-- Mathlib/MeasureTheory/Function/UniformIntegrable.lean
theorem uniformIntegrable_of [IsFiniteMeasure μ] (hp : 1 ≤ p) (hp' : p ≠ ∞)
    (hf : ∀ i, AEStronglyMeasurable (f i) μ)
    (h : ∀ ε : ℝ, 0 < ε → ∃ C : ℝ≥0,
      ∀ i, eLpNorm ({ x | C ≤ ‖f i x‖₊ }.indicator (f i)) p μ ≤ ENNReal.ofReal ε) :
    UniformIntegrable f p μ

Proof state:
case neg
α : Type u_1
β : Type u_2
ι : Type u_3
m : MeasurableSpace α
μ : Measure α
inst✝¹ : NormedAddCommGroup β
p : ℝ≥0∞
f : ι → α → β
inst✝ : IsFiniteMeasure μ
hp : 1 ≤ p
hp' : p ≠ ⊤
hf : ∀ (i : ι), AEStronglyMeasurable (f i) μ
h : ∀ (ε : ℝ), 0 < ε → ∃ C, ∀ (i : ι), eLpNorm ({x | C ≤ ‖f i x‖₊}.indicator (f i)) p μ ≤ ENNReal.ofReal ε
g : ι → α → β := fun i => Exists.choose ⋯
hgmeas : ∀ (i : ι), StronglyMeasurable (g i)
hgeq : ∀ (i : ι), g i =ᶠ[ae μ] f i
ε : ℝ
hε : 0 < ε
C : ℝ≥0
hC : ∀ (i : ι), eLpNorm ({x | C ≤ ‖f i x‖₊}.indicator (f i)) p μ ≤ ENNReal.ofReal ε
i : ι
x : α
hx : f i x = Exists.choose ⋯ x
hfx : x ∉ {x | C ≤ ‖f i x‖₊}
⊢ {x | C ≤ ‖g i x‖₊}.indicator (g i) x = {x | C ≤ ‖f i x‖₊}.indicator (f i) x

Tactic:
rw [Set.indicator_of_not_mem hfx, Set.indicator_of_not_mem]

Result:
case neg.h
α : Type u_1
β : Type u_2
ι : Type u_3
m : MeasurableSpace α
μ : Measure α
inst✝¹ : NormedAddCommGroup β
p : ℝ≥0∞
f : ι → α → β
inst✝ : IsFiniteMeasure μ
hp : 1 ≤ p
hp' : p ≠ ⊤
hf : ∀ (i : ι), AEStronglyMeasurable (f i) μ
h : ∀ (ε : ℝ), 0 < ε → ∃ C, ∀ (i : ι), eLpNorm ({x | C ≤ ‖f i x‖₊}.indicator (f i)) p μ ≤ ENNReal.ofReal ε
g : ι → α → β := fun i => Exists.choose ⋯
hgmeas : ∀ (i : ι), StronglyMeasurable (g i)
hgeq : ∀ (i : ι), g i =ᶠ[ae μ] f i
ε : ℝ
hε : 0 < ε
C : ℝ≥0
hC : ∀ (i : ι), eLpNorm ({x | C ≤ ‖f i x‖₊}.indicator (f i)) p μ ≤ ENNReal.ofReal ε
i : ι
x : α
hx : f i x = Exists.choose ⋯ x
hfx : x ∉ {x | C ≤ ‖f i x‖₊}
⊢ x ∉ {x | C ≤ ‖g i x‖₊}

Proof state:
case intro
α : Type u_1
β : Type u_2
ι : Type u_3
m : MeasurableSpace α
μ : Measure α
inst✝¹ : NormedAddCommGroup β
p : ℝ≥0∞
f : ι → α → β
inst✝ : IsFiniteMeasure μ
hp : 1 ≤ p
hp' : p ≠ ⊤
hf : ∀ (i : ι), AEStronglyMeasurable (f i) μ
h : ∀ (ε : ℝ), 0 < ε → ∃ C, ∀ (i : ι), eLpNorm ({x | C ≤ ‖f i x‖₊}.indicator (f i)) p μ ≤ ENNReal.ofReal ε
g : ι → α → β := fun i => Exists.choose ⋯
hgmeas : ∀ (i : ι), StronglyMeasurable (g i)
hgeq : ∀ (i : ι), g i =ᶠ[ae μ] f i
ε : ℝ
hε : 0 < ε
C : ℝ≥0
hC : ∀ (i : ι), eLpNorm ({x | C ≤ ‖f i x‖₊}.indicator (f i)) p μ ≤ ENNReal.ofReal ε
⊢ ∃ C, ∀ (i : ι), eLpNorm ({x | C ≤ ‖g i x‖₊}.indicator (g i)) p μ ≤ ENNReal.ofReal ε

