-- Mathlib/CategoryTheory/Galois/EssSurj.lean
/-- For every open subgroup `V` of `Aut F`, there exists an `X : C` such that
`F.obj X ≅ Aut F ⧸ V` as `Aut F`-sets. -/
lemma exists_lift_of_quotient_openSubgroup (V : OpenSubgroup (Aut F)) :
    ∃ (X : C), Nonempty ((functorToAction F).obj X ≅ Aut F ⧸ₐ V.toSubgroup)

Proof state:
case intro.intro.intro
C : Type u₁
inst✝² : Category.{u₂, u₁} C
F : C ⥤ FintypeCat
inst✝¹ : GaloisCategory C
inst✝ : FiberFunctor F
V : OpenSubgroup (Aut F)
I : Set C
hf : Fintype ↑I
hc : ∀ X ∈ I, IsConnected X
hi : ∀ (σ : Aut F), (∀ (X : ↑I), σ.hom.app ↑X = 𝟙 (F.obj ↑X)) → σ ∈ ↑V
⊢ ∃ X, Nonempty ((functorToAction F).obj X ≅ Action.FintypeCat.ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ ↑V)))

Tactic:
haveI (X : I) : IsConnected X.val := hc X X.property

Result:
case intro.intro.intro
C : Type u₁
inst✝² : Category.{u₂, u₁} C
F : C ⥤ FintypeCat
inst✝¹ : GaloisCategory C
inst✝ : FiberFunctor F
V : OpenSubgroup (Aut F)
I : Set C
hf : Fintype ↑I
hc : ∀ X ∈ I, IsConnected X
hi : ∀ (σ : Aut F), (∀ (X : ↑I), σ.hom.app ↑X = 𝟙 (F.obj ↑X)) → σ ∈ ↑V
this : ∀ (X : ↑I), IsConnected ↑X
⊢ ∃ X, Nonempty ((functorToAction F).obj X ≅ Action.FintypeCat.ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ ↑V)))

Proof state:
case intro.intro.intro.intro.intro.intro
C : Type u₁
inst✝² : Category.{u₂, u₁} C
F : C ⥤ FintypeCat
inst✝¹ : GaloisCategory C
inst✝ : FiberFunctor F
V : OpenSubgroup (Aut F)
I : Set C
hf : Fintype ↑I
hc : ∀ X ∈ I, IsConnected X
hi : ∀ (σ : Aut F), (∀ (X : ↑I), σ.hom.app ↑X = 𝟙 (F.obj ↑X)) → σ ∈ ↑V
this✝ : ∀ (X : ↑I), IsConnected ↑X
this : ∀ (X : ↑I), Nonempty (F.obj ↑X).carrier
hn : Nonempty (F.obj (∏ᶜ fun X => ↑X)).carrier
A : C
f : A ⟶ ∏ᶜ fun X => ↑X
hgal : IsGalois A
a : (F.obj A).carrier
U : OpenSubgroup (Aut F) := { toSubgroup := MulAction.stabilizer (Aut F) a, isOpen' := ⋯ }
u : (functorToAction F).obj A ≅
  Action.FintypeCat.ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ MulAction.stabilizer (Aut F) a)) :=
  fiberIsoQuotientStabilizer A a
hUnormal : (↑U).Normal
h1 : ∀ σ ∈ U, σ.hom.app A = 𝟙 (F.obj A)
⊢ ∃ X, Nonempty ((functorToAction F).obj X ≅ Action.FintypeCat.ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ ↑V)))

Tactic:
have h2 (σ : Aut F) (σinU : σ ∈ U) : ∀ X : I, σ.hom.app X = 𝟙 (F.obj X) := by
  intro ⟨X, hX⟩
  ext (x : F.obj X)
  let p : A ⟶ X := f ≫ Pi.π (fun Z : I => (Z : C)) ⟨X, hX⟩
  have : IsConnected X := hc X hX
  obtain ⟨a, rfl⟩ := surjective_of_nonempty_fiber_of_isConnected F p x
  simp only [FintypeCat.id_apply, FunctorToFintypeCat.naturality, h1 σ σinU]

