Module: Mathlib.MeasureTheory.Function.AbsolutelyContinuous

Goal state before:
pp: f : ℝ → ℝ
a b c : ℝ
h : IntervalIntegrable f volume a b
hc : c ∈ uIcc a b
s : ℕ × (ℕ → ℝ × ℝ) → Set ℝ := fun E ↦ ⋃ i ∈ Finset.range E.1, uIoc (E.2 i).1 (E.2 i).2
this✝¹ :
  Tendsto (fun i ↦ ∫⁻ (x : ℝ) in s i, ‖f x‖ₑ ∂volume.restrict (uIoc a b)) (totalLengthFilter ⊓ 𝓟 (disjWithin a b)) (𝓝 0)
this✝ :
  Tendsto (ENNReal.toReal ∘ fun i ↦ ∫⁻ (x : ℝ) in s i, ‖f x‖ₑ ∂volume.restrict (uIoc a b))
    (totalLengthFilter ⊓ 𝓟 (disjWithin a b)) (𝓝 0)
this : ∀ᶠ (E : ℕ × (ℕ → ℝ × ℝ)) in totalLengthFilter ⊓ 𝓟 (disjWithin a b), E ∈ disjWithin a b
n : ℕ
I : ℕ → ℝ × ℝ
hnI : (n, I) ∈ disjWithin a b
hnI1 : ∀ i ∈ Finset.range (n, I).1, ((n, I).2 i).1 ∈ uIcc a b ∧ ((n, I).2 i).2 ∈ uIcc a b
hnI2 : (↑(Finset.range (n, I).1)).PairwiseDisjoint fun i ↦ uIoc ((n, I).2 i).1 ((n, I).2 i).2
⊢ ∑ x ∈ Finset.range n, dist (∫ (v : ℝ) in c..(I x).1, f v) (∫ (v : ℝ) in c..(I x).2, f v) ≤
    ∑ i ∈ Finset.range (n, I).1, ∫ (x : ℝ) in uIoc ((n, I).2 i).1 ((n, I).2 i).2, ‖f x‖ ∂volume.restrict (uIoc a b)

pp: f : ℝ → ℝ
a b c : ℝ
h : IntervalIntegrable f volume a b
hc : c ∈ uIcc a b
s : ℕ × (ℕ → ℝ × ℝ) → Set ℝ := fun E ↦ ⋃ i ∈ Finset.range E.1, uIoc (E.2 i).1 (E.2 i).2
this✝¹ :
  Tendsto (fun i ↦ ∫⁻ (x : ℝ) in s i, ‖f x‖ₑ ∂volume.restrict (uIoc a b)) (totalLengthFilter ⊓ 𝓟 (disjWithin a b)) (𝓝 0)
this✝ :
  Tendsto (ENNReal.toReal ∘ fun i ↦ ∫⁻ (x : ℝ) in s i, ‖f x‖ₑ ∂volume.restrict (uIoc a b))
    (totalLengthFilter ⊓ 𝓟 (disjWithin a b)) (𝓝 0)
this : ∀ᶠ (E : ℕ × (ℕ → ℝ × ℝ)) in totalLengthFilter ⊓ 𝓟 (disjWithin a b), E ∈ disjWithin a b
n : ℕ
I : ℕ → ℝ × ℝ
hnI : (n, I) ∈ disjWithin a b
hnI1 : ∀ i ∈ Finset.range (n, I).1, ((n, I).2 i).1 ∈ uIcc a b ∧ ((n, I).2 i).2 ∈ uIcc a b
hnI2 : (↑(Finset.range (n, I).1)).PairwiseDisjoint fun i ↦ uIoc ((n, I).2 i).1 ((n, I).2 i).2
⊢ ∀ i ∈ Finset.range (n, I).1,
    IntegrableOn (fun x ↦ ‖f x‖) (uIoc ((n, I).2 i).1 ((n, I).2 i).2) (volume.restrict (uIoc a b))

Tactic:
· refine Finset.sum_le_sum (fun i hi ↦ ?_)
    rw [Real.dist_eq,
        intervalIntegral.integral_interval_sub_left
          (by apply IntervalIntegrable.mono_set' h; grind [uIoc, uIcc])
          (by apply IntervalIntegrable.mono_set' h; grind [uIoc, uIcc]),
        Measure.restrict_restrict_of_subset
          (uIoc_subset_of_mem_disjWithin hnI (Finset.mem_range.mp hi)),
        intervalIntegral.integral_symm, abs_neg,
        intervalIntegral.abs_intervalIntegral_eq]
    exact abs_integral_le_integral_abs

Goal state after:
f : ℝ → ℝ
a b c : ℝ
h : IntervalIntegrable f volume a b
hc : c ∈ uIcc a b
s : ℕ × (ℕ → ℝ × ℝ) → Set ℝ := fun E ↦ ⋃ i ∈ Finset.range E.1, uIoc (E.2 i).1 (E.2 i).2
this✝¹ :
  Tendsto (fun i ↦ ∫⁻ (x : ℝ) in s i, ‖f x‖ₑ ∂volume.restrict (uIoc a b)) (totalLengthFilter ⊓ 𝓟 (disjWithin a b)) (𝓝 0)
this✝ :
  Tendsto (ENNReal.toReal ∘ fun i ↦ ∫⁻ (x : ℝ) in s i, ‖f x‖ₑ ∂volume.restrict (uIoc a b))
    (totalLengthFilter ⊓ 𝓟 (disjWithin a b)) (𝓝 0)
this : ∀ᶠ (E : ℕ × (ℕ → ℝ × ℝ)) in totalLengthFilter ⊓ 𝓟 (disjWithin a b), E ∈ disjWithin a b
n : ℕ
I : ℕ → ℝ × ℝ
hnI : (n, I) ∈ disjWithin a b
hnI1 : ∀ i ∈ Finset.range (n, I).1, ((n, I).2 i).1 ∈ uIcc a b ∧ ((n, I).2 i).2 ∈ uIcc a b
hnI2 : (↑(Finset.range (n, I).1)).PairwiseDisjoint fun i ↦ uIoc ((n, I).2 i).1 ((n, I).2 i).2
⊢ ∀ i ∈ Finset.range (n, I).1,
    IntegrableOn (fun x ↦ ‖f x‖) (uIoc ((n, I).2 i).1 ((n, I).2 i).2) (volume.restrict (uIoc a b))