-- Mathlib/Analysis/Complex/Polynomial/Basic.lean
theorem card_complex_roots_eq_card_real_add_card_not_gal_inv (p : ℚ[X]) :
    (p.rootSet ℂ).toFinset.card =
      (p.rootSet ℝ).toFinset.card +
        (galActionHom p ℂ (restrict p ℂ
        (AlgEquiv.restrictScalars ℚ Complex.conjAe))).support.card

Proof state:
p : ℚ[X]
hp : ¬p = 0
inj : Function.Injective ⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)
a : Finset ℂ := (p.rootSet ℂ).toFinset
b : Finset ℂ := Finset.image (⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)) (p.rootSet ℝ).toFinset
c : Finset ℂ :=
  Finset.image (fun a => ↑a) ((galActionHom p ℂ) ((restrict p ℂ) (AlgEquiv.restrictScalars ℚ conjAe))).support
ha : ∀ (z : ℂ), z ∈ a ↔ (aeval z) p = 0
hb : ∀ (z : ℂ), z ∈ b ↔ (aeval z) p = 0 ∧ z.im = 0
hc0 :
  ∀ (w : ↑(p.rootSet ℂ)), ((galActionHom p ℂ) ((restrict p ℂ) (AlgEquiv.restrictScalars ℚ conjAe))) w = w ↔ (↑w).im = 0
z : ℂ
⊢ z ∈ c ↔ (aeval z) p = 0 ∧ z.im ≠ 0

Tactic:
simp_rw [c, Finset.mem_image]

Result:
p : ℚ[X]
hp : ¬p = 0
inj : Function.Injective ⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)
a : Finset ℂ := (p.rootSet ℂ).toFinset
b : Finset ℂ := Finset.image (⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)) (p.rootSet ℝ).toFinset
c : Finset ℂ :=
  Finset.image (fun a => ↑a) ((galActionHom p ℂ) ((restrict p ℂ) (AlgEquiv.restrictScalars ℚ conjAe))).support
ha : ∀ (z : ℂ), z ∈ a ↔ (aeval z) p = 0
hb : ∀ (z : ℂ), z ∈ b ↔ (aeval z) p = 0 ∧ z.im = 0
hc0 :
  ∀ (w : ↑(p.rootSet ℂ)), ((galActionHom p ℂ) ((restrict p ℂ) (AlgEquiv.restrictScalars ℚ conjAe))) w = w ↔ (↑w).im = 0
z : ℂ
⊢ (∃ a ∈ ((galActionHom p ℂ) ((restrict p ℂ) (AlgEquiv.restrictScalars ℚ conjAe))).support, ↑a = z) ↔
    (aeval z) p = 0 ∧ z.im ≠ 0

Proof state:
case a
p : ℚ[X]
hp : ¬p = 0
inj : Function.Injective ⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)
a : Finset ℂ := (p.rootSet ℂ).toFinset
b : Finset ℂ := Finset.image (⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)) (p.rootSet ℝ).toFinset
c : Finset ℂ :=
  Finset.image (fun a => ↑a) ((galActionHom p ℂ) ((restrict p ℂ) (AlgEquiv.restrictScalars ℚ conjAe))).support
ha : ∀ (z : ℂ), z ∈ a ↔ (aeval z) p = 0
z : ℂ
hz1 : (aeval z) p = 0
hz2 : z.im = 0
⊢ ((IsScalarTower.toAlgHom ℚ ℝ ℂ) z.re).im = z.im

Tactic:
rw [hz2]

Result:
case a
p : ℚ[X]
hp : ¬p = 0
inj : Function.Injective ⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)
a : Finset ℂ := (p.rootSet ℂ).toFinset
b : Finset ℂ := Finset.image (⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)) (p.rootSet ℝ).toFinset
c : Finset ℂ :=
  Finset.image (fun a => ↑a) ((galActionHom p ℂ) ((restrict p ℂ) (AlgEquiv.restrictScalars ℚ conjAe))).support
ha : ∀ (z : ℂ), z ∈ a ↔ (aeval z) p = 0
z : ℂ
hz1 : (aeval z) p = 0
hz2 : z.im = 0
⊢ ((IsScalarTower.toAlgHom ℚ ℝ ℂ) z.re).im = 0

Proof state:
p : ℚ[X]
hp : p = 0
⊢ IsEmpty ↑∅

Tactic:
infer_instance

Result:
no goals

Proof state:
p : ℚ[X]
hp : ¬p = 0
inj : Function.Injective ⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)
a : Finset ℂ := (p.rootSet ℂ).toFinset
b : Finset ℂ := Finset.image (⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)) (p.rootSet ℝ).toFinset
c : Finset ℂ :=
  Finset.image (fun a => ↑a) ((galActionHom p ℂ) ((restrict p ℂ) (AlgEquiv.restrictScalars ℚ conjAe))).support
ha : ∀ (z : ℂ), z ∈ a ↔ (aeval z) p = 0
hb : ∀ (z : ℂ), z ∈ b ↔ (aeval z) p = 0 ∧ z.im = 0
w : ↑(p.rootSet ℂ)
⊢ (AlgEquiv.restrictScalars ℚ conjAe) ↑w = ↑w ↔ (↑w).im = 0

Tactic:
exact Complex.conj_eq_iff_im

Result:
no goals

Proof state:
case neg
p : ℚ[X]
hp : ¬p = 0
inj : Function.Injective ⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)
⊢ (p.rootSet ℂ).toFinset.card =
    (p.rootSet ℝ).toFinset.card + ((galActionHom p ℂ) ((restrict p ℂ) (AlgEquiv.restrictScalars ℚ conjAe))).support.card

