-- Mathlib/MeasureTheory/Measure/Regular.lean
theorem weaklyRegular_of_finite [BorelSpace α] (μ : Measure α) [IsFiniteMeasure μ]
    (H : InnerRegularWRT μ IsClosed IsOpen) : WeaklyRegular μ

Proof state:
case compl
α : Type u_1
inst✝³ : MeasurableSpace α
inst✝² : TopologicalSpace α
inst✝¹ : BorelSpace α
μ : Measure α
inst✝ : IsFiniteMeasure μ
H✝ : μ.InnerRegularWRT IsClosed IsOpen
hfin : ∀ {s : Set α}, μ s ≠ ⊤
s : Set α
hs : MeasurableSet s
H : ∀ (ε : ℝ≥0∞), ε ≠ 0 → ∃ F ⊆ s, ∃ U ⊇ s, IsClosed F ∧ IsOpen U ∧ μ s ≤ μ F + ε ∧ μ U ≤ μ s + ε
ε : ℝ≥0∞
hε : ε ≠ 0
⊢ ∃ F ⊆ sᶜ, ∃ U ⊇ sᶜ, IsClosed F ∧ IsOpen U ∧ μ sᶜ ≤ μ F + ε ∧ μ U ≤ μ sᶜ + ε

Tactic:
rcases H ε hε with ⟨F, hFs, U, hsU, hFc, hUo, hF, hU⟩

Result:
case compl.intro.intro.intro.intro.intro.intro.intro
α : Type u_1
inst✝³ : MeasurableSpace α
inst✝² : TopologicalSpace α
inst✝¹ : BorelSpace α
μ : Measure α
inst✝ : IsFiniteMeasure μ
H✝ : μ.InnerRegularWRT IsClosed IsOpen
hfin : ∀ {s : Set α}, μ s ≠ ⊤
s : Set α
hs : MeasurableSet s
H : ∀ (ε : ℝ≥0∞), ε ≠ 0 → ∃ F ⊆ s, ∃ U ⊇ s, IsClosed F ∧ IsOpen U ∧ μ s ≤ μ F + ε ∧ μ U ≤ μ s + ε
ε : ℝ≥0∞
hε : ε ≠ 0
F : Set α
hFs : F ⊆ s
U : Set α
hsU : U ⊇ s
hFc : IsClosed F
hUo : IsOpen U
hF : μ s ≤ μ F + ε
hU : μ U ≤ μ s + ε
⊢ ∃ F ⊆ sᶜ, ∃ U ⊇ sᶜ, IsClosed F ∧ IsOpen U ∧ μ sᶜ ≤ μ F + ε ∧ μ U ≤ μ sᶜ + ε

Proof state:
α : Type u_1
inst✝³ : MeasurableSpace α
inst✝² : TopologicalSpace α
inst✝¹ : BorelSpace α
μ : Measure α
inst✝ : IsFiniteMeasure μ
H : μ.InnerRegularWRT IsClosed IsOpen
hfin : ∀ {s : Set α}, μ s ≠ ⊤
⊢ ∀ (s : Set α),
    MeasurableSet s → ∀ (ε : ℝ≥0∞), ε ≠ 0 → ∃ F ⊆ s, ∃ U ⊇ s, IsClosed F ∧ IsOpen U ∧ μ s ≤ μ F + ε ∧ μ U ≤ μ s + ε

Tactic:
apply MeasurableSet.induction_on_open

Result:
case isOpen
α : Type u_1
inst✝³ : MeasurableSpace α
inst✝² : TopologicalSpace α
inst✝¹ : BorelSpace α
μ : Measure α
inst✝ : IsFiniteMeasure μ
H : μ.InnerRegularWRT IsClosed IsOpen
hfin : ∀ {s : Set α}, μ s ≠ ⊤
⊢ ∀ (U : Set α),
    IsOpen U → ∀ (ε : ℝ≥0∞), ε ≠ 0 → ∃ F ⊆ U, ∃ U_1 ⊇ U, IsClosed F ∧ IsOpen U_1 ∧ μ U ≤ μ F + ε ∧ μ U_1 ≤ μ U + ε

case compl
α : Type u_1
inst✝³ : MeasurableSpace α
inst✝² : TopologicalSpace α
inst✝¹ : BorelSpace α
μ : Measure α
inst✝ : IsFiniteMeasure μ
H : μ.InnerRegularWRT IsClosed IsOpen
hfin : ∀ {s : Set α}, μ s ≠ ⊤
⊢ ∀ (t : Set α),
    MeasurableSet t →
      (∀ (ε : ℝ≥0∞), ε ≠ 0 → ∃ F ⊆ t, ∃ U ⊇ t, IsClosed F ∧ IsOpen U ∧ μ t ≤ μ F + ε ∧ μ U ≤ μ t + ε) →
        ∀ (ε : ℝ≥0∞), ε ≠ 0 → ∃ F ⊆ tᶜ, ∃ U ⊇ tᶜ, IsClosed F ∧ IsOpen U ∧ μ tᶜ ≤ μ F + ε ∧ μ U ≤ μ tᶜ + ε

case iUnion
α : Type u_1
inst✝³ : MeasurableSpace α
inst✝² : TopologicalSpace α
inst✝¹ : BorelSpace α
μ : Measure α
inst✝ : IsFiniteMeasure μ
H : μ.InnerRegularWRT IsClosed IsOpen
hfin : ∀ {s : Set α}, μ s ≠ ⊤
⊢ ∀ (f : ℕ → Set α),
    Pairwise (Function.onFun Disjoint f) →
      (∀ (i : ℕ), MeasurableSet (f i)) →
        (∀ (i : ℕ) (ε : ℝ≥0∞),
            ε ≠ 0 → ∃ F ⊆ f i, ∃ U ⊇ f i, IsClosed F ∧ IsOpen U ∧ μ (f i) ≤ μ F + ε ∧ μ U ≤ μ (f i) + ε) →
          ∀ (ε : ℝ≥0∞),
            ε ≠ 0 →
              ∃ F ⊆ ⋃ i, f i, ∃ U ⊇ ⋃ i, f i, IsClosed F ∧ IsOpen U ∧ μ (⋃ i, f i) ≤ μ F + ε ∧ μ U ≤ μ (⋃ i, f i) + ε

Proof state:
case isOpen
α : Type u_1
inst✝³ : MeasurableSpace α
inst✝² : TopologicalSpace α
inst✝¹ : BorelSpace α
μ : Measure α
inst✝ : IsFiniteMeasure μ
H : μ.InnerRegularWRT IsClosed IsOpen
hfin : ∀ {s : Set α}, μ s ≠ ⊤
⊢ ∀ (U : Set α),
    IsOpen U → ∀ (ε : ℝ≥0∞), ε ≠ 0 → ∃ F ⊆ U, ∃ U_1 ⊇ U, IsClosed F ∧ IsOpen U_1 ∧ μ U ≤ μ F + ε ∧ μ U_1 ≤ μ U + ε

Tactic:
intro U hU ε hε

Result:
case isOpen
α : Type u_1
inst✝³ : MeasurableSpace α
inst✝² : TopologicalSpace α
inst✝¹ : BorelSpace α
μ : Measure α
inst✝ : IsFiniteMeasure μ
H : μ.InnerRegularWRT IsClosed IsOpen
hfin : ∀ {s : Set α}, μ s ≠ ⊤
U : Set α
hU : IsOpen U
ε : ℝ≥0∞
hε : ε ≠ 0
⊢ ∃ F ⊆ U, ∃ U_1 ⊇ U, IsClosed F ∧ IsOpen U_1 ∧ μ U ≤ μ F + ε ∧ μ U_1 ≤ μ U + ε

Proof state:
α : Type u_1
inst✝³ : MeasurableSpace α
inst✝² : TopologicalSpace α
inst✝¹ : BorelSpace α
μ : Measure α
inst✝ : IsFiniteMeasure μ
H✝ : μ.InnerRegularWRT IsClosed IsOpen
hfin : ∀ {s : Set α}, μ s ≠ ⊤
s : ℕ → Set α
hsd : Pairwise (Function.onFun Disjoint s)
hsm : ∀ (i : ℕ), MeasurableSet (s i)
H : ∀ (i : ℕ) (ε : ℝ≥0∞), ε ≠ 0 → ∃ F ⊆ s i, ∃ U ⊇ s i, IsClosed F ∧ IsOpen U ∧ μ (s i) ≤ μ F + ε ∧ μ U ≤ μ (s i) + ε
ε : ℝ≥0∞
ε0 : ε ≠ 0
ε0' : ε / 2 ≠ 0
δ : ℕ → ℝ≥0∞
δ0 : ∀ (i : ℕ), 0 < δ i
hδε : ∑' (i : ℕ), δ i < ε / 2
F : ℕ → Set α
hFs : ∀ (n : ℕ), F n ⊆ s n
U : ℕ → Set α
hsU : ∀ (n : ℕ), U n ⊇ s n
hFc : ∀ (n : ℕ), IsClosed (F n)
hUo : ∀ (n : ℕ), IsOpen (U n)
hF : ∀ (n : ℕ), μ (s n) ≤ μ (F n) + δ n
hU : ∀ (n : ℕ), μ (U n) ≤ μ (s n) + δ n
⊢ Tendsto (fun t => ∑ k ∈ t, μ (s k) + ε / 2) atTop (𝓝 (∑' (i : ℕ), μ (s i) + ε / 2))

Tactic:
exact Tendsto.add ENNReal.summable.hasSum tendsto_const_nhds

Result:
no goals