-- Mathlib/Topology/NoetherianSpace.lean
theorem NoetherianSpace.exists_finite_set_closeds_irreducible [NoetherianSpace α] (s : Closeds α) :
    ∃ S : Set (Closeds α), S.Finite ∧ (∀ t ∈ S, IsIrreducible (t : Set α)) ∧ s = sSup S

Proof state:
case neg.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : NoetherianSpace α
s : Closeds α
H : ∀ y < s, ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ y = sSup S
h₀ : s ≠ ⊥
z₁ : Closeds α
hz₁' : ¬↑s ⊆ ↑z₁
z₂ : Closeds α
hz₂' : ¬↑s ⊆ ↑z₂
h : ↑s ⊆ ↑z₁ ∪ ↑z₂
S₁ : Set (Closeds α)
hSf₁ : S₁.Finite
hS₁ : ∀ t ∈ S₁, IsIrreducible ↑t
h₁ : s ⊓ z₁ = sSup S₁
S₂ : Set (Closeds α)
hSf₂ : S₂.Finite
hS₂ : ∀ t ∈ S₂, IsIrreducible ↑t
h₂ : s ⊓ z₂ = sSup S₂
⊢ ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ s = sSup S

Tactic:
refine ⟨S₁ ∪ S₂, hSf₁.union hSf₂, Set.union_subset_iff.2 ⟨hS₁, hS₂⟩, ?_⟩

Result:
case neg.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : NoetherianSpace α
s : Closeds α
H : ∀ y < s, ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ y = sSup S
h₀ : s ≠ ⊥
z₁ : Closeds α
hz₁' : ¬↑s ⊆ ↑z₁
z₂ : Closeds α
hz₂' : ¬↑s ⊆ ↑z₂
h : ↑s ⊆ ↑z₁ ∪ ↑z₂
S₁ : Set (Closeds α)
hSf₁ : S₁.Finite
hS₁ : ∀ t ∈ S₁, IsIrreducible ↑t
h₁ : s ⊓ z₁ = sSup S₁
S₂ : Set (Closeds α)
hSf₂ : S₂.Finite
hS₂ : ∀ t ∈ S₂, IsIrreducible ↑t
h₂ : s ⊓ z₂ = sSup S₂
⊢ s = sSup (S₁ ∪ S₂)

Proof state:
case neg
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : NoetherianSpace α
s : Closeds α
H : ∀ y < s, ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ y = sSup S
h₀ : s ≠ ⊥
h₁ : ¬IsPreirreducible ↑s
⊢ ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ s = sSup S

Tactic:
simp only [isPreirreducible_iff_isClosed_union_isClosed, not_forall, not_or] at h₁

Result:
case neg
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : NoetherianSpace α
s : Closeds α
H : ∀ y < s, ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ y = sSup S
h₀ : s ≠ ⊥
h₁ : ∃ x x_1, ∃ (_ : IsClosed x) (_ : IsClosed x_1) (_ : ↑s ⊆ x ∪ x_1), ¬↑s ⊆ x ∧ ¬↑s ⊆ x_1
⊢ ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ s = sSup S

Proof state:
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : NoetherianSpace α
s : Closeds α
⊢ ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ s = sSup S

Tactic:
apply wellFounded_lt.induction s

Result:
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : NoetherianSpace α
s : Closeds α
⊢ ∀ (x : Closeds α),
    (∀ y < x, ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ y = sSup S) →
      ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ x = sSup S

Proof state:
case neg.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : NoetherianSpace α
s : Closeds α
H : ∀ y < s, ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ y = sSup S
h₀ : s ≠ ⊥
z₁ : Closeds α
hz₁' : ¬↑s ⊆ ↑z₁
z₂ : Closeds α
hz₂' : ¬↑s ⊆ ↑z₂
h : ↑s ⊆ ↑z₁ ∪ ↑z₂
S₁ : Set (Closeds α)
hSf₁ : S₁.Finite
hS₁ : ∀ t ∈ S₁, IsIrreducible ↑t
h₁ : s ⊓ z₁ = sSup S₁
⊢ ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ s = sSup S

Tactic:
rcases H (s ⊓ z₂) (inf_lt_left.2 hz₂') with ⟨S₂, hSf₂, hS₂, h₂⟩

Result:
case neg.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : NoetherianSpace α
s : Closeds α
H : ∀ y < s, ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ y = sSup S
h₀ : s ≠ ⊥
z₁ : Closeds α
hz₁' : ¬↑s ⊆ ↑z₁
z₂ : Closeds α
hz₂' : ¬↑s ⊆ ↑z₂
h : ↑s ⊆ ↑z₁ ∪ ↑z₂
S₁ : Set (Closeds α)
hSf₁ : S₁.Finite
hS₁ : ∀ t ∈ S₁, IsIrreducible ↑t
h₁ : s ⊓ z₁ = sSup S₁
S₂ : Set (Closeds α)
hSf₂ : S₂.Finite
hS₂ : ∀ t ∈ S₂, IsIrreducible ↑t
h₂ : s ⊓ z₂ = sSup S₂
⊢ ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ s = sSup S

Proof state:
case neg
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : NoetherianSpace α
s : Closeds α
H : ∀ y < s, ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ y = sSup S
h₀ : s ≠ ⊥
h₁ : ∃ x x_1, ∃ (_ : IsClosed x) (_ : IsClosed x_1) (_ : ↑s ⊆ x ∪ x_1), ¬↑s ⊆ x ∧ ¬↑s ⊆ x_1
⊢ ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ s = sSup S

Tactic:
obtain ⟨z₁, z₂, hz₁, hz₂, h, hz₁', hz₂'⟩ := h₁

Result:
case neg.intro.intro.intro.intro.intro.intro
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : NoetherianSpace α
s : Closeds α
H : ∀ y < s, ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ y = sSup S
h₀ : s ≠ ⊥
z₁ z₂ : Set α
hz₁ : IsClosed z₁
hz₂ : IsClosed z₂
h : ↑s ⊆ z₁ ∪ z₂
hz₁' : ¬↑s ⊆ z₁
hz₂' : ¬↑s ⊆ z₂
⊢ ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ s = sSup S

Proof state:
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : NoetherianSpace α
⊢ ∀ (x : Closeds α),
    (∀ y < x, ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ y = sSup S) →
      ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ x = sSup S

Tactic:
intro s H

Result:
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : NoetherianSpace α
s : Closeds α
H : ∀ y < s, ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ y = sSup S
⊢ ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ s = sSup S

Proof state:
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : NoetherianSpace α
s : Closeds α
⊢ ∀ (x : Closeds α),
    (∀ y < x, ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ y = sSup S) →
      ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ x = sSup S

Tactic:
clear s

Result:
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : NoetherianSpace α
⊢ ∀ (x : Closeds α),
    (∀ y < x, ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ y = sSup S) →
      ∃ S, S.Finite ∧ (∀ t ∈ S, IsIrreducible ↑t) ∧ x = sSup S