-- Mathlib/MeasureTheory/Function/UnifTight.lean
theorem add (hf : UnifTight f p μ) (hg : UnifTight g p μ)
    (hf_meas : ∀ i, AEStronglyMeasurable (f i) μ) (hg_meas : ∀ i, AEStronglyMeasurable (g i) μ) :
    UnifTight (f + g) p μ := fun ε hε ↦ by
  rcases exists_Lp_half β μ p (coe_ne_zero.mpr hε.ne') with ⟨η, hη_pos, hη⟩
  by_cases hη_top : η = ∞
  · replace hη := hη_top ▸ hη
    refine ⟨∅, (by measurability), fun i ↦ ?_⟩
    simp only [compl_empty, indicator_univ, Pi.add_apply]
    exact (hη (f i) (g i) (hf_meas i) (hg_meas i) le_top le_top).le
  obtain ⟨s, hμs, hsm, hfs, hgs⟩ :
      ∃ s ∈ μ.cofinite, MeasurableSet s ∧
        (∀ i, eLpNorm (s.indicator (f i)) p μ ≤ η) ∧
        (∀ i, eLpNorm (s.indicator (g i)) p μ ≤ η) :=
    ((hf.eventually_cofinite_indicator hη_pos.ne').and
      (hg.eventually_cofinite_indicator hη_pos.ne')).exists_measurable_mem_of_smallSets
  refine ⟨sᶜ, ne_of_lt hμs, fun i ↦ ?_⟩
  have η_cast : ↑η.toNNReal = η := coe_toNNReal hη_top
  calc
    eLpNorm (indicator sᶜᶜ (f i + g i)) p μ
      = eLpNorm (indicator s (f i) + indicator s (g i)) 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✝ : NormedAddCommGroup β
f g : ι → α → β
p : ℝ≥0∞
hf : UnifTight f p μ
hg : UnifTight g p μ
hf_meas : ∀ (i : ι), AEStronglyMeasurable (f i) μ
hg_meas : ∀ (i : ι), AEStronglyMeasurable (g i) μ
ε : ℝ≥0
hε : 0 < ε
η : ℝ≥0∞
hη_pos : 0 < η
hη :
  ∀ (f g : α → β),
    AEStronglyMeasurable f μ →
      AEStronglyMeasurable g μ → eLpNorm f p μ ≤ η → eLpNorm g p μ ≤ η → eLpNorm (f + g) p μ < ↑ε
hη_top : ¬η = ⊤
⊢ ∃ s, μ s ≠ ⊤ ∧ ∀ (i : ι), eLpNorm (sᶜ.indicator ((f + g) i)) p μ ≤ ↑ε

Tactic:
obtain ⟨s, hμs, hsm, hfs, hgs⟩ :
    ∃ s ∈ μ.cofinite, MeasurableSet s ∧
      (∀ i, eLpNorm (s.indicator (f i)) p μ ≤ η) ∧
      (∀ i, eLpNorm (s.indicator (g i)) p μ ≤ η) :=
  ((hf.eventually_cofinite_indicator hη_pos.ne').and
    (hg.eventually_cofinite_indicator hη_pos.ne')).exists_measurable_mem_of_smallSets

Result:
case neg.intro.intro.intro.intro
α : Type u_1
β : Type u_2
ι : Type u_3
m : MeasurableSpace α
μ : Measure α
inst✝ : NormedAddCommGroup β
f g : ι → α → β
p : ℝ≥0∞
hf : UnifTight f p μ
hg : UnifTight g p μ
hf_meas : ∀ (i : ι), AEStronglyMeasurable (f i) μ
hg_meas : ∀ (i : ι), AEStronglyMeasurable (g i) μ
ε : ℝ≥0
hε : 0 < ε
η : ℝ≥0∞
hη_pos : 0 < η
hη :
  ∀ (f g : α → β),
    AEStronglyMeasurable f μ →
      AEStronglyMeasurable g μ → eLpNorm f p μ ≤ η → eLpNorm g p μ ≤ η → eLpNorm (f + g) p μ < ↑ε
hη_top : ¬η = ⊤
s : Set α
hμs : s ∈ μ.cofinite
hsm : MeasurableSet s
hfs : ∀ (i : ι), eLpNorm (s.indicator (f i)) p μ ≤ η
hgs : ∀ (i : ι), eLpNorm (s.indicator (g i)) p μ ≤ η
⊢ ∃ s, μ s ≠ ⊤ ∧ ∀ (i : ι), eLpNorm (sᶜ.indicator ((f + g) i)) p μ ≤ ↑ε

