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

Proof state:
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
p_linear : Polynomial.map (algebraMap (↥F⟮γ⟯) E) p = C h.leadingCoeff * (X - C β)
finale : β = (algebraMap (↥F⟮γ⟯) E) (-p.coeff 0 / p.coeff 1)
⊢ (algebraMap (↥F⟮γ⟯) E) (-p.coeff 0 / p.coeff 1) ∈ F⟮γ⟯

Tactic:
exact Subtype.mem (-p.coeff 0 / p.coeff 1)

Result:
no goals

Proof state:
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
x : (Polynomial.map ιFE g).SplittingField
hx : eval₂ ιEE' x h = 0
f_root : eval₂ ιEE' x ((Polynomial.map ιFE f).comp (C γ - C (ιFE c) * X)) = 0
⊢ ιEE' γ - ιEE' (ιFE c) * x ∈ (Polynomial.map (ιEE'.comp ιFE) f).roots

Tactic:
rw [eval₂_comp, eval₂_sub, eval₂_mul, eval₂_C, eval₂_C, eval₂_X, eval₂_map] at f_root

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
x : (Polynomial.map ιFE g).SplittingField
hx : eval₂ ιEE' x h = 0
f_root : eval₂ (ιEE'.comp ιFE) (ιEE' γ - ιEE' (ιFE c) * x) f = 0
⊢ ιEE' γ - ιEE' (ιFE c) * x ∈ (Polynomial.map (ιEE'.comp ιFE) f).roots

Proof state:
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
⊢ eval β h = 0

Tactic:
apply eval_gcd_eq_zero

Result:
case hf
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
⊢ eval β ((Polynomial.map ιFE f).comp (C γ - C (ιFE c) * X)) = 0

case hg
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
⊢ eval β (Polynomial.map ιFE g) = 0