-- Mathlib/Topology/Sheaves/LocallySurjective.lean
theorem locally_surjective_iff_surjective_on_stalks (T : ℱ ⟶ 𝒢) :
    IsLocallySurjective T ↔ ∀ x : X, Function.Surjective ((stalkFunctor C x).map T)

Proof state:
case mp
C : Type u
inst✝⁴ : Category.{v, u} C
FC : C → C → Type u_1
CC : C → Type v
inst✝³ : (X Y : C) → FunLike (FC X Y) (CC X) (CC Y)
inst✝² : ConcreteCategory C FC
X : TopCat
ℱ 𝒢 : Presheaf C X
inst✝¹ : Limits.HasColimits C
inst✝ : Limits.PreservesFilteredColimits (forget C)
T : ℱ ⟶ 𝒢
hT : IsLocallySurjective T
x : ↑X
g : CC ((stalkFunctor C x).obj 𝒢)
⊢ ∃ a, (ConcreteCategory.hom ((stalkFunctor C x).map T)) a = g

Tactic:
obtain ⟨U, hxU, t, rfl⟩ := 𝒢.germ_exist x g

Result:
case mp.intro.intro.intro
C : Type u
inst✝⁴ : Category.{v, u} C
FC : C → C → Type u_1
CC : C → Type v
inst✝³ : (X Y : C) → FunLike (FC X Y) (CC X) (CC Y)
inst✝² : ConcreteCategory C FC
X : TopCat
ℱ 𝒢 : Presheaf C X
inst✝¹ : Limits.HasColimits C
inst✝ : Limits.PreservesFilteredColimits (forget C)
T : ℱ ⟶ 𝒢
hT : IsLocallySurjective T
x : ↑X
U : Opens ↑X
hxU : x ∈ U
t : ToType (𝒢.obj (op U))
⊢ ∃ a, (ConcreteCategory.hom ((stalkFunctor C x).map T)) a = (ConcreteCategory.hom (𝒢.germ U x hxU)) t

Proof state:
case mpr.imageSieve_mem
C : Type u
inst✝⁴ : Category.{v, u} C
FC : C → C → Type u_1
CC : C → Type v
inst✝³ : (X Y : C) → FunLike (FC X Y) (CC X) (CC Y)
inst✝² : ConcreteCategory C FC
X : TopCat
ℱ 𝒢 : Presheaf C X
inst✝¹ : Limits.HasColimits C
inst✝ : Limits.PreservesFilteredColimits (forget C)
T : ℱ ⟶ 𝒢
hT : ∀ (x : ↑X), Function.Surjective ⇑(ConcreteCategory.hom ((stalkFunctor C x).map T))
⊢ ∀ {U : Opens ↑X} (s : ToType (𝒢.obj (op U))), Presheaf.imageSieve T s ∈ (Opens.grothendieckTopology ↑X) U

Tactic:
intro U t x hxU

Result:
case mpr.imageSieve_mem
C : Type u
inst✝⁴ : Category.{v, u} C
FC : C → C → Type u_1
CC : C → Type v
inst✝³ : (X Y : C) → FunLike (FC X Y) (CC X) (CC Y)
inst✝² : ConcreteCategory C FC
X : TopCat
ℱ 𝒢 : Presheaf C X
inst✝¹ : Limits.HasColimits C
inst✝ : Limits.PreservesFilteredColimits (forget C)
T : ℱ ⟶ 𝒢
hT : ∀ (x : ↑X), Function.Surjective ⇑(ConcreteCategory.hom ((stalkFunctor C x).map T))
U : Opens ↑X
t : ToType (𝒢.obj (op U))
x : ↑X
hxU : x ∈ U
⊢ ∃ U_1 f, (Presheaf.imageSieve T t).arrows f ∧ x ∈ U_1

Proof state:
case mpr.imageSieve_mem
C : Type u
inst✝⁴ : Category.{v, u} C
FC : C → C → Type u_1
CC : C → Type v
inst✝³ : (X Y : C) → FunLike (FC X Y) (CC X) (CC Y)
inst✝² : ConcreteCategory C FC
X : TopCat
ℱ 𝒢 : Presheaf C X
inst✝¹ : Limits.HasColimits C
inst✝ : Limits.PreservesFilteredColimits (forget C)
T : ℱ ⟶ 𝒢
hT : ∀ (x : ↑X), Function.Surjective ⇑(ConcreteCategory.hom ((stalkFunctor C x).map T))
U : Opens ↑X
t : ToType (𝒢.obj (op U))
x : ↑X
hxU : x ∈ U
t_x : CC (𝒢.stalk x) := (ConcreteCategory.hom (𝒢.germ U x hxU)) t
ht_x : t_x = (ConcreteCategory.hom (𝒢.germ U x hxU)) t
⊢ ∃ U_1 f, (Presheaf.imageSieve T t).arrows f ∧ x ∈ U_1

