Theorem: PolishSpace.IsClopenable.iUnion
File: Mathlib/Topology/MetricSpace/Polish.lean

tactic: choose m mt m_polish _ m_open using hs
state_before: α : Type u_1
β : Type ?u.68249
t : TopologicalSpace α
inst✝ : PolishSpace α
s : ℕ → Set α
hs : ∀ (n : ℕ), IsClopenable (s n)
⊢ IsClopenable (⋃ (n : ℕ), s n)

tactic: obtain ⟨t', t'm, -, t'_polish⟩ :
  ∃ t' : TopologicalSpace α, (∀ n : ℕ, t' ≤ m n) ∧ t' ≤ t ∧ @PolishSpace α t' :=
  exists_polishSpace_forall_le m mt m_polish
state_before: α : Type u_1
β : Type ?u.68249
t : TopologicalSpace α
inst✝ : PolishSpace α
s : ℕ → Set α
m : ℕ → TopologicalSpace α
mt : ∀ (n : ℕ), m n ≤ t
m_polish : ∀ (n : ℕ), PolishSpace α
h✝ : ∀ (n : ℕ), IsClosed (s n)
m_open : ∀ (n : ℕ), IsOpen (s n)
⊢ IsClopenable (⋃ (n : ℕ), s n)

tactic: have A : IsOpen[t'] (⋃ n, s n) := by
  apply isOpen_iUnion
  intro n
  apply t'm n
  exact m_open n
state_before: case intro.intro.intro
α : Type u_1
β : Type ?u.68249
t : TopologicalSpace α
inst✝ : PolishSpace α
s : ℕ → Set α
m : ℕ → TopologicalSpace α
mt : ∀ (n : ℕ), m n ≤ t
m_polish : ∀ (n : ℕ), PolishSpace α
h✝ : ∀ (n : ℕ), IsClosed (s n)
m_open : ∀ (n : ℕ), IsOpen (s n)
t' : TopologicalSpace α
t'm : ∀ (n : ℕ), t' ≤ m n
t'_polish : PolishSpace α
⊢ IsClopenable (⋃ (n : ℕ), s n)

tactic: obtain ⟨t'', t''_le, t''_polish, h1, h2⟩ : ∃ t'' : TopologicalSpace α,
    t'' ≤ t' ∧ @PolishSpace α t'' ∧ IsClosed[t''] (⋃ n, s n) ∧ IsOpen[t''] (⋃ n, s n) :=
  @IsOpen.isClopenable α t' t'_polish _ A
state_before: case intro.intro.intro
α : Type u_1
β : Type ?u.68249
t : TopologicalSpace α
inst✝ : PolishSpace α
s : ℕ → Set α
m : ℕ → TopologicalSpace α
mt : ∀ (n : ℕ), m n ≤ t
m_polish : ∀ (n : ℕ), PolishSpace α
h✝ : ∀ (n : ℕ), IsClosed (s n)
m_open : ∀ (n : ℕ), IsOpen (s n)
t' : TopologicalSpace α
t'm : ∀ (n : ℕ), t' ≤ m n
t'_polish : PolishSpace α
A : IsOpen (⋃ (n : ℕ), s n)
⊢ IsClopenable (⋃ (n : ℕ), s n)

tactic: exact ⟨t'', t''_le.trans ((t'm 0).trans (mt 0)), t''_polish, h1, h2⟩
state_before: case intro.intro.intro.intro.intro.intro.intro
α : Type u_1
β : Type ?u.68249
t : TopologicalSpace α
inst✝ : PolishSpace α
s : ℕ → Set α
m : ℕ → TopologicalSpace α
mt : ∀ (n : ℕ), m n ≤ t
m_polish : ∀ (n : ℕ), PolishSpace α
h✝ : ∀ (n : ℕ), IsClosed (s n)
m_open : ∀ (n : ℕ), IsOpen (s n)
t' : TopologicalSpace α
t'm : ∀ (n : ℕ), t' ≤ m n
t'_polish : PolishSpace α
A : IsOpen (⋃ (n : ℕ), s n)
t'' : TopologicalSpace α
t''_le : t'' ≤ t'
t''_polish : PolishSpace α
h1 : IsClosed (⋃ (n : ℕ), s n)
h2 : IsOpen (⋃ (n : ℕ), s n)
⊢ IsClopenable (⋃ (n : ℕ), s n)

tactic: apply isOpen_iUnion
state_before: α : Type u_1
β : Type ?u.68249
t : TopologicalSpace α
inst✝ : PolishSpace α
s : ℕ → Set α
m : ℕ → TopologicalSpace α
mt : ∀ (n : ℕ), m n ≤ t
m_polish : ∀ (n : ℕ), PolishSpace α
h✝ : ∀ (n : ℕ), IsClosed (s n)
m_open : ∀ (n : ℕ), IsOpen (s n)
t' : TopologicalSpace α
t'm : ∀ (n : ℕ), t' ≤ m n
t'_polish : PolishSpace α
⊢ IsOpen (⋃ (n : ℕ), s n)

tactic: intro n
state_before: case h
α : Type u_1
β : Type ?u.68249
t : TopologicalSpace α
inst✝ : PolishSpace α
s : ℕ → Set α
m : ℕ → TopologicalSpace α
mt : ∀ (n : ℕ), m n ≤ t
m_polish : ∀ (n : ℕ), PolishSpace α
h✝ : ∀ (n : ℕ), IsClosed (s n)
m_open : ∀ (n : ℕ), IsOpen (s n)
t' : TopologicalSpace α
t'm : ∀ (n : ℕ), t' ≤ m n
t'_polish : PolishSpace α
⊢ ∀ (i : ℕ), IsOpen (s i)

tactic: apply t'm n
state_before: case h
α : Type u_1
β : Type ?u.68249
t : TopologicalSpace α
inst✝ : PolishSpace α
s : ℕ → Set α
m : ℕ → TopologicalSpace α
mt : ∀ (n : ℕ), m n ≤ t
m_polish : ∀ (n : ℕ), PolishSpace α
h✝ : ∀ (n : ℕ), IsClosed (s n)
m_open : ∀ (n : ℕ), IsOpen (s n)
t' : TopologicalSpace α
t'm : ∀ (n : ℕ), t' ≤ m n
t'_polish : PolishSpace α
n : ℕ
⊢ IsOpen (s n)

tactic: exact m_open n
state_before: case h.a
α : Type u_1
β : Type ?u.68249
t : TopologicalSpace α
inst✝ : PolishSpace α
s : ℕ → Set α
m : ℕ → TopologicalSpace α
mt : ∀ (n : ℕ), m n ≤ t
m_polish : ∀ (n : ℕ), PolishSpace α
h✝ : ∀ (n : ℕ), IsClosed (s n)
m_open : ∀ (n : ℕ), IsOpen (s n)
t' : TopologicalSpace α
t'm : ∀ (n : ℕ), t' ≤ m n
t'_polish : PolishSpace α
n : ℕ
⊢ IsOpen (s n)