-- Mathlib/Probability/IdentDistrib.lean
theorem MemLp.uniformIntegrable_of_identDistrib_aux {ι : Type*} {f : ι → α → E} {j : ι} {p : ℝ≥0∞}
    (hp : 1 ≤ p) (hp' : p ≠ ∞) (hℒp : MemLp (f j) p μ) (hfmeas : ∀ i, StronglyMeasurable (f i))
    (hf : ∀ i, IdentDistrib (f i) (f j) μ μ) : UniformIntegrable f p μ

Proof state:
case pos.intro.intro
α : Type u_1
inst✝⁴ : MeasurableSpace α
E : Type u_5
inst✝³ : MeasurableSpace E
inst✝² : NormedAddCommGroup E
inst✝¹ : BorelSpace E
μ : Measure α
inst✝ : IsFiniteMeasure μ
ι : Type u_6
f : ι → α → E
j : ι
p : ℝ≥0∞
hp : 1 ≤ p
hp' : p ≠ ⊤
hℒp : MemLp (f j) p μ
hfmeas : ∀ (i : ι), StronglyMeasurable (f i)
hf : ∀ (i : ι), IdentDistrib (f i) (f j) μ μ
ε : ℝ
hε : 0 < ε
hι : Nonempty ι
C : ℝ
hC₁ : 0 < C
hC₂ : eLpNorm ({x | C ≤ ↑‖f j x‖₊}.indicator (f j)) p μ ≤ ENNReal.ofReal ε
i : ι
this : {x | ⟨C, ⋯⟩ ≤ ‖f i x‖₊} = {x | C ≤ ‖f i x‖}
⊢ eLpNorm ({x | ⟨C, ⋯⟩ ≤ ‖f i x‖₊}.indicator (f i)) p μ = eLpNorm ({x | C ≤ ↑‖f j x‖₊}.indicator (f j)) p μ

Tactic:
rw [this, ← eLpNorm_norm, ← eLpNorm_norm (Set.indicator _ _)]

Result:
case pos.intro.intro
α : Type u_1
inst✝⁴ : MeasurableSpace α
E : Type u_5
inst✝³ : MeasurableSpace E
inst✝² : NormedAddCommGroup E
inst✝¹ : BorelSpace E
μ : Measure α
inst✝ : IsFiniteMeasure μ
ι : Type u_6
f : ι → α → E
j : ι
p : ℝ≥0∞
hp : 1 ≤ p
hp' : p ≠ ⊤
hℒp : MemLp (f j) p μ
hfmeas : ∀ (i : ι), StronglyMeasurable (f i)
hf : ∀ (i : ι), IdentDistrib (f i) (f j) μ μ
ε : ℝ
hε : 0 < ε
hι : Nonempty ι
C : ℝ
hC₁ : 0 < C
hC₂ : eLpNorm ({x | C ≤ ↑‖f j x‖₊}.indicator (f j)) p μ ≤ ENNReal.ofReal ε
i : ι
this : {x | ⟨C, ⋯⟩ ≤ ‖f i x‖₊} = {x | C ≤ ‖f i x‖}
⊢ eLpNorm (fun x => ‖{x | C ≤ ‖f i x‖}.indicator (f i) x‖) p μ =
    eLpNorm (fun x => ‖{x | C ≤ ↑‖f j x‖₊}.indicator (f j) x‖) p μ

Proof state:
case pos.intro.intro
α : Type u_1
inst✝⁴ : MeasurableSpace α
E : Type u_5
inst✝³ : MeasurableSpace E
inst✝² : NormedAddCommGroup E
inst✝¹ : BorelSpace E
μ : Measure α
inst✝ : IsFiniteMeasure μ
ι : Type u_6
f : ι → α → E
j : ι
p : ℝ≥0∞
hp : 1 ≤ p
hp' : p ≠ ⊤
hℒp : MemLp (f j) p μ
hfmeas : ∀ (i : ι), StronglyMeasurable (f i)
hf : ∀ (i : ι), IdentDistrib (f i) (f j) μ μ
ε : ℝ
hε : 0 < ε
hι : Nonempty ι
C : ℝ
hC₁ : 0 < C
hC₂ : eLpNorm ({x | C ≤ ↑‖f j x‖₊}.indicator (f j)) p μ ≤ ENNReal.ofReal ε
⊢ ∃ C, ∀ (i : ι), eLpNorm ({x | C ≤ ‖f i x‖₊}.indicator (f i)) p μ ≤ ENNReal.ofReal ε