Tactic:
refine ⟨C, fun i => le_trans (le_of_eq <| eLpNorm_congr_ae ?_) (hC i)⟩

Result:
case intro
α : Type u_1
β : Type u_2
ι : Type u_3
m : MeasurableSpace α
μ : Measure α
inst✝¹ : NormedAddCommGroup β
p : ℝ≥0∞
f : ι → α → β
inst✝ : IsFiniteMeasure μ
hp : 1 ≤ p
hp' : p ≠ ⊤
hf : ∀ (i : ι), AEStronglyMeasurable (f i) μ
h : ∀ (ε : ℝ), 0 < ε → ∃ C, ∀ (i : ι), eLpNorm ({x | C ≤ ‖f i x‖₊}.indicator (f i)) p μ ≤ ENNReal.ofReal ε
g : ι → α → β := fun i => Exists.choose ⋯
hgmeas : ∀ (i : ι), StronglyMeasurable (g i)
hgeq : ∀ (i : ι), g i =ᶠ[ae μ] f i
ε : ℝ
hε : 0 < ε
C : ℝ≥0
hC : ∀ (i : ι), eLpNorm ({x | C ≤ ‖f i x‖₊}.indicator (f i)) p μ ≤ ENNReal.ofReal ε
i : ι
⊢ {x | C ≤ ‖g i x‖₊}.indicator (g i) =ᶠ[ae μ] {x | C ≤ ‖f i x‖₊}.indicator (f i)

Proof state:
case h
α : Type u_1
β : Type u_2
ι : Type u_3
m : MeasurableSpace α
μ : Measure α
inst✝¹ : NormedAddCommGroup β
p : ℝ≥0∞
f : ι → α → β
inst✝ : IsFiniteMeasure μ
hp : 1 ≤ p
hp' : p ≠ ⊤
hf : ∀ (i : ι), AEStronglyMeasurable (f i) μ
h : ∀ (ε : ℝ), 0 < ε → ∃ C, ∀ (i : ι), eLpNorm ({x | C ≤ ‖f i x‖₊}.indicator (f i)) p μ ≤ ENNReal.ofReal ε
g : ι → α → β := fun i => Exists.choose ⋯
hgmeas : ∀ (i : ι), StronglyMeasurable (g i)
hgeq : ∀ (i : ι), g i =ᶠ[ae μ] f i
ε : ℝ
hε : 0 < ε
C : ℝ≥0
hC : ∀ (i : ι), eLpNorm ({x | C ≤ ‖f i x‖₊}.indicator (f i)) p μ ≤ ENNReal.ofReal ε
i : ι
x : α
hx : f i x = Exists.choose ⋯ x
⊢ {x | C ≤ ‖g i x‖₊}.indicator (g i) x = {x | C ≤ ‖f i x‖₊}.indicator (f i) x

Tactic:
by_cases hfx : x ∈ { x | C ≤ ‖f i x‖₊ }

Result:
case pos
α : Type u_1
β : Type u_2
ι : Type u_3
m : MeasurableSpace α
μ : Measure α
inst✝¹ : NormedAddCommGroup β
p : ℝ≥0∞
f : ι → α → β
inst✝ : IsFiniteMeasure μ
hp : 1 ≤ p
hp' : p ≠ ⊤
hf : ∀ (i : ι), AEStronglyMeasurable (f i) μ
h : ∀ (ε : ℝ), 0 < ε → ∃ C, ∀ (i : ι), eLpNorm ({x | C ≤ ‖f i x‖₊}.indicator (f i)) p μ ≤ ENNReal.ofReal ε
g : ι → α → β := fun i => Exists.choose ⋯
hgmeas : ∀ (i : ι), StronglyMeasurable (g i)
hgeq : ∀ (i : ι), g i =ᶠ[ae μ] f i
ε : ℝ
hε : 0 < ε
C : ℝ≥0
hC : ∀ (i : ι), eLpNorm ({x | C ≤ ‖f i x‖₊}.indicator (f i)) p μ ≤ ENNReal.ofReal ε
i : ι
x : α
hx : f i x = Exists.choose ⋯ x
hfx : x ∈ {x | C ≤ ‖f i x‖₊}
⊢ {x | C ≤ ‖g i x‖₊}.indicator (g i) x = {x | C ≤ ‖f i x‖₊}.indicator (f i) x