Tactic:
obtain ⟨s_x, hs_x : ((stalkFunctor C x).map T) s_x = t_x⟩ := hT x t_x

Result:
case mpr.imageSieve_mem.intro
C : Type u
inst✝⁴ : Category.{v, u} C
FC : C → C → Type u_1
CC : C → Type v
inst✝³ : (X Y : C) → FunLike (FC X Y) (CC X) (CC Y)
inst✝² : ConcreteCategory C FC
X : TopCat
ℱ 𝒢 : Presheaf C X
inst✝¹ : Limits.HasColimits C
inst✝ : Limits.PreservesFilteredColimits (forget C)
T : ℱ ⟶ 𝒢
hT : ∀ (x : ↑X), Function.Surjective ⇑(ConcreteCategory.hom ((stalkFunctor C x).map T))
U : Opens ↑X
t : ToType (𝒢.obj (op U))
x : ↑X
hxU : x ∈ U
t_x : CC (𝒢.stalk x) := (ConcreteCategory.hom (𝒢.germ U x hxU)) t
ht_x : t_x = (ConcreteCategory.hom (𝒢.germ U x hxU)) t
s_x : CC ((stalkFunctor C x).obj ℱ)
hs_x : (ConcreteCategory.hom ((stalkFunctor C x).map T)) s_x = t_x
⊢ ∃ U_1 f, (Presheaf.imageSieve T t).arrows f ∧ x ∈ U_1

Proof state:
case mp
C : Type u
inst✝⁴ : Category.{v, u} C
FC : C → C → Type u_1
CC : C → Type v
inst✝³ : (X Y : C) → FunLike (FC X Y) (CC X) (CC Y)
inst✝² : ConcreteCategory C FC
X : TopCat
ℱ 𝒢 : Presheaf C X
inst✝¹ : Limits.HasColimits C
inst✝ : Limits.PreservesFilteredColimits (forget C)
T : ℱ ⟶ 𝒢
hT : IsLocallySurjective T
⊢ ∀ (x : ↑X), Function.Surjective ⇑(ConcreteCategory.hom ((stalkFunctor C x).map T))

Tactic:
intro x g

Result:
case mp
C : Type u
inst✝⁴ : Category.{v, u} C
FC : C → C → Type u_1
CC : C → Type v
inst✝³ : (X Y : C) → FunLike (FC X Y) (CC X) (CC Y)
inst✝² : ConcreteCategory C FC
X : TopCat
ℱ 𝒢 : Presheaf C X
inst✝¹ : Limits.HasColimits C
inst✝ : Limits.PreservesFilteredColimits (forget C)
T : ℱ ⟶ 𝒢
hT : IsLocallySurjective T
x : ↑X
g : CC ((stalkFunctor C x).obj 𝒢)
⊢ ∃ a, (ConcreteCategory.hom ((stalkFunctor C x).map T)) a = g

Proof state:
C : Type u
inst✝⁴ : Category.{v, u} C
FC : C → C → Type u_1
CC : C → Type v
inst✝³ : (X Y : C) → FunLike (FC X Y) (CC X) (CC Y)
inst✝² : ConcreteCategory C FC
X : TopCat
ℱ 𝒢 : Presheaf C X
inst✝¹ : Limits.HasColimits C
inst✝ : Limits.PreservesFilteredColimits (forget C)
T : ℱ ⟶ 𝒢
⊢ IsLocallySurjective T ↔ ∀ (x : ↑X), Function.Surjective ⇑(ConcreteCategory.hom ((stalkFunctor C x).map T))

