-- 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:
case neg.refine_2
p : ℚ[X]
hp : ¬p = 0
inj : Function.Injective ⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)
a : Finset ℂ := ?neg.refine_1✝
⊢ (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

case neg.refine_1
p : ℚ[X]
hp : ¬p = 0
inj : Function.Injective ⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)
⊢ Finset ℂ

Tactic:
on_goal 1 => let b : Finset ℂ := ?_

Result:
case neg.refine_2.refine_2
p : ℚ[X]
hp : ¬p = 0
inj : Function.Injective ⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)
a : Finset ℂ := ?neg.refine_1✝
b : Finset ℂ := ?neg.refine_2.refine_1✝
⊢ (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

case neg.refine_2.refine_1
p : ℚ[X]
hp : ¬p = 0
inj : Function.Injective ⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)
a : Finset ℂ := ?neg.refine_1✝
⊢ Finset ℂ

case neg.refine_1
p : ℚ[X]
hp : ¬p = 0
inj : Function.Injective ⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)
⊢ Finset ℂ

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
⊢ a.card = b.card + c.card

Tactic:
have ha : ∀ z : ℂ, z ∈ a ↔ aeval z p = 0 := by
  intro z; rw [Set.mem_toFinset, mem_rootSet_of_ne hp]

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
⊢ a.card = b.card + c.card

Proof state:
p : ℚ[X]
hp : ¬p = 0
inj : Function.Injective ⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)
a : Finset ℂ := ?neg.refine_1✝
b : Finset ℂ := ?neg.refine_2.refine_1✝
c : Finset ℂ := ?neg.refine_2.refine_2.refine_1✝
this : a.card = b.card + c.card
⊢ (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

Tactic:
exact this

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
z : ℂ
hz1 : (aeval z) p = 0
hz2 : z.im = 0
key : (IsScalarTower.toAlgHom ℚ ℝ ℂ) z.re = z
⊢ ∃ a, (aeval a) p = 0 ∧ (IsScalarTower.toAlgHom ℚ ℝ ℂ) a = z

Tactic:
exact ⟨z.re, inj (by rwa [← aeval_algHom_apply, key, map_zero]), key⟩

Result:
no goals

Proof state:
case mpr
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
⊢ (aeval z) p = 0 ∧ z.im = 0 → ∃ a, (aeval a) p = 0 ∧ (IsScalarTower.toAlgHom ℚ ℝ ℂ) a = z

Tactic:
rintro ⟨hz1, hz2⟩

Result:
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
z : ℂ
hz1 : (aeval z) p = 0
hz2 : z.im = 0
⊢ ∃ a, (aeval a) p = 0 ∧ (IsScalarTower.toAlgHom ℚ ℝ ℂ) a = z

Proof state:
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

Tactic:
let a : Finset ℂ := ?_

Result:
case neg.refine_2
p : ℚ[X]
hp : ¬p = 0
inj : Function.Injective ⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)
a : Finset ℂ := ?neg.refine_1✝
⊢ (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

case neg.refine_1
p : ℚ[X]
hp : ¬p = 0
inj : Function.Injective ⇑(IsScalarTower.toAlgHom ℚ ℝ ℂ)
⊢ Finset ℂ

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).re = z.re

Tactic:
rfl

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
z : ℂ
⊢ (∃ a, (aeval a) p = 0 ∧ (IsScalarTower.toAlgHom ℚ ℝ ℂ) a = z) ↔ (aeval z) p = 0 ∧ z.im = 0

Tactic:
constructor

Result:
case mp
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
⊢ (∃ a, (aeval a) p = 0 ∧ (IsScalarTower.toAlgHom ℚ ℝ ℂ) a = z) → (aeval z) p = 0 ∧ z.im = 0

case mpr
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
⊢ (aeval z) p = 0 ∧ z.im = 0 → ∃ a, (aeval a) p = 0 ∧ (IsScalarTower.toAlgHom ℚ ℝ ℂ) a = z