Result:
case intro.intro.intro.intro.intro.intro
C : Type u₁
inst✝² : Category.{u₂, u₁} C
F : C ⥤ FintypeCat
inst✝¹ : GaloisCategory C
inst✝ : FiberFunctor F
V : OpenSubgroup (Aut F)
I : Set C
hf : Fintype ↑I
hc : ∀ X ∈ I, IsConnected X
hi : ∀ (σ : Aut F), (∀ (X : ↑I), σ.hom.app ↑X = 𝟙 (F.obj ↑X)) → σ ∈ ↑V
this✝ : ∀ (X : ↑I), IsConnected ↑X
this : ∀ (X : ↑I), Nonempty (F.obj ↑X).carrier
hn : Nonempty (F.obj (∏ᶜ fun X => ↑X)).carrier
A : C
f : A ⟶ ∏ᶜ fun X => ↑X
hgal : IsGalois A
a : (F.obj A).carrier
U : OpenSubgroup (Aut F) := { toSubgroup := MulAction.stabilizer (Aut F) a, isOpen' := ⋯ }
u : (functorToAction F).obj A ≅
  Action.FintypeCat.ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ MulAction.stabilizer (Aut F) a)) :=
  fiberIsoQuotientStabilizer A a
hUnormal : (↑U).Normal
h1 : ∀ σ ∈ U, σ.hom.app A = 𝟙 (F.obj A)
h2 : ∀ σ ∈ U, ∀ (X : ↑I), σ.hom.app ↑X = 𝟙 (F.obj ↑X)
⊢ ∃ X, Nonempty ((functorToAction F).obj X ≅ Action.FintypeCat.ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ ↑V)))

Proof state:
case h.a
C : Type u₁
inst✝² : Category.{u₂, u₁} C
F : C ⥤ FintypeCat
inst✝¹ : GaloisCategory C
inst✝ : FiberFunctor F
V : OpenSubgroup (Aut F)
I : Set C
hf : Fintype ↑I
hc : ∀ X ∈ I, IsConnected X
hi : ∀ (σ : Aut F), (∀ (X : ↑I), σ.hom.app ↑X = 𝟙 (F.obj ↑X)) → σ ∈ ↑V
this✝ : ∀ (X : ↑I), IsConnected ↑X
this : ∀ (X : ↑I), Nonempty (F.obj ↑X).carrier
hn : Nonempty (F.obj (∏ᶜ fun X => ↑X)).carrier
A : C
f : A ⟶ ∏ᶜ fun X => ↑X
hgal : IsGalois A
a : (F.obj A).carrier
U : OpenSubgroup (Aut F) := { toSubgroup := MulAction.stabilizer (Aut F) a, isOpen' := ⋯ }
u : (functorToAction F).obj A ≅
  Action.FintypeCat.ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ MulAction.stabilizer (Aut F) a)) :=
  fiberIsoQuotientStabilizer A a
hUnormal : (↑U).Normal
σ : Aut F
σinU : σ ∈ U
τ : Aut F
⊢ τ⁻¹ * σ⁻¹ * τ ∈ MulAction.stabilizer (Aut F) a

Tactic:
exact Subgroup.Normal.conj_mem hUnormal _ (Subgroup.inv_mem U.toSubgroup σinU) _

Result:
no goals

Proof state:
case intro.intro.intro.intro.intro
C : Type u₁
inst✝² : Category.{u₂, u₁} C
F : C ⥤ FintypeCat
inst✝¹ : GaloisCategory C
inst✝ : FiberFunctor F
V : OpenSubgroup (Aut F)
I : Set C
hf : Fintype ↑I
hc : ∀ X ∈ I, IsConnected X
hi : ∀ (σ : Aut F), (∀ (X : ↑I), σ.hom.app ↑X = 𝟙 (F.obj ↑X)) → σ ∈ ↑V
this✝ : ∀ (X : ↑I), IsConnected ↑X
this : ∀ (X : ↑I), Nonempty (F.obj ↑X).carrier
hn : Nonempty (F.obj (∏ᶜ fun X => ↑X)).carrier
A : C
f : A ⟶ ∏ᶜ fun X => ↑X
hgal : IsGalois A
⊢ ∃ X, Nonempty ((functorToAction F).obj X ≅ Action.FintypeCat.ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ ↑V)))

