Module: Mathlib.MeasureTheory.Function.UniformIntegrable

Goal state before:
pp: α : Type u_1
β : Type u_2
ι : Type u_3
m : MeasurableSpace α
μ : Measure α
inst✝ : NormedAddCommGroup β
p : ℝ≥0∞
hp : 1 ≤ p
hp' : p ≠ ∞
f : ι → α → β
hf : ∀ (i : ι), StronglyMeasurable (f i)
h : ∀ (ε : ℝ), 0 < ε → ∃ C, 0 < C ∧ ∀ (i : ι), eLpNorm ({x | C ≤ ‖f i x‖₊}.indicator (f i)) p μ ≤ ENNReal.ofReal ε
⊢ UnifIntegrable f p μ

Tactic:
have hpzero := (lt_of_lt_of_le zero_lt_one hp).ne.symm
  by_cases hμ : μ Set.univ = 0
  · rw [Measure.measure_univ_eq_zero] at hμ
    exact hμ.symm ▸ unifIntegrable_zero_meas
  intro ε hε
  obtain ⟨C, hCpos, hC⟩ := h (ε / 2) (half_pos hε)
  refine ⟨(ε / (2 * C)) ^ ENNReal.toReal p,
    Real.rpow_pos_of_pos (div_pos hε (mul_pos two_pos (NNReal.coe_pos.2 hCpos))) _,
    fun i s hs hμs => ?_⟩
  by_cases hμs' : μ s = 0
  · rw [(eLpNorm_eq_zero_iff ((hf i).indicator hs).aestronglyMeasurable hpzero).2
        (indicator_meas_zero hμs')]
    simp
  calc
    eLpNorm (Set.indicator s (f i)) p μ ≤
        eLpNorm (Set.indicator (s ∩ { x | C ≤ ‖f i x‖₊ }) (f i)) p μ +
          eLpNorm (Set.indicator (s ∩ { x | ‖f i x‖₊ < C }) (f i)) p μ := by
      refine le_trans (Eq.le ?_) (eLpNorm_add_le
        (StronglyMeasurable.aestronglyMeasurable
          ((hf i).indicator (hs.inter (stronglyMeasurable_const.measurableSet_le (hf i).nnnorm))))
        (StronglyMeasurable.aestronglyMeasurable
          ((hf i).indicator (hs.inter ((hf i).nnnorm.measurableSet_lt stronglyMeasurable_const))))
        hp)
      congr
      change _ = fun x => (s ∩ { x : α | C ≤ ‖f i x‖₊ }).indicator (f i) x +
        (s ∩ { x : α | ‖f i x‖₊ < C }).indicator (f i) x
      rw [← Set.indicator_union_of_disjoint]
      · rw [← Set.inter_union_distrib_left, (by ext; simp [le_or_gt] :
            { x : α | C ≤ ‖f i x‖₊ } ∪ { x : α | ‖f i x‖₊ < C } = Set.univ),
          Set.inter_univ]
      · refine (Disjoint.inf_right' _ ?_).inf_left' _
        rw [disjoint_iff_inf_le]
        rintro x ⟨hx₁, hx₂⟩
        rw [Set.mem_ofPred_eq] at hx₁ hx₂
        exact False.elim (hx₂.ne (eq_of_le_of_not_lt hx₁ (not_lt.2 hx₂.le)).symm)
    _ ≤ eLpNorm (Set.indicator { x | C ≤ ‖f i x‖₊ } (f i)) p μ +
        (C : ℝ≥0∞) * μ s ^ (1 / ENNReal.toReal p) := by
      refine add_le_add
        (eLpNorm_mono fun x => norm_indicator_le_of_subset Set.inter_subset_right _ _) ?_
      rw [← Set.indicator_indicator]
      rw [eLpNorm_indicator_eq_eLpNorm_restrict hs]
      have : ∀ᵐ x ∂μ.restrict s, ‖{ x : α | ‖f i x‖₊ < C }.indicator (f i) x‖ ≤ C := by
        filter_upwards
        simp_rw [norm_indicator_eq_indicator_norm]
        exact Set.indicator_le' (fun x (hx : _ < _) => hx.le) fun _ _ => NNReal.coe_nonneg _
      refine le_trans (eLpNorm_le_of_ae_bound this) ?_
      rw [mul_comm, Measure.restrict_apply' hs, Set.univ_inter, ENNReal.ofReal_coe_nnreal, one_div]
    _ ≤ ENNReal.ofReal (ε / 2) + C * ENNReal.ofReal (ε / (2 * C)) := by
      grw [hC i]
      gcongr
      rwa [one_div, ENNReal.rpow_inv_le_iff (ENNReal.toReal_pos hpzero hp'),
        ENNReal.ofReal_rpow_of_pos (div_pos hε (mul_pos two_pos (NNReal.coe_pos.2 hCpos)))]
    _ ≤ ENNReal.ofReal (ε / 2) + ENNReal.ofReal (ε / 2) := by
      gcongr
      rw [← ENNReal.ofReal_coe_nnreal, ← ENNReal.ofReal_mul (NNReal.coe_nonneg _), ← div_div,
        mul_div_cancel₀ _ (NNReal.coe_pos.2 hCpos).ne.symm]
    _ ≤ ENNReal.ofReal ε := by
      rw [← ENNReal.ofReal_add (half_pos hε).le (half_pos hε).le, add_halves]

Goal state after: