-- Mathlib/MeasureTheory/Decomposition/Lebesgue.lean
theorem exists_positive_of_not_mutuallySingular (μ ν : Measure α) [IsFiniteMeasure μ]
    [IsFiniteMeasure ν] (h : ¬ μ ⟂ₘ ν) :
    ∃ ε : ℝ≥0, 0 < ε ∧
      ∃