Tactic:
refine ⟨⟨C, hC₁.le⟩, fun i => le_trans (le_of_eq ?_) hC₂⟩

Result:
case pos.intro.intro
α : Type u_1
inst✝⁴ : MeasurableSpace α
E : Type u_5
inst✝³ : MeasurableSpace E
inst✝² : NormedAddCommGroup E
inst✝¹ : BorelSpace E
μ : Measure α
inst✝ : IsFiniteMeasure μ
ι : Type u_6
f : ι → α → E
j : ι
p : ℝ≥0∞
hp : 1 ≤ p
hp' : p ≠ ⊤
hℒp : MemLp (f j) p μ
hfmeas : ∀ (i : ι), StronglyMeasurable (f i)
hf : ∀ (i : ι), IdentDistrib (f i) (f j) μ μ
ε : ℝ
hε : 0 < ε
hι : Nonempty ι
C : ℝ
hC₁ : 0 < C
hC₂ : eLpNorm ({x | C ≤ ↑‖f j x‖₊}.indicator (f j)) p μ ≤ ENNReal.ofReal ε
i : ι
⊢ eLpNorm ({x | ⟨C, ⋯⟩ ≤ ‖f i x‖₊}.indicator (f i)) p μ = eLpNorm ({x | C ≤ ↑‖f j x‖₊}.indicator (f j)) p μ

Proof state:
case pos.intro.intro
α : Type u_1
inst✝⁴ : MeasurableSpace α
E : Type u_5
inst✝³ : MeasurableSpace E
inst✝² : NormedAddCommGroup E
inst✝¹ : BorelSpace E
μ : Measure α
inst✝ : IsFiniteMeasure μ
ι : Type u_6
f : ι → α → E
j : ι
p : ℝ≥0∞
hp : 1 ≤ p
hp' : p ≠ ⊤
hℒp : MemLp (f j) p μ
hfmeas : ∀ (i : ι), StronglyMeasurable (f i)
hf : ∀ (i : ι), IdentDistrib (f i) (f j) μ μ
ε : ℝ
hε : 0 < ε
hι : Nonempty ι
C : ℝ
hC₁ : 0 < C
hC₂ : eLpNorm ({x | C ≤ ↑‖f j x‖₊}.indicator (f j)) p μ ≤ ENNReal.ofReal ε
i : ι
this✝ : {x | ⟨C, ⋯⟩ ≤ ‖f i x‖₊} = {x | C ≤ ‖f i x‖}
F : E → ℝ := fun x => if ⟨C, ⋯⟩ ≤ ‖x‖₊ then ‖x‖ else 0
F_meas : Measurable F
this : ∀ (k : ι), (fun x => {x | C ≤ ‖f k x‖}.indicator (fun a => ‖f k a‖) x) = F ∘ f k
⊢ eLpNorm (fun x => {x | C ≤ ‖f i x‖}.indicator (fun a => ‖f i a‖) x) p μ =
    eLpNorm (fun x => {x | C ≤ ‖f j x‖}.indicator (fun a => ‖f j a‖) x) p μ

Tactic:
rw [this, this, ← eLpNorm_map_measure F_meas.aestronglyMeasurable (hf i).aemeasurable_fst,
  (hf i).map_eq, eLpNorm_map_measure F_meas.aestronglyMeasurable (hf j).aemeasurable_fst]

Result:
no goals