-- Mathlib/Topology/ExtremallyDisconnected.lean
private lemma ExtremallyDisconnected.homeoCompactToT2_injective [ExtremallyDisconnected A]
    [T2Space A] [T2Space E] [CompactSpace E] {ρ : E → A} (ρ_cont : Continuous ρ)
    (ρ_surj : ρ.Surjective) (zorn_subset : ∀ E₀ : Set E, E₀ ≠ univ → IsClosed E₀ → ρ '' E₀ ≠ univ) :
    ρ.Injective

Proof state:
case intro.intro.intro.intro.intro.intro
A E : Type u
inst✝⁵ : TopologicalSpace A
inst✝⁴ : TopologicalSpace E
inst✝³ : ExtremallyDisconnected A
inst✝² : T2Space A
inst✝¹ : T2Space E
inst✝ : CompactSpace E
ρ : E → A
ρ_cont : Continuous ρ
ρ_surj : Surjective ρ
zorn_subset : ∀ (E₀ : Set E), E₀ ≠ univ → IsClosed E₀ → ρ '' E₀ ≠ univ
x₁ x₂ : E
hρx : ρ x₁ = ρ x₂
hx : ¬x₁ = x₂
G₁ G₂ : Set E
G₁_open : IsOpen G₁
G₂_open : IsOpen G₂
hx₁ : x₁ ∈ G₁
hx₂ : x₂ ∈ G₂
disj : Disjoint G₁ G₂
G₁_comp : IsCompact G₁ᶜ
G₂_comp : IsCompact G₂ᶜ
G₁_open' : IsOpen (ρ '' G₁ᶜ)ᶜ
⊢ False

Tactic:
have G₂_open' : IsOpen (ρ '' G₂ᶜ)ᶜ := (G₂_comp.image ρ_cont).isClosed.isOpen_compl

Result:
case intro.intro.intro.intro.intro.intro
A E : Type u
inst✝⁵ : TopologicalSpace A
inst✝⁴ : TopologicalSpace E
inst✝³ : ExtremallyDisconnected A
inst✝² : T2Space A
inst✝¹ : T2Space E
inst✝ : CompactSpace E
ρ : E → A
ρ_cont : Continuous ρ
ρ_surj : Surjective ρ
zorn_subset : ∀ (E₀ : Set E), E₀ ≠ univ → IsClosed E₀ → ρ '' E₀ ≠ univ
x₁ x₂ : E
hρx : ρ x₁ = ρ x₂
hx : ¬x₁ = x₂
G₁ G₂ : Set E
G₁_open : IsOpen G₁
G₂_open : IsOpen G₂
hx₁ : x₁ ∈ G₁
hx₂ : x₂ ∈ G₂
disj : Disjoint G₁ G₂
G₁_comp : IsCompact G₁ᶜ
G₂_comp : IsCompact G₂ᶜ
G₁_open' : IsOpen (ρ '' G₁ᶜ)ᶜ
G₂_open' : IsOpen (ρ '' G₂ᶜ)ᶜ
⊢ False

Proof state:
A E : Type u
inst✝⁵ : TopologicalSpace A
inst✝⁴ : TopologicalSpace E
inst✝³ : ExtremallyDisconnected A
inst✝² : T2Space A
inst✝¹ : T2Space E
inst✝ : CompactSpace E
ρ : E → A
ρ_cont : Continuous ρ
ρ_surj : Surjective ρ
zorn_subset : ∀ (E₀ : Set E), E₀ ≠ univ → IsClosed E₀ → ρ '' E₀ ≠ univ
x₁ x₂ : E
hρx : ρ x₁ = ρ x₂
⊢ x₁ = x₂

Tactic:
by_contra hx

Result:
A E : Type u
inst✝⁵ : TopologicalSpace A
inst✝⁴ : TopologicalSpace E
inst✝³ : ExtremallyDisconnected A
inst✝² : T2Space A
inst✝¹ : T2Space E
inst✝ : CompactSpace E
ρ : E → A
ρ_cont : Continuous ρ
ρ_surj : Surjective ρ
zorn_subset : ∀ (E₀ : Set E), E₀ ≠ univ → IsClosed E₀ → ρ '' E₀ ≠ univ
x₁ x₂ : E
hρx : ρ x₁ = ρ x₂
hx : ¬x₁ = x₂
⊢ False

Proof state:
case intro.intro.intro.intro.intro.intro
A E : Type u
inst✝⁵ : TopologicalSpace A
inst✝⁴ : TopologicalSpace E
inst✝³ : ExtremallyDisconnected A
inst✝² : T2Space A
inst✝¹ : T2Space E
inst✝ : CompactSpace E
ρ : E → A
ρ_cont : Continuous ρ
ρ_surj : Surjective ρ
zorn_subset : ∀ (E₀ : Set E), E₀ ≠ univ → IsClosed E₀ → ρ '' E₀ ≠ univ
x₁ x₂ : E
hρx : ρ x₁ = ρ x₂
hx : ¬x₁ = x₂
G₁ G₂ : Set E
G₁_open : IsOpen G₁
G₂_open : IsOpen G₂
hx₁ : x₁ ∈ G₁
hx₂ : x₂ ∈ G₂
disj : Disjoint G₁ G₂
G₁_comp : IsCompact G₁ᶜ
G₂_comp : IsCompact G₂ᶜ
⊢ False