Proof state:
case neg.intro.intro.intro.intro
α : Type u_1
β : Type u_2
ι : Type u_3
m : MeasurableSpace α
μ : Measure α
inst✝ : NormedAddCommGroup β
f g : ι → α → β
p : ℝ≥0∞
hf : UnifTight f p μ
hg : UnifTight g p μ
hf_meas : ∀ (i : ι), AEStronglyMeasurable (f i) μ
hg_meas : ∀ (i : ι), AEStronglyMeasurable (g i) μ
ε : ℝ≥0
hε : 0 < ε
η : ℝ≥0∞
hη_pos : 0 < η
hη :
  ∀ (f g : α → β),
    AEStronglyMeasurable f μ →
      AEStronglyMeasurable g μ → eLpNorm f p μ ≤ η → eLpNorm g p μ ≤ η → eLpNorm (f + g) p μ < ↑ε
hη_top : ¬η = ⊤
s : Set α
hμs : s ∈ μ.cofinite
hsm : MeasurableSet s
hfs : ∀ (i : ι), eLpNorm (s.indicator (f i)) p μ ≤ η
hgs : ∀ (i : ι), eLpNorm (s.indicator (g i)) p μ ≤ η
⊢ ∃ s, μ s ≠ ⊤ ∧ ∀ (i : ι), eLpNorm (sᶜ.indicator ((f + g) i)) p μ ≤ ↑ε

Tactic:
refine ⟨sᶜ, ne_of_lt hμs, fun i ↦ ?_⟩

Result:
case neg.intro.intro.intro.intro
α : Type u_1
β : Type u_2
ι : Type u_3
m : MeasurableSpace α
μ : Measure α
inst✝ : NormedAddCommGroup β
f g : ι → α → β
p : ℝ≥0∞
hf : UnifTight f p μ
hg : UnifTight g p μ
hf_meas : ∀ (i : ι), AEStronglyMeasurable (f i) μ
hg_meas : ∀ (i : ι), AEStronglyMeasurable (g i) μ
ε : ℝ≥0
hε : 0 < ε
η : ℝ≥0∞
hη_pos : 0 < η
hη :
  ∀ (f g : α → β),
    AEStronglyMeasurable f μ →
      AEStronglyMeasurable g μ → eLpNorm f p μ ≤ η → eLpNorm g p μ ≤ η → eLpNorm (f + g) p μ < ↑ε
hη_top : ¬η = ⊤
s : Set α
hμs : s ∈ μ.cofinite
hsm : MeasurableSet s
hfs : ∀ (i : ι), eLpNorm (s.indicator (f i)) p μ ≤ η
hgs : ∀ (i : ι), eLpNorm (s.indicator (g i)) p μ ≤ η
i : ι
⊢ eLpNorm (sᶜᶜ.indicator ((f + g) i)) p μ ≤ ↑ε

Proof state:
case intro.intro
α : Type u_1
β : Type u_2
ι : Type u_3
m : MeasurableSpace α
μ : Measure α
inst✝ : NormedAddCommGroup β
f g : ι → α → β
p : ℝ≥0∞
hf : UnifTight f p μ
hg : UnifTight g p μ
hf_meas : ∀ (i : ι), AEStronglyMeasurable (f i) μ
hg_meas : ∀ (i : ι), AEStronglyMeasurable (g i) μ
ε : ℝ≥0
hε : 0 < ε
η : ℝ≥0∞
hη_pos : 0 < η
hη :
  ∀ (f g : α → β),
    AEStronglyMeasurable f μ →
      AEStronglyMeasurable g μ → eLpNorm f p μ ≤ η → eLpNorm g p μ ≤ η → eLpNorm (f + g) p μ < ↑ε
⊢ ∃ s, μ s ≠ ⊤ ∧ ∀ (i : ι), eLpNorm (sᶜ.indicator ((f + g) i)) p μ ≤ ↑ε

Tactic:
by_cases hη_top : η = ∞

Result:
case pos
α : Type u_1
β : Type u_2
ι : Type u_3
m : MeasurableSpace α
μ : Measure α
inst✝ : NormedAddCommGroup β
f g : ι → α → β
p : ℝ≥0∞
hf : UnifTight f p μ
hg : UnifTight g p μ
hf_meas : ∀ (i : ι), AEStronglyMeasurable (f i) μ
hg_meas : ∀ (i : ι), AEStronglyMeasurable (g i) μ
ε : ℝ≥0
hε : 0 < ε
η : ℝ≥0∞
hη_pos : 0 < η
hη :
  ∀ (f g : α → β),
    AEStronglyMeasurable f μ →
      AEStronglyMeasurable g μ → eLpNorm f p μ ≤ η → eLpNorm g p μ ≤ η → eLpNorm (f + g) p μ < ↑ε
hη_top : η = ⊤
⊢ ∃ s, μ s ≠ ⊤ ∧ ∀ (i : ι), eLpNorm (sᶜ.indicator ((f + g) i)) p μ ≤ ↑ε

