-- 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:
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
⊢ σ.hom.app A = 𝟙 (F.obj A)

Tactic:
have hi : (Aut F ⧸ₐ MulAction.stabilizer (Aut F) a).ρ σ = 𝟙 _ := by
  refine FintypeCat.hom_ext _ _ (fun x ↦ ?_)
  induction' x using Quotient.inductionOn with τ
  show ⟦σ * τ⟧ = ⟦τ⟧
  apply Quotient.sound
  apply (QuotientGroup.leftRel_apply).mpr
  simp only [mul_inv_rev]
  exact Subgroup.Normal.conj_mem hUnormal _ (Subgroup.inv_mem U.toSubgroup σinU) _

Result:
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
hi :
  (Action.FintypeCat.ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ MulAction.stabilizer (Aut F) a))).ρ σ =
    𝟙 (Action.FintypeCat.ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ MulAction.stabilizer (Aut F) a))).V
⊢ σ.hom.app A = 𝟙 (F.obj A)

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' := ⋯ }
⊢ ∃ X, Nonempty ((functorToAction F).obj X ≅ Action.FintypeCat.ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ ↑V)))

Tactic:
let u := fiberIsoQuotientStabilizer A 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
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
⊢ ∃ 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
⊢ ∃ X, Nonempty ((functorToAction F).obj X ≅ Action.FintypeCat.ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ ↑V)))

Tactic:
let U : OpenSubgroup (Aut F) := ⟨MulAction.stabilizer (Aut F) a, stabilizer_isOpen (Aut 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
U : OpenSubgroup (Aut F) := { toSubgroup := MulAction.stabilizer (Aut F) a, isOpen' := ⋯ }
⊢ ∃ X, Nonempty ((functorToAction F).obj X ≅ Action.FintypeCat.ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ ↑V)))

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
this✝ : ∀ (X : ↑I), IsConnected ↑X
this : ∀ (X : ↑I), Nonempty (F.obj ↑X).carrier
⊢ ∃ X, Nonempty ((functorToAction F).obj X ≅ Action.FintypeCat.ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ ↑V)))

Tactic:
have hn : Nonempty (F.obj <| (∏ᶜ fun X : I => X)) := nonempty_fiber_pi_of_nonempty_of_finite F _

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
this : ∀ (X : ↑I), Nonempty (F.obj ↑X).carrier
hn : Nonempty (F.obj (∏ᶜ fun X => ↑X)).carrier
⊢ ∃ X, Nonempty ((functorToAction F).obj X ≅ Action.FintypeCat.ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ ↑V)))

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
this✝ : ∀ (X : ↑I), IsConnected ↑X
this : ∀ (X : ↑I), Nonempty (F.obj ↑X).carrier
hn : Nonempty (F.obj (∏ᶜ fun X => ↑X)).carrier
⊢ ∃ X, Nonempty ((functorToAction F).obj X ≅ Action.FintypeCat.ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ ↑V)))

Tactic:
obtain ⟨A, f, hgal⟩ := exists_hom_from_galois_of_fiber_nonempty F (∏ᶜ fun X : I => X) hn

Result:
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)))