-- Mathlib/MeasureTheory/Function/UniformIntegrable.lean
theorem MemLp.eLpNorm_indicator_le_of_meas (hp_one : 1 ≤ p) (hp_top : p ≠ ∞) (hf : MemLp f p μ)
    (hmeas : StronglyMeasurable f) {ε : ℝ} (hε : 0 < ε) :
    ∃ (δ : ℝ) (_ : 0 < δ), ∀ s, MeasurableSet s → μ s ≤ ENNReal.ofReal δ →
      eLpNorm (s.indicator f) p μ ≤ ENNReal.ofReal ε

Proof state:
α : Type u_1
β : Type u_2
m : MeasurableSpace α
μ : Measure α
inst✝ : NormedAddCommGroup β
p : ℝ≥0∞
f : α → β
hp_one : 1 ≤ p
hp_top : p ≠ ⊤
hf : MemLp f p μ
hmeas : StronglyMeasurable f
ε : ℝ
hε : 0 < ε
⊢ ∃ δ,
    ∃ (_ : 0 < δ),
      ∀ (s : Set α), MeasurableSet s → μ s ≤ ENNReal.ofReal δ → eLpNorm (s.indicator f) p μ ≤ ENNReal.ofReal ε

Tactic:
obtain ⟨δ, hδpos, hδ⟩ := hf.eLpNorm_indicator_le' hp_one hp_top hmeas (half_pos hε)

Result:
case intro.intro
α : Type u_1
β : Type u_2
m : MeasurableSpace α
μ : Measure α
inst✝ : NormedAddCommGroup β
p : ℝ≥0∞
f : α → β
hp_one : 1 ≤ p
hp_top : p ≠ ⊤
hf : MemLp f p μ
hmeas : StronglyMeasurable f
ε : ℝ
hε : 0 < ε
δ : ℝ
hδpos : 0 < δ
hδ : ∀ (s : Set α), MeasurableSet s → μ s ≤ ENNReal.ofReal δ → eLpNorm (s.indicator f) p μ ≤ 2 * ENNReal.ofReal (ε / 2)
⊢ ∃ δ,
    ∃ (_ : 0 < δ),
      ∀ (s : Set α), MeasurableSet s → μ s ≤ ENNReal.ofReal δ → eLpNorm (s.indicator f) p μ ≤ ENNReal.ofReal ε

Proof state:
α : Type u_1
β : Type u_2
m : MeasurableSpace α
μ : Measure α
inst✝ : NormedAddCommGroup β
p : ℝ≥0∞
f : α → β
hp_one : 1 ≤ p
hp_top : p ≠ ⊤
hf : MemLp f p μ
hmeas : StronglyMeasurable f
ε : ℝ
hε : 0 < ε
δ : ℝ
hδpos : 0 < δ
hδ : ∀ (s : Set α), MeasurableSet s → μ s ≤ ENNReal.ofReal δ → eLpNorm (s.indicator f) p μ ≤ 2 * ENNReal.ofReal (ε / 2)
s : Set α
hs : MeasurableSet s
hμs : μ s ≤ ENNReal.ofReal δ
⊢ ENNReal.ofReal 2 = 2

Tactic:
norm_num

Result:
no goals

Proof state:
case intro.intro
α : Type u_1
β : Type u_2
m : MeasurableSpace α
μ : Measure α
inst✝ : NormedAddCommGroup β
p : ℝ≥0∞
f : α → β
hp_one : 1 ≤ p
hp_top : p ≠ ⊤
hf : MemLp f p μ
hmeas : StronglyMeasurable f
ε : ℝ
hε : 0 < ε
δ : ℝ
hδpos : 0 < δ
hδ : ∀ (s : Set α), MeasurableSet s → μ s ≤ ENNReal.ofReal δ → eLpNorm (s.indicator f) p μ ≤ 2 * ENNReal.ofReal (ε / 2)
s : Set α
hs : MeasurableSet s
hμs : μ s ≤ ENNReal.ofReal δ
⊢ 2 * ENNReal.ofReal (ε / 2) ≤ ENNReal.ofReal ε

Tactic:
rw [ENNReal.ofReal_div_of_pos zero_lt_two, (by norm_num : ENNReal.ofReal 2 = 2),
    ENNReal.mul_div_cancel] <;>
  norm_num

Result:
no goals

Proof state:
case intro.intro
α : Type u_1
β : Type u_2
m : MeasurableSpace α
μ : Measure α
inst✝ : NormedAddCommGroup β
p : ℝ≥0∞
f : α → β
hp_one : 1 ≤ p
hp_top : p ≠ ⊤
hf : MemLp f p μ
hmeas : StronglyMeasurable f
ε : ℝ
hε : 0 < ε
δ : ℝ
hδpos : 0 < δ
hδ : ∀ (s : Set α), MeasurableSet s → μ s ≤ ENNReal.ofReal δ → eLpNorm (s.indicator f) p μ ≤ 2 * ENNReal.ofReal (ε / 2)
⊢ ∃ δ,
    ∃ (_ : 0 < δ),
      ∀ (s : Set α), MeasurableSet s → μ s ≤ ENNReal.ofReal δ → eLpNorm (s.indicator f) p μ ≤ ENNReal.ofReal ε

Tactic:
refine ⟨δ, hδpos, fun s hs hμs => le_trans (hδ s hs hμs) ?_⟩

Result:
case intro.intro
α : Type u_1
β : Type u_2
m : MeasurableSpace α
μ : Measure α
inst✝ : NormedAddCommGroup β
p : ℝ≥0∞
f : α → β
hp_one : 1 ≤ p
hp_top : p ≠ ⊤
hf : MemLp f p μ
hmeas : StronglyMeasurable f
ε : ℝ
hε : 0 < ε
δ : ℝ
hδpos : 0 < δ
hδ : ∀ (s : Set α), MeasurableSet s → μ s ≤ ENNReal.ofReal δ → eLpNorm (s.indicator f) p μ ≤ 2 * ENNReal.ofReal (ε / 2)
s : Set α
hs : MeasurableSet s
hμs : μ s ≤ ENNReal.ofReal δ
⊢ 2 * ENNReal.ofReal (ε / 2) ≤ ENNReal.ofReal ε