-- Mathlib/AlgebraicGeometry/Morphisms/Separated.lean
/--
Suppose `X` is a reduced scheme and that `f g : X ⟶ Y` agree over some separated `Y ⟶ Z`.
Then `f = g` if `ι ≫ f = ι ≫ g` for some dominant `ι`.
-/
lemma ext_of_isDominant_of_isSeparated [IsReduced X] {f g : X ⟶ Y}
    (s : Y ⟶ Z) [IsSeparated s] (h : f ≫ s = g ≫ s)
    (ι : W ⟶ X) [IsDominant ι] (hU : ι ≫ f = ι ≫ g) : f = g

Proof state:
case h
W X Y Z : Scheme
inst✝² : IsReduced X
f g : X ⟶ Y
s : Y ⟶ Z
inst✝¹ : IsSeparated s
h : f ≫ s = g ≫ s
ι : W ⟶ X
inst✝ : IsDominant ι
hU : ι ≫ f = ι ≫ g
X' : Over Z := Over.mk (f ≫ s)
Y' : Over Z := Over.mk s
U' : Over Z := Over.mk (ι ≫ f ≫ s)
f' : X' ⟶ Y' := Over.homMk f ⋯
g' : X' ⟶ Y' := Over.homMk g ⋯
ι' : U' ⟶ X' := Over.homMk ι ⋯
this : IsSeparated Y'.hom
⊢ (ι' ≫ f').left = (ι' ≫ g').left

Tactic:
exact hU

Result:
no goals

Proof state:
W X Y Z : Scheme
inst✝² : IsReduced X
f g : X ⟶ Y
s : Y ⟶ Z
inst✝¹ : IsSeparated s
h : f ≫ s = g ≫ s
ι : W ⟶ X
inst✝ : IsDominant ι
hU : ι ≫ f = ι ≫ g
X' : Over Z := Over.mk (f ≫ s)
Y' : Over Z := Over.mk s
U' : Over Z := Over.mk (ι ≫ f ≫ s)
f' : X' ⟶ Y' := Over.homMk f ⋯
g' : X' ⟶ Y' := Over.homMk g ⋯
ι' : U' ⟶ X' := Over.homMk ι ⋯
this✝ : IsSeparated Y'.hom
this : IsDominant (equalizer.ι f' g').left
⊢ f = g

Tactic:
have : Surjective (equalizer.ι f' g').left :=
  surjective_of_isDominant_of_isClosed_range _ IsClosedImmersion.base_closed.2

Result:
W X Y Z : Scheme
inst✝² : IsReduced X
f g : X ⟶ Y
s : Y ⟶ Z
inst✝¹ : IsSeparated s
h : f ≫ s = g ≫ s
ι : W ⟶ X
inst✝ : IsDominant ι
hU : ι ≫ f = ι ≫ g
X' : Over Z := Over.mk (f ≫ s)
Y' : Over Z := Over.mk s
U' : Over Z := Over.mk (ι ≫ f ≫ s)
f' : X' ⟶ Y' := Over.homMk f ⋯
g' : X' ⟶ Y' := Over.homMk g ⋯
ι' : U' ⟶ X' := Over.homMk ι ⋯
this✝¹ : IsSeparated Y'.hom
this✝ : IsDominant (equalizer.ι f' g').left
this : Surjective (equalizer.ι f' g').left
⊢ f = g

Proof state:
W X Y Z : Scheme
inst✝² : IsReduced X
f g : X ⟶ Y
s : Y ⟶ Z
inst✝¹ : IsSeparated s
h : f ≫ s = g ≫ s
ι : W ⟶ X
inst✝ : IsDominant ι
hU : ι ≫ f = ι ≫ g
X' : Over Z := Over.mk (f ≫ s)
Y' : Over Z := Over.mk s
U' : Over Z := Over.mk (ι ≫ f ≫ s)
f' : X' ⟶ Y' := Over.homMk f ⋯
g' : X' ⟶ Y' := Over.homMk g ⋯
ι' : U' ⟶ X' := Over.homMk ι ⋯
this✝² : IsSeparated Y'.hom
this✝¹ : IsDominant (equalizer.ι f' g').left
this✝ : Surjective (equalizer.ι f' g').left
this : IsIso (equalizer.ι f' g').left
⊢ f = g

Tactic:
rw [← cancel_epi (equalizer.ι f' g').left]

Result:
W X Y Z : Scheme
inst✝² : IsReduced X
f g : X ⟶ Y
s : Y ⟶ Z
inst✝¹ : IsSeparated s
h : f ≫ s = g ≫ s
ι : W ⟶ X
inst✝ : IsDominant ι
hU : ι ≫ f = ι ≫ g
X' : Over Z := Over.mk (f ≫ s)
Y' : Over Z := Over.mk s
U' : Over Z := Over.mk (ι ≫ f ≫ s)
f' : X' ⟶ Y' := Over.homMk f ⋯
g' : X' ⟶ Y' := Over.homMk g ⋯
ι' : U' ⟶ X' := Over.homMk ι ⋯
this✝² : IsSeparated Y'.hom
this✝¹ : IsDominant (equalizer.ι f' g').left
this✝ : Surjective (equalizer.ι f' g').left
this : IsIso (equalizer.ι f' g').left
⊢ (equalizer.ι f' g').left ≫ f = (equalizer.ι f' g').left ≫ g