case neg
α : Type u_1
β : Type u_2
ι : Type u_3
m : MeasurableSpace α
μ : Measure α
inst✝ : NormedAddCommGroup β
f g : ι → α → β
p : ℝ≥0∞
hf : UnifTight f p μ
hg : UnifTight g p μ
hf_meas : ∀ (i : ι), AEStronglyMeasurable (f i) μ
hg_meas : ∀ (i : ι), AEStronglyMeasurable (g i) μ
ε : ℝ≥0
hε : 0 < ε
η : ℝ≥0∞
hη_pos : 0 < η
hη :
  ∀ (f g : α → β),
    AEStronglyMeasurable f μ →
      AEStronglyMeasurable g μ → eLpNorm f p μ ≤ η → eLpNorm g p μ ≤ η → eLpNorm (f + g) p μ < ↑ε
hη_top : ¬η = ⊤
⊢ ∃ s, μ s ≠ ⊤ ∧ ∀ (i : ι), eLpNorm (sᶜ.indicator ((f + g) i)) p μ ≤ ↑ε

Proof state:
case pos
α : Type u_1
β : Type u_2
ι : Type u_3
m : MeasurableSpace α
μ : Measure α
inst✝ : NormedAddCommGroup β
f g : ι → α → β
p : ℝ≥0∞
hf : UnifTight f p μ
hg : UnifTight g p μ
hf_meas : ∀ (i : ι), AEStronglyMeasurable (f i) μ
hg_meas : ∀ (i : ι), AEStronglyMeasurable (g i) μ
ε : ℝ≥0
hε : 0 < ε
η : ℝ≥0∞
hη_pos : 0 < η
hη :
  ∀ (f g : α → β),
    AEStronglyMeasurable f μ →
      AEStronglyMeasurable g μ → eLpNorm f p μ ≤ η → eLpNorm g p μ ≤ η → eLpNorm (f + g) p μ < ↑ε
hη_top : η = ⊤
⊢ ∃ s, μ s ≠ ⊤ ∧ ∀ (i : ι), eLpNorm (sᶜ.indicator ((f + g) i)) p μ ≤ ↑ε

Tactic:
replace hη := hη_top ▸ hη

Result:
case pos
α : Type u_1
β : Type u_2
ι : Type u_3
m : MeasurableSpace α
μ : Measure α
inst✝ : NormedAddCommGroup β
f g : ι → α → β
p : ℝ≥0∞
hf : UnifTight f p μ
hg : UnifTight g p μ
hf_meas : ∀ (i : ι), AEStronglyMeasurable (f i) μ
hg_meas : ∀ (i : ι), AEStronglyMeasurable (g i) μ
ε : ℝ≥0
hε : 0 < ε
η : ℝ≥0∞
hη_pos : 0 < η
hη_top : η = ⊤
hη :
  ∀ (f g : α → β),
    AEStronglyMeasurable f μ →
      AEStronglyMeasurable g μ → eLpNorm f p μ ≤ ⊤ → eLpNorm g p μ ≤ ⊤ → eLpNorm (f + g) p μ < ↑ε
⊢ ∃ s, μ s ≠ ⊤ ∧ ∀ (i : ι), eLpNorm (sᶜ.indicator ((f + g) i)) p μ ≤ ↑ε

Proof state:
α : Type u_1
β : Type u_2
ι : Type u_3
m : MeasurableSpace α
μ : Measure α
inst✝ : NormedAddCommGroup β
f g : ι → α → β
p : ℝ≥0∞
hf : UnifTight f p μ
hg : UnifTight g p μ
hf_meas : ∀ (i : ι), AEStronglyMeasurable (f i) μ
hg_meas : ∀ (i : ι), AEStronglyMeasurable (g i) μ
ε : ℝ≥0
hε : 0 < ε
η : ℝ≥0∞
hη_pos : 0 < η
hη_top : η = ⊤
hη :
  ∀ (f g : α → β),
    AEStronglyMeasurable f μ →
      AEStronglyMeasurable g μ → eLpNorm f p μ ≤ ⊤ → eLpNorm g p μ ≤ ⊤ → eLpNorm (f + g) p μ < ↑ε
⊢ μ ∅ ≠ ⊤

Tactic:
measurability

Result:
no goals