Tactic:
obtain ⟨a⟩ := nonempty_fiber_of_isConnected F A

Result:
case intro.intro.intro.intro.intro.intro
C : Type u₁
inst✝² : Category.{u₂, u₁} C
F : C ⥤ FintypeCat
inst✝¹ : GaloisCategory C
inst✝ : FiberFunctor F
V : OpenSubgroup (Aut F)
I : Set C
hf : Fintype ↑I
hc : ∀ X ∈ I, IsConnected X
hi : ∀ (σ : Aut F), (∀ (X : ↑I), σ.hom.app ↑X = 𝟙 (F.obj ↑X)) → σ ∈ ↑V
this✝ : ∀ (X : ↑I), IsConnected ↑X
this : ∀ (X : ↑I), Nonempty (F.obj ↑X).carrier
hn : Nonempty (F.obj (∏ᶜ fun X => ↑X)).carrier
A : C
f : A ⟶ ∏ᶜ fun X => ↑X
hgal : IsGalois A
a : (F.obj A).carrier
⊢ ∃ X, Nonempty ((functorToAction F).obj X ≅ Action.FintypeCat.ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ ↑V)))

Proof state:
case h.a
C : Type u₁
inst✝² : Category.{u₂, u₁} C
F : C ⥤ FintypeCat
inst✝¹ : GaloisCategory C
inst✝ : FiberFunctor F
V : OpenSubgroup (Aut F)
I : Set C
hf : Fintype ↑I
hc : ∀ X ∈ I, IsConnected X
hi : ∀ (σ : Aut F), (∀ (X : ↑I), σ.hom.app ↑X = 𝟙 (F.obj ↑X)) → σ ∈ ↑V
this✝ : ∀ (X : ↑I), IsConnected ↑X
this : ∀ (X : ↑I), Nonempty (F.obj ↑X).carrier
hn : Nonempty (F.obj (∏ᶜ fun X => ↑X)).carrier
A : C
f : A ⟶ ∏ᶜ fun X => ↑X
hgal : IsGalois A
a : (F.obj A).carrier
U : OpenSubgroup (Aut F) := { toSubgroup := MulAction.stabilizer (Aut F) a, isOpen' := ⋯ }
u : (functorToAction F).obj A ≅
  Action.FintypeCat.ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ MulAction.stabilizer (Aut F) a)) :=
  fiberIsoQuotientStabilizer A a
hUnormal : (↑U).Normal
σ : Aut F
σinU : σ ∈ U
τ : Aut F
⊢ (σ * τ)⁻¹ * τ ∈ MulAction.stabilizer (Aut F) a

Tactic:
simp only [mul_inv_rev]

Result:
case h.a
C : Type u₁
inst✝² : Category.{u₂, u₁} C
F : C ⥤ FintypeCat
inst✝¹ : GaloisCategory C
inst✝ : FiberFunctor F
V : OpenSubgroup (Aut F)
I : Set C
hf : Fintype ↑I
hc : ∀ X ∈ I, IsConnected X
hi : ∀ (σ : Aut F), (∀ (X : ↑I), σ.hom.app ↑X = 𝟙 (F.obj ↑X)) → σ ∈ ↑V
this✝ : ∀ (X : ↑I), IsConnected ↑X
this : ∀ (X : ↑I), Nonempty (F.obj ↑X).carrier
hn : Nonempty (F.obj (∏ᶜ fun X => ↑X)).carrier
A : C
f : A ⟶ ∏ᶜ fun X => ↑X
hgal : IsGalois A
a : (F.obj A).carrier
U : OpenSubgroup (Aut F) := { toSubgroup := MulAction.stabilizer (Aut F) a, isOpen' := ⋯ }
u : (functorToAction F).obj A ≅
  Action.FintypeCat.ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ MulAction.stabilizer (Aut F) a)) :=
  fiberIsoQuotientStabilizer A a
hUnormal : (↑U).Normal
σ : Aut F
σinU : σ ∈ U
τ : Aut F
⊢ τ⁻¹ * σ⁻¹ * τ ∈ MulAction.stabilizer (Aut F) a