Proof state:
W X Y Z : Scheme
inst✝² : IsReduced X
f g : X ⟶ Y
s : Y ⟶ Z
inst✝¹ : IsSeparated s
h : f ≫ s = g ≫ s
ι : W ⟶ X
inst✝ : IsDominant ι
hU : ι ≫ f = ι ≫ g
X' : Over Z := Over.mk (f ≫ s)
Y' : Over Z := Over.mk s
U' : Over Z := Over.mk (ι ≫ f ≫ s)
f' : X' ⟶ Y' := Over.homMk f ⋯
g' : X' ⟶ Y' := Over.homMk g ⋯
ι' : U' ⟶ X' := Over.homMk ι ⋯
this : IsSeparated Y'.hom
⊢ IsDominant (equalizer.ι f' g').left

Tactic:
apply (config := { allowSynthFailures := true }) IsDominant.of_comp (equalizer.lift ι' ?_).left

Result:
case H
W X Y Z : Scheme
inst✝² : IsReduced X
f g : X ⟶ Y
s : Y ⟶ Z
inst✝¹ : IsSeparated s
h : f ≫ s = g ≫ s
ι : W ⟶ X
inst✝ : IsDominant ι
hU : ι ≫ f = ι ≫ g
X' : Over Z := Over.mk (f ≫ s)
Y' : Over Z := Over.mk s
U' : Over Z := Over.mk (ι ≫ f ≫ s)
f' : X' ⟶ Y' := Over.homMk f ⋯
g' : X' ⟶ Y' := Over.homMk g ⋯
ι' : U' ⟶ X' := Over.homMk ι ⋯
this : IsSeparated Y'.hom
⊢ IsDominant ((equalizer.lift ι' ?m.81244).left ≫ (equalizer.ι f' g').left)

W X Y Z : Scheme
inst✝² : IsReduced X
f g : X ⟶ Y
s : Y ⟶ Z
inst✝¹ : IsSeparated s
h : f ≫ s = g ≫ s
ι : W ⟶ X
inst✝ : IsDominant ι
hU : ι ≫ f = ι ≫ g
X' : Over Z := Over.mk (f ≫ s)
Y' : Over Z := Over.mk s
U' : Over Z := Over.mk (ι ≫ f ≫ s)
f' : X' ⟶ Y' := Over.homMk f ⋯
g' : X' ⟶ Y' := Over.homMk g ⋯
ι' : U' ⟶ X' := Over.homMk ι ⋯
this : IsSeparated Y'.hom
⊢ ι' ≫ f' = ι' ≫ g'

Proof state:
case H
W X Y Z : Scheme
inst✝² : IsReduced X
f g : X ⟶ Y
s : Y ⟶ Z
inst✝¹ : IsSeparated s
h : f ≫ s = g ≫ s
ι : W ⟶ X
inst✝ : IsDominant ι
hU : ι ≫ f = ι ≫ g
X' : Over Z := Over.mk (f ≫ s)
Y' : Over Z := Over.mk s
U' : Over Z := Over.mk (ι ≫ f ≫ s)
f' : X' ⟶ Y' := Over.homMk f ⋯
g' : X' ⟶ Y' := Over.homMk g ⋯
ι' : U' ⟶ X' := Over.homMk ι ⋯
this : IsSeparated Y'.hom
⊢ IsDominant ((equalizer.lift ι' ?m.81244).left ≫ (equalizer.ι f' g').left)

Tactic:
rwa [← Over.comp_left, equalizer.lift_ι]

Result:
no goals

Proof state:
W X Y Z : Scheme
inst✝² : IsReduced X
f g : X ⟶ Y
s : Y ⟶ Z
inst✝¹ : IsSeparated s
h : f ≫ s = g ≫ s
ι : W ⟶ X
inst✝ : IsDominant ι
hU : ι ≫ f = ι ≫ g
X' : Over Z := Over.mk (f ≫ s)
Y' : Over Z := Over.mk s
U' : Over Z := Over.mk (ι ≫ f ≫ s)
f' : X' ⟶ Y' := Over.homMk f ⋯
g' : X' ⟶ Y' := Over.homMk g ⋯
ι' : U' ⟶ X' := Over.homMk ι ⋯
this✝¹ : IsSeparated Y'.hom
this✝ : IsDominant (equalizer.ι f' g').left
this : Surjective (equalizer.ι f' g').left
⊢ f = g