case neg
α : Type u_1
β : Type u_2
ι : Type u_3
m : MeasurableSpace α
μ : Measure α
inst✝¹ : NormedAddCommGroup β
p : ℝ≥0∞
f : ι → α → β
inst✝ : IsFiniteMeasure μ
hp : 1 ≤ p
hp' : p ≠ ⊤
hf : ∀ (i : ι), AEStronglyMeasurable (f i) μ
h : ∀ (ε : ℝ), 0 < ε → ∃ C, ∀ (i : ι), eLpNorm ({x | C ≤ ‖f i x‖₊}.indicator (f i)) p μ ≤ ENNReal.ofReal ε
g : ι → α → β := fun i => Exists.choose ⋯
hgmeas : ∀ (i : ι), StronglyMeasurable (g i)
hgeq : ∀ (i : ι), g i =ᶠ[ae μ] f i
ε : ℝ
hε : 0 < ε
C : ℝ≥0
hC : ∀ (i : ι), eLpNorm ({x | C ≤ ‖f i x‖₊}.indicator (f i)) p μ ≤ ENNReal.ofReal ε
i : ι
x : α
hx : f i x = Exists.choose ⋯ x
hfx : x ∉ {x | C ≤ ‖f i x‖₊}
⊢ {x | C ≤ ‖g i x‖₊}.indicator (g i) x = {x | C ≤ ‖f i x‖₊}.indicator (f i) x

Proof state:
α : Type u_1
β : Type u_2
ι : Type u_3
m : MeasurableSpace α
μ : Measure α
inst✝¹ : NormedAddCommGroup β
p : ℝ≥0∞
f : ι → α → β
inst✝ : IsFiniteMeasure μ
hp : 1 ≤ p
hp' : p ≠ ⊤
hf : ∀ (i : ι), AEStronglyMeasurable (f i) μ
h : ∀ (ε : ℝ), 0 < ε → ∃ C, ∀ (i : ι), eLpNorm ({x | C ≤ ‖f i x‖₊}.indicator (f i)) p μ ≤ ENNReal.ofReal ε
g : ι → α → β := fun i => Exists.choose ⋯
⊢ UniformIntegrable f p μ

Tactic:
have hgmeas : ∀ i, StronglyMeasurable (g i) := fun i => (Exists.choose_spec <| hf i).1

Result:
α : Type u_1
β : Type u_2
ι : Type u_3
m : MeasurableSpace α
μ : Measure α
inst✝¹ : NormedAddCommGroup β
p : ℝ≥0∞
f : ι → α → β
inst✝ : IsFiniteMeasure μ
hp : 1 ≤ p
hp' : p ≠ ⊤
hf : ∀ (i : ι), AEStronglyMeasurable (f i) μ
h : ∀ (ε : ℝ), 0 < ε → ∃ C, ∀ (i : ι), eLpNorm ({x | C ≤ ‖f i x‖₊}.indicator (f i)) p μ ≤ ENNReal.ofReal ε
g : ι → α → β := fun i => Exists.choose ⋯
hgmeas : ∀ (i : ι), StronglyMeasurable (g i)
⊢ UniformIntegrable f p μ

Proof state:
α : Type u_1
β : Type u_2
ι : Type u_3
m : MeasurableSpace α
μ : Measure α
inst✝¹ : NormedAddCommGroup β
p : ℝ≥0∞
f : ι → α → β
inst✝ : IsFiniteMeasure μ
hp : 1 ≤ p
hp' : p ≠ ⊤
hf : ∀ (i : ι), AEStronglyMeasurable (f i) μ
h : ∀ (ε : ℝ), 0 < ε → ∃ C, ∀ (i : ι), eLpNorm ({x | C ≤ ‖f i x‖₊}.indicator (f i)) p μ ≤ ENNReal.ofReal ε
g : ι → α → β := fun i => Exists.choose ⋯
hgmeas : ∀ (i : ι), StronglyMeasurable (g i)
⊢ UniformIntegrable f p μ

Tactic:
have hgeq : ∀ i, g i =ᵐ[μ] f i := fun i => (Exists.choose_spec <| hf i).2.symm

Result:
α : Type u_1
β : Type u_2
ι : Type u_3
m : MeasurableSpace α
μ : Measure α
inst✝¹ : NormedAddCommGroup β
p : ℝ≥0∞
f : ι → α → β
inst✝ : IsFiniteMeasure μ
hp : 1 ≤ p
hp' : p ≠ ⊤
hf : ∀ (i : ι), AEStronglyMeasurable (f i) μ
h : ∀ (ε : ℝ), 0 < ε → ∃ C, ∀ (i : ι), eLpNorm ({x | C ≤ ‖f i x‖₊}.indicator (f i)) p μ ≤ ENNReal.ofReal ε
g : ι → α → β := fun i => Exists.choose ⋯
hgmeas : ∀ (i : ι), StronglyMeasurable (g i)
hgeq : ∀ (i : ι), g i =ᶠ[ae μ] f i
⊢ UniformIntegrable f p μ