Tactic:
rw [← Finset.card_image_of_injective _ Subtype.coe_injective, ←
  Finset.card_image_of_injective _ inj]

Result:
case neg
p : ℚ[X]
hp : ¬p = 0
inj : Function.Injective ⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)
⊢ (p.rootSet ℂ).toFinset.card =
    (Finset.image (⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)) (p.rootSet ℝ).toFinset).card +
      (Finset.image (fun a => ↑a)
          ((galActionHom p ℂ) ((restrict p ℂ) (AlgEquiv.restrictScalars ℚ conjAe))).support).card

Proof state:
case neg.refine_2.refine_2.refine_2
p : ℚ[X]
hp : ¬p = 0
inj : Function.Injective ⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)
a : Finset ℂ := (p.rootSet ℂ).toFinset
b : Finset ℂ := Finset.image (⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)) (p.rootSet ℝ).toFinset
c : Finset ℂ :=
  Finset.image (fun a => ↑a) ((galActionHom p ℂ) ((restrict p ℂ) (AlgEquiv.restrictScalars ℚ conjAe))).support
ha : ∀ (z : ℂ), z ∈ a ↔ (aeval z) p = 0
hb : ∀ (z : ℂ), z ∈ b ↔ (aeval z) p = 0 ∧ z.im = 0
hc0 :
  ∀ (w : ↑(p.rootSet ℂ)), ((galActionHom p ℂ) ((restrict p ℂ) (AlgEquiv.restrictScalars ℚ conjAe))) w = w ↔ (↑w).im = 0
hc : ∀ (z : ℂ), z ∈ c ↔ (aeval z) p = 0 ∧ z.im ≠ 0
⊢ ∀ (a : ℂ), (aeval a) p = 0 ↔ (aeval a) p = 0 ∧ a.im = 0 ∨ (aeval a) p = 0 ∧ a.im ≠ 0

Tactic:
tauto

Result:
no goals

Proof state:
case mpr.intro
p : ℚ[X]
hp : ¬p = 0
inj : Function.Injective ⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)
a : Finset ℂ := (p.rootSet ℂ).toFinset
b : Finset ℂ := Finset.image (⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)) (p.rootSet ℝ).toFinset
c : Finset ℂ :=
  Finset.image (fun a => ↑a) ((galActionHom p ℂ) ((restrict p ℂ) (AlgEquiv.restrictScalars ℚ conjAe))).support
ha : ∀ (z : ℂ), z ∈ a ↔ (aeval z) p = 0
hb : ∀ (z : ℂ), z ∈ b ↔ (aeval z) p = 0 ∧ z.im = 0
hc0 :
  ∀ (w : ↑(p.rootSet ℂ)), ((galActionHom p ℂ) ((restrict p ℂ) (AlgEquiv.restrictScalars ℚ conjAe))) w = w ↔ (↑w).im = 0
z : ℂ
hz1 : (aeval z) p = 0
hz2 : z.im ≠ 0
⊢ ∃ a ∈ ((galActionHom p ℂ) ((restrict p ℂ) (AlgEquiv.restrictScalars ℚ conjAe))).support, ↑a = z

Tactic:
exact ⟨⟨z, mem_rootSet.mpr ⟨hp, hz1⟩⟩, Equiv.Perm.mem_support.mpr (mt (hc0 _).mp hz2), rfl⟩

Result:
no goals

Proof state:
case neg.refine_2.refine_2.refine_2
p : ℚ[X]
hp : ¬p = 0
inj : Function.Injective ⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)
a : Finset ℂ := (p.rootSet ℂ).toFinset
b : Finset ℂ := Finset.image (⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)) (p.rootSet ℝ).toFinset
c : Finset ℂ :=
  Finset.image (fun a => ↑a) ((galActionHom p ℂ) ((restrict p ℂ) (AlgEquiv.restrictScalars ℚ conjAe))).support
ha : ∀ (z : ℂ), z ∈ a ↔ (aeval z) p = 0
hb : ∀ (z : ℂ), z ∈ b ↔ (aeval z) p = 0 ∧ z.im = 0
hc0 :
  ∀ (w : ↑(p.rootSet ℂ)), ((galActionHom p ℂ) ((restrict p ℂ) (AlgEquiv.restrictScalars ℚ conjAe))) w = w ↔ (↑w).im = 0
hc : ∀ (z : ℂ), z ∈ c ↔ (aeval z) p = 0 ∧ z.im ≠ 0
⊢ ∀ ⦃a : ℂ⦄, a ∈ b → a ∉ c

Tactic:
intro z

Result:
case neg.refine_2.refine_2.refine_2
p : ℚ[X]
hp : ¬p = 0
inj : Function.Injective ⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)
a : Finset ℂ := (p.rootSet ℂ).toFinset
b : Finset ℂ := Finset.image (⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)) (p.rootSet ℝ).toFinset
c : Finset ℂ :=
  Finset.image (fun a => ↑a) ((galActionHom p ℂ) ((restrict p ℂ) (AlgEquiv.restrictScalars ℚ conjAe))).support
ha : ∀ (z : ℂ), z ∈ a ↔ (aeval z) p = 0
hb : ∀ (z : ℂ), z ∈ b ↔ (aeval z) p = 0 ∧ z.im = 0
hc0 :
  ∀ (w : ↑(p.rootSet ℂ)), ((galActionHom p ℂ) ((restrict p ℂ) (AlgEquiv.restrictScalars ℚ conjAe))) w = w ↔ (↑w).im = 0
hc : ∀ (z : ℂ), z ∈ c ↔ (aeval z) p = 0 ∧ z.im ≠ 0
z : ℂ
⊢ z ∈ b → z ∉ c