Tactic:
constructor <;> intro hT

Result:
case mp
C : Type u
inst✝⁴ : Category.{v, u} C
FC : C → C → Type u_1
CC : C → Type v
inst✝³ : (X Y : C) → FunLike (FC X Y) (CC X) (CC Y)
inst✝² : ConcreteCategory C FC
X : TopCat
ℱ 𝒢 : Presheaf C X
inst✝¹ : Limits.HasColimits C
inst✝ : Limits.PreservesFilteredColimits (forget C)
T : ℱ ⟶ 𝒢
hT : IsLocallySurjective T
⊢ ∀ (x : ↑X), Function.Surjective ⇑(ConcreteCategory.hom ((stalkFunctor C x).map T))

case mpr
C : Type u
inst✝⁴ : Category.{v, u} C
FC : C → C → Type u_1
CC : C → Type v
inst✝³ : (X Y : C) → FunLike (FC X Y) (CC X) (CC Y)
inst✝² : ConcreteCategory C FC
X : TopCat
ℱ 𝒢 : Presheaf C X
inst✝¹ : Limits.HasColimits C
inst✝ : Limits.PreservesFilteredColimits (forget C)
T : ℱ ⟶ 𝒢
hT : ∀ (x : ↑X), Function.Surjective ⇑(ConcreteCategory.hom ((stalkFunctor C x).map T))
⊢ IsLocallySurjective T

Proof state:
case h.e'_2.h
C : Type u
inst✝⁴ : Category.{v, u} C
FC : C → C → Type u_1
CC : C → Type v
inst✝³ : (X Y : C) → FunLike (FC X Y) (CC X) (CC Y)
inst✝² : ConcreteCategory C FC
X : TopCat
ℱ 𝒢 : Presheaf C X
inst✝¹ : Limits.HasColimits C
inst✝ : Limits.PreservesFilteredColimits (forget C)
T : ℱ ⟶ 𝒢
hT : ∀ (x : ↑X), Function.Surjective ⇑(ConcreteCategory.hom ((stalkFunctor C x).map T))
U : Opens ↑X
t : ToType (𝒢.obj (op U))
x : ↑X
hxU : x ∈ U
t_x : CC (𝒢.stalk x) := (ConcreteCategory.hom (𝒢.germ U x hxU)) t
ht_x : t_x = (ConcreteCategory.hom (𝒢.germ U x hxU)) t
V : Opens ↑X
hxV : x ∈ V
s : ToType (ℱ.obj (op V))
hs_x : (ConcreteCategory.hom ((stalkFunctor C x).map T)) ((ConcreteCategory.hom (ℱ.germ V x hxV)) s) = t_x
e_1✝ : CC (𝒢.stalk x) = CC ((stalkFunctor C x).obj 𝒢)
⊢ (ConcreteCategory.hom (𝒢.germ V x hxV)) ((ConcreteCategory.hom (T.app (op V))) s) =
    (ConcreteCategory.hom ((stalkFunctor C x).map T)) ((ConcreteCategory.hom (ℱ.germ V x hxV)) s)

Tactic:
symm

Result:
case h.e'_2.h
C : Type u
inst✝⁴ : Category.{v, u} C
FC : C → C → Type u_1
CC : C → Type v
inst✝³ : (X Y : C) → FunLike (FC X Y) (CC X) (CC Y)
inst✝² : ConcreteCategory C FC
X : TopCat
ℱ 𝒢 : Presheaf C X
inst✝¹ : Limits.HasColimits C
inst✝ : Limits.PreservesFilteredColimits (forget C)
T : ℱ ⟶ 𝒢
hT : ∀ (x : ↑X), Function.Surjective ⇑(ConcreteCategory.hom ((stalkFunctor C x).map T))
U : Opens ↑X
t : ToType (𝒢.obj (op U))
x : ↑X
hxU : x ∈ U
t_x : CC (𝒢.stalk x) := (ConcreteCategory.hom (𝒢.germ U x hxU)) t
ht_x : t_x = (ConcreteCategory.hom (𝒢.germ U x hxU)) t
V : Opens ↑X
hxV : x ∈ V
s : ToType (ℱ.obj (op V))
hs_x : (ConcreteCategory.hom ((stalkFunctor C x).map T)) ((ConcreteCategory.hom (ℱ.germ V x hxV)) s) = t_x
e_1✝ : CC (𝒢.stalk x) = CC ((stalkFunctor C x).obj 𝒢)
⊢ (ConcreteCategory.hom ((stalkFunctor C x).map T)) ((ConcreteCategory.hom (ℱ.germ V x hxV)) s) =
    (ConcreteCategory.hom (𝒢.germ V x hxV)) ((ConcreteCategory.hom (T.app (op V))) s)