEE' h
h_roots : ∀ x ∈ (Polynomial.map ιEE' h).roots, x = ιEE' β
⊢ E[X]

Proof state:
case intro
F : Type u_1
inst✝⁴ : Field F
inst✝³ : Infinite F
E : Type u_2
inst✝² : Field E
α β : E
inst✝¹ : Algebra F E
inst✝ : Algebra.IsSeparable F E
hα : IsIntegral F α
hβ : IsIntegral F β
f : F[X] := minpoly F α
g : F[X] := minpoly F β
ιFE : F →+* E := algebraMap F E
ιEE' : E →+* (Polynomial.map ιFE g).SplittingField := algebraMap E (Polynomial.map ιFE g).SplittingField
c : F
hc :
  ∀ α' ∈ (Polynomial.map (ιEE'.comp ιFE) f).roots,
    ∀ β' ∈ (Polynomial.map (ιEE'.comp ιFE) g).roots, -(α' - ιEE' α) / (β' - ιEE' β) ≠ (ιEE'.comp ιFE) c
γ : E := α + c • β
⊢ β ∈ F⟮γ⟯

Tactic:
let p := EuclideanDomain.gcd ((f.map (algebraMap F F⟮γ⟯)).comp
  (C (AdjoinSimple.gen F γ) - (C ↑c : F⟮γ⟯[X]) * X)) (g.map (algebraMap F F⟮γ⟯))

Result:
case intro
F : Type u_1
inst✝⁴ : Field F
inst✝³ : Infinite F
E : Type u_2
inst✝² : Field E
α β : E
inst✝¹ : Algebra F E
inst✝ : Algebra.IsSeparable F E
hα : IsIntegral F α
hβ : IsIntegral F β
f : F[X] := minpoly F α
g : F[X] := minpoly F β
ιFE : F →+* E := algebraMap F E
ιEE' : E →+* (Polynomial.map ιFE g).SplittingField := algebraMap E (Polynomial.map ιFE g).SplittingField
c : F
hc :
  ∀ α' ∈ (Polynomial.map (ιEE'.comp ιFE) f).roots,
    ∀ β' ∈ (Polynomial.map (ιEE'.comp ιFE) g).roots, -(α' - ιEE' α) / (β' - ιEE' β) ≠ (ιEE'.comp ιFE) c
γ : E := α + c • β
p : (↥F⟮γ⟯)[X] :=
  EuclideanDomain.gcd
    ((Polynomial.map (algebraMap F ↥F⟮γ⟯) f).comp (C (AdjoinSimple.gen F γ) - C ⟨(algebraMap F E) c, ⋯⟩ * X))
    (Polynomial.map (algebraMap F ↥F⟮γ⟯) g)
⊢ β ∈ F⟮γ⟯

Proof state:
case h.a
F : Type u_1
inst✝⁴ : Field F
inst✝³ : Infinite F
E : Type u_2
inst✝² : Field E
α β : E
inst✝¹ : Algebra F E
inst✝ : Algebra.IsSeparable F E
hα : IsIntegral F α
hβ : IsIntegral F β
f : F[X] := minpoly F α
g : F[X] := minpoly F β
ιFE : F →+* E := algebraMap F E
ιEE' : E →+* (Polynomial.map ιFE g).SplittingField := algebraMap E (Polynomial.map ιFE g).SplittingField
c : F
hc :
  ∀ α' ∈ (Polynomial.map (ιEE'.comp ιFE) f).roots,
    ∀ β' ∈ (Polynomial.map (ιEE'.comp ιFE) g).roots, -(α' - ιEE' α) / (β' - ιEE' β) ≠ (ιEE'.comp ιFE) c
γ : E := α + c • β
β_in_Fγ : β ∈ F⟮γ⟯
α_in_Fγ : α ∈ F⟮γ⟯
⊢ {α, β} ⊆ ↑F⟮γ⟯

Tactic:
rintro x (rfl | rfl) <;> assumption

Result:
no goals