Tactic:
have := isIso_of_isClosedImmersion_of_surjective (Y := X) (equalizer.ι f' g').left

Result:
W X Y Z : Scheme
inst✝² : IsReduced X
f g : X ⟶ Y
s : Y ⟶ Z
inst✝¹ : IsSeparated s
h : f ≫ s = g ≫ s
ι : W ⟶ X
inst✝ : IsDominant ι
hU : ι ≫ f = ι ≫ g
X' : Over Z := Over.mk (f ≫ s)
Y' : Over Z := Over.mk s
U' : Over Z := Over.mk (ι ≫ f ≫ s)
f' : X' ⟶ Y' := Over.homMk f ⋯
g' : X' ⟶ Y' := Over.homMk g ⋯
ι' : U' ⟶ X' := Over.homMk ι ⋯
this✝² : IsSeparated Y'.hom
this✝¹ : IsDominant (equalizer.ι f' g').left
this✝ : Surjective (equalizer.ι f' g').left
this : IsIso (equalizer.ι f' g').left
⊢ f = g

Proof state:
W X Y Z : Scheme
inst✝² : IsReduced X
f g : X ⟶ Y
s : Y ⟶ Z
inst✝¹ : IsSeparated s
h : f ≫ s = g ≫ s
ι : W ⟶ X
inst✝ : IsDominant ι
hU : ι ≫ f = ι ≫ g
X' : Over Z := Over.mk (f ≫ s)
Y' : Over Z := Over.mk s
U' : Over Z := Over.mk (ι ≫ f ≫ s)
f' : X' ⟶ Y' := Over.homMk f ⋯
g' : X' ⟶ Y' := Over.homMk g ⋯
ι' : U' ⟶ X' := Over.homMk ι ⋯
⊢ f = g

Tactic:
have : IsSeparated Y'.hom := ‹_›

Result:
W X Y Z : Scheme
inst✝² : IsReduced X
f g : X ⟶ Y
s : Y ⟶ Z
inst✝¹ : IsSeparated s
h : f ≫ s = g ≫ s
ι : W ⟶ X
inst✝ : IsDominant ι
hU : ι ≫ f = ι ≫ g
X' : Over Z := Over.mk (f ≫ s)
Y' : Over Z := Over.mk s
U' : Over Z := Over.mk (ι ≫ f ≫ s)
f' : X' ⟶ Y' := Over.homMk f ⋯
g' : X' ⟶ Y' := Over.homMk g ⋯
ι' : U' ⟶ X' := Over.homMk ι ⋯
this : IsSeparated Y'.hom
⊢ f = g

Proof state:
W X Y Z : Scheme
inst✝² : IsReduced X
f g : X ⟶ Y
s : Y ⟶ Z
inst✝¹ : IsSeparated s
h : f ≫ s = g ≫ s
ι : W ⟶ X
inst✝ : IsDominant ι
hU : ι ≫ f = ι ≫ g
X' : Over Z := Over.mk (f ≫ s)
Y' : Over Z := Over.mk s
U' : Over Z := Over.mk (ι ≫ f ≫ s)
f' : X' ⟶ Y' := Over.homMk f ⋯
⊢ f = g

Tactic:
let g' : X' ⟶ Y' := Over.homMk g

Result:
W X Y Z : Scheme
inst✝² : IsReduced X
f g : X ⟶ Y
s : Y ⟶ Z
inst✝¹ : IsSeparated s
h : f ≫ s = g ≫ s
ι : W ⟶ X
inst✝ : IsDominant ι
hU : ι ≫ f = ι ≫ g
X' : Over Z := Over.mk (f ≫ s)
Y' : Over Z := Over.mk s
U' : Over Z := Over.mk (ι ≫ f ≫ s)
f' : X' ⟶ Y' := Over.homMk f ⋯
g' : X' ⟶ Y' := Over.homMk g ⋯
⊢ f = g

Proof state:
W X Y Z : Scheme
inst✝² : IsReduced X
f g : X ⟶ Y
s : Y ⟶ Z
inst✝¹ : IsSeparated s
h : f ≫ s = g ≫ s
ι : W ⟶ X
inst✝ : IsDominant ι
hU : ι ≫ f = ι ≫ g
X' : Over Z := Over.mk (f ≫ s)
Y' : Over Z := Over.mk s
⊢ f = g

Tactic:
let U' : Over Z := Over.mk (ι ≫ f ≫ s)

Result:
W X Y Z : Scheme
inst✝² : IsReduced X
f g : X ⟶ Y
s : Y ⟶ Z
inst✝¹ : IsSeparated s
h : f ≫ s = g ≫ s
ι : W ⟶ X
inst✝ : IsDominant ι
hU : ι ≫ f = ι ≫ g
X' : Over Z := Over.mk (f ≫ s)
Y' : Over Z := Over.mk s
U' : Over Z := Over.mk (ι ≫ f ≫ s)
⊢ f = g