Tactic:
have G₁_open' : IsOpen (ρ '' G₁ᶜ)ᶜ := (G₁_comp.image ρ_cont).isClosed.isOpen_compl

Result:
case intro.intro.intro.intro.intro.intro
A E : Type u
inst✝⁵ : TopologicalSpace A
inst✝⁴ : TopologicalSpace E
inst✝³ : ExtremallyDisconnected A
inst✝² : T2Space A
inst✝¹ : T2Space E
inst✝ : CompactSpace E
ρ : E → A
ρ_cont : Continuous ρ
ρ_surj : Surjective ρ
zorn_subset : ∀ (E₀ : Set E), E₀ ≠ univ → IsClosed E₀ → ρ '' E₀ ≠ univ
x₁ x₂ : E
hρx : ρ x₁ = ρ x₂
hx : ¬x₁ = x₂
G₁ G₂ : Set E
G₁_open : IsOpen G₁
G₂_open : IsOpen G₂
hx₁ : x₁ ∈ G₁
hx₂ : x₂ ∈ G₂
disj : Disjoint G₁ G₂
G₁_comp : IsCompact G₁ᶜ
G₂_comp : IsCompact G₂ᶜ
G₁_open' : IsOpen (ρ '' G₁ᶜ)ᶜ
⊢ False

Proof state:
case intro.intro.intro.intro.intro.intro
A E : Type u
inst✝⁵ : TopologicalSpace A
inst✝⁴ : TopologicalSpace E
inst✝³ : ExtremallyDisconnected A
inst✝² : T2Space A
inst✝¹ : T2Space E
inst✝ : CompactSpace E
ρ : E → A
ρ_cont : Continuous ρ
ρ_surj : Surjective ρ
zorn_subset : ∀ (E₀ : Set E), E₀ ≠ univ → IsClosed E₀ → ρ '' E₀ ≠ univ
x₁ x₂ : E
hρx : ρ x₁ = ρ x₂
hx : ¬x₁ = x₂
G₁ G₂ : Set E
G₁_open : IsOpen G₁
G₂_open : IsOpen G₂
hx₁ : x₁ ∈ G₁
hx₂ : x₂ ∈ G₂
disj : Disjoint G₁ G₂
G₁_comp : IsCompact G₁ᶜ
G₂_comp : IsCompact G₂ᶜ
G₁_open' : IsOpen (ρ '' G₁ᶜ)ᶜ
G₂_open' : IsOpen (ρ '' G₂ᶜ)ᶜ
disj' : Disjoint (ρ '' G₁ᶜ)ᶜ (ρ '' G₂ᶜ)ᶜ
⊢ False

Tactic:
have disj'' : Disjoint (closure (ρ '' G₁ᶜ)ᶜ) (closure (ρ '' G₂ᶜ)ᶜ) :=
  disjoint_closure_of_disjoint_isOpen disj' G₁_open' G₂_open'

Result:
case intro.intro.intro.intro.intro.intro
A E : Type u
inst✝⁵ : TopologicalSpace A
inst✝⁴ : TopologicalSpace E
inst✝³ : ExtremallyDisconnected A
inst✝² : T2Space A
inst✝¹ : T2Space E
inst✝ : CompactSpace E
ρ : E → A
ρ_cont : Continuous ρ
ρ_surj : Surjective ρ
zorn_subset : ∀ (E₀ : Set E), E₀ ≠ univ → IsClosed E₀ → ρ '' E₀ ≠ univ
x₁ x₂ : E
hρx : ρ x₁ = ρ x₂
hx : ¬x₁ = x₂
G₁ G₂ : Set E
G₁_open : IsOpen G₁
G₂_open : IsOpen G₂
hx₁ : x₁ ∈ G₁
hx₂ : x₂ ∈ G₂
disj : Disjoint G₁ G₂
G₁_comp : IsCompact G₁ᶜ
G₂_comp : IsCompact G₂ᶜ
G₁_open' : IsOpen (ρ '' G₁ᶜ)ᶜ
G₂_open' : IsOpen (ρ '' G₂ᶜ)ᶜ
disj' : Disjoint (ρ '' G₁ᶜ)ᶜ (ρ '' G₂ᶜ)ᶜ
disj'' : Disjoint (closure (ρ '' G₁ᶜ)ᶜ) (closure (ρ '' G₂ᶜ)ᶜ)
⊢ False

Proof state:
case intro.intro.intro.intro.intro.intro
A E : Type u
inst✝⁵ : TopologicalSpace A
inst✝⁴ : TopologicalSpace E
inst✝³ : ExtremallyDisconnected A
inst✝² : T2Space A
inst✝¹ : T2Space E
inst✝ : CompactSpace E
ρ : E → A
ρ_cont : Continuous ρ
ρ_surj : Surjective ρ
zorn_subset : ∀ (E₀ : Set E), E₀ ≠ univ → IsClosed E₀ → ρ '' E₀ ≠ univ
x₁ x₂ : E
hρx : ρ x₁ = ρ x₂
hx : ¬x₁ = x₂
G₁ G₂ : Set E
G₁_open : IsOpen G₁
G₂_open : IsOpen G₂
hx₁ : x₁ ∈ G₁
hx₂ : x₂ ∈ G₂
disj : Disjoint G₁ G₂
G₁_comp : IsCompact G₁ᶜ
G₂_comp : IsCompact G₂ᶜ
G₁_open' : IsOpen (ρ '' G₁ᶜ)ᶜ
G₂_open' : IsOpen (ρ '' G₂ᶜ)ᶜ
disj' : Disjoint (ρ '' G₁ᶜ)ᶜ (ρ '' G₂ᶜ)ᶜ
disj'' : Disjoint (closure (ρ '' G₁ᶜ)ᶜ) (closure (ρ '' G₂ᶜ)ᶜ)
hx₁' : ρ x₁ ∈ closure (ρ '' G₁ᶜ)ᶜ
⊢ False

Tactic:
have hx₂' := image_subset_closure_compl_image_compl_of_isOpen ρ_cont ρ_surj zorn_subset G₂_open <|
  mem_image_of_mem ρ hx₂

Result:
case intro.intro.intro.intro.intro.intro
A E : Type u
inst✝⁵ : TopologicalSpace A
inst✝⁴ : TopologicalSpace E
inst✝³ : ExtremallyDisconnected A
inst✝² : T2Space A
inst✝¹ : T2Space E
inst✝ : CompactSpace E
ρ : E → A
ρ_cont : Continuous ρ
ρ_surj : Surjective ρ
zorn_subset : ∀ (E₀ : Set E), E₀ ≠ univ → IsClosed E₀ → ρ '' E₀ ≠ univ
x₁ x₂ : E
hρx : ρ x₁ = ρ x₂
hx : ¬x₁ = x₂
G₁ G₂ : Set E
G₁_open : IsOpen G₁
G₂_open : IsOpen G₂
hx₁ : x₁ ∈ G₁
hx₂ : x₂ ∈ G₂
disj : Disjoint G₁ G₂
G₁_comp : IsCompact G₁ᶜ
G₂_comp : IsCompact G₂ᶜ
G₁_open' : IsOpen (ρ '' G₁ᶜ)ᶜ
G₂_open' : IsOpen (ρ '' G₂ᶜ)ᶜ
disj' : Disjoint (ρ '' G₁ᶜ)ᶜ (ρ '' G₂ᶜ)ᶜ
disj'' : Disjoint (closure (ρ '' G₁ᶜ)ᶜ) (closure (ρ '' G₂ᶜ)ᶜ)
hx₁' : ρ x₁ ∈ closure (ρ '' G₁ᶜ)ᶜ
hx₂' : ρ x₂ ∈ closure (ρ '' G₂ᶜ)ᶜ
⊢ False

Proof state:
A E : Type u
inst✝⁵ : TopologicalSpace A
inst✝⁴ : TopologicalSpace E
inst✝³ : ExtremallyDisconnected A
inst✝² : T2Space A
inst✝¹ : T2Space E
inst✝ : CompactSpace E
ρ : E → A
ρ_cont : Continuous ρ
ρ_surj : Surjective ρ
zorn_subset : ∀ (E₀ : Set E), E₀ ≠ univ → IsClosed E₀ → ρ '' E₀ ≠ univ
x₁ x₂ : E
hρx : ρ x₁ = ρ x₂
hx : ¬x₁ = x₂
⊢ False

Tactic:
rcases t2_separation hx with ⟨G₁, G₂, G₁_open, G₂_open, hx₁, hx₂, disj⟩

Result:
case intro.intro.intro.intro.intro.intro
A E : Type u
inst✝⁵ : TopologicalSpace A
inst✝⁴ : TopologicalSpace E
inst✝³ : ExtremallyDisconnected A
inst✝² : T2Space A
inst✝¹ : T2Space E
inst✝ : CompactSpace E
ρ : E → A
ρ_cont : Continuous ρ
ρ_surj : Surjective ρ
zorn_subset : ∀ (E₀ : Set E), E₀ ≠ univ → IsClosed E₀ → ρ '' E₀ ≠ univ
x₁ x₂ : E
hρx : ρ x₁ = ρ x₂
hx : ¬x₁ = x₂
G₁ G₂ : Set E
G₁_open : IsOpen G₁
G₂_open : IsOpen G₂
hx₁ : x₁ ∈ G₁
hx₂ : x₂ ∈ G₂
disj : Disjoint G₁ G₂
⊢ False