-- Mathlib/Topology/UnitInterval.lean
/-- Any open cover `c` of a closed interval `[a, b]` in ℝ
can be refined to a finite partition into subintervals. -/
lemma exists_monotone_Icc_subset_open_cover_Icc {ι} {a b : ℝ} (h : a ≤ b) {c : ι → Set (Icc a b)}
    (hc₁ : ∀ i, IsOpen (c i)) (hc₂ : univ ⊆ ⋃ i, c i) : ∃ t : ℕ → Icc a b, t 0 = a ∧
      Monotone t ∧ (∃ m, ∀ n ≥ m, t n = b) ∧ ∀ n, ∃ i, Icc (t n) (t (n + 1)) ⊆ c i

Proof state:
case intro.intro
ι : Sort u_1
a b : ℝ
h : a ≤ b
c : ι → Set ↑(Icc a b)
hc₁ : ∀ (i : ι), IsOpen (c i)
hc₂ : univ ⊆ ⋃ i, c i
δ : ℝ
δ_pos : δ > 0
ball_subset : ∀ x ∈ univ, ∃ i, Metric.ball x δ ⊆ c i
⊢ ∃ t, ↑(t 0) = a ∧ Monotone t ∧ (∃ m, ∀ n ≥ m, ↑(t n) = b) ∧ ∀ (n : ℕ), ∃ i, Icc (t n) (t (n + 1)) ⊆ c i

Tactic:
have hδ := half_pos δ_pos

Result:
case intro.intro
ι : Sort u_1
a b : ℝ
h : a ≤ b
c : ι → Set ↑(Icc a b)
hc₁ : ∀ (i : ι), IsOpen (c i)
hc₂ : univ ⊆ ⋃ i, c i
δ : ℝ
δ_pos : δ > 0
ball_subset : ∀ x ∈ univ, ∃ i, Metric.ball x δ ⊆ c i
hδ : 0 < δ / 2
⊢ ∃ t, ↑(t 0) = a ∧ Monotone t ∧ (∃ m, ∀ n ≥ m, ↑(t n) = b) ∧ ∀ (n : ℕ), ∃ i, Icc (t n) (t (n + 1)) ⊆ c i

Proof state:
ι : Sort u_1
a b : ℝ
h : a ≤ b
c : ι → Set ↑(Icc a b)
hc₁ : ∀ (i : ι), IsOpen (c i)
hc₂ : univ ⊆ ⋃ i, c i
⊢ ∃ t, ↑(t 0) = a ∧ Monotone t ∧ (∃ m, ∀ n ≥ m, ↑(t n) = b) ∧ ∀ (n : ℕ), ∃ i, Icc (t n) (t (n + 1)) ⊆ c i

Tactic:
obtain ⟨δ, δ_pos, ball_subset⟩ := lebesgue_number_lemma_of_metric isCompact_univ hc₁ hc₂

Result:
case intro.intro
ι : Sort u_1
a b : ℝ
h : a ≤ b
c : ι → Set ↑(Icc a b)
hc₁ : ∀ (i : ι), IsOpen (c i)
hc₂ : univ ⊆ ⋃ i, c i
δ : ℝ
δ_pos : δ > 0
ball_subset : ∀ x ∈ univ, ∃ i, Metric.ball x δ ⊆ c i
⊢ ∃ t, ↑(t 0) = a ∧ Monotone t ∧ (∃ m, ∀ n ≥ m, ↑(t n) = b) ∧ ∀ (n : ℕ), ∃ i, Icc (t n) (t (n + 1)) ⊆ c i

Proof state:
case intro.intro
ι : Sort u_1
a b : ℝ
h : a ≤ b
c : ι → Set ↑(Icc a b)
hc₁ : ∀ (i : ι), IsOpen (c i)
hc₂ : univ ⊆ ⋃ i, c i
δ : ℝ
δ_pos : δ > 0
ball_subset : ∀ x ∈ univ, ∃ i, Metric.ball x δ ⊆ c i
hδ : 0 < δ / 2
⊢ ∃ t, ↑(t 0) = a ∧ Monotone t ∧ (∃ m, ∀ n ≥ m, ↑(t n) = b) ∧ ∀ (n : ℕ), ∃ i, Icc (t n) (t (n + 1)) ⊆ c i

Tactic:
refine ⟨addNSMul h (δ/2), addNSMul_zero h,
  monotone_addNSMul h hδ.le, addNSMul_eq_right h hδ, fun n ↦ ?_⟩

Result:
case intro.intro
ι : Sort u_1
a b : ℝ
h : a ≤ b
c : ι → Set ↑(Icc a b)
hc₁ : ∀ (i : ι), IsOpen (c i)
hc₂ : univ ⊆ ⋃ i, c i
δ : ℝ
δ_pos : δ > 0
ball_subset : ∀ x ∈ univ, ∃ i, Metric.ball x δ ⊆ c i
hδ : 0 < δ / 2
n : ℕ
⊢ ∃ i, Icc (addNSMul h (δ / 2) n) (addNSMul h (δ / 2) (n + 1)) ⊆ c i

Proof state:
case intro.intro
ι : Sort u_1
a b : ℝ
h : a ≤ b
c : ι → Set ↑(Icc a b)
hc₁ : ∀ (i : ι), IsOpen (c i)
hc₂ : univ ⊆ ⋃ i, c i
δ : ℝ
δ_pos : δ > 0
ball_subset : ∀ x ∈ univ, ∃ i, Metric.ball x δ ⊆ c i
hδ : 0 < δ / 2
n : ℕ
⊢ ∃ i, Icc (addNSMul h (δ / 2) n) (addNSMul h (δ / 2) (n + 1)) ⊆ c i

Tactic:
obtain ⟨i, hsub⟩ := ball_subset (addNSMul h (δ/2) n) trivial

Result:
case intro.intro.intro
ι : Sort u_1
a b : ℝ
h : a ≤ b
c : ι → Set ↑(Icc a b)
hc₁ : ∀ (i : ι), IsOpen (c i)
hc₂ : univ ⊆ ⋃ i, c i
δ : ℝ
δ_pos : δ > 0
ball_subset : ∀ x ∈ univ, ∃ i, Metric.ball x δ ⊆ c i
hδ : 0 < δ / 2
n : ℕ
i : ι
hsub : Metric.ball (addNSMul h (δ / 2) n) δ ⊆ c i
⊢ ∃ i, Icc (addNSMul h (δ / 2) n) (addNSMul h (δ / 2) (n + 1)) ⊆ c i

Proof state:
case intro.intro.intro
ι : Sort u_1
a b : ℝ
h : a ≤ b
c : ι → Set ↑(Icc a b)
hc₁ : ∀ (i : ι), IsOpen (c i)
hc₂ : univ ⊆ ⋃ i, c i
δ : ℝ
δ_pos : δ > 0
ball_subset : ∀ x ∈ univ, ∃ i, Metric.ball x δ ⊆ c i
hδ : 0 < δ / 2
n : ℕ
i : ι
hsub : Metric.ball (addNSMul h (δ / 2) n) δ ⊆ c i
⊢ ∃ i, Icc (addNSMul h (δ / 2) n) (addNSMul h (δ / 2) (n + 1)) ⊆ c i

Tactic:
exact ⟨i, fun t ht ↦ hsub ((abs_sub_addNSMul_le h hδ.le n ht).trans_lt <| half_lt_self δ_pos)⟩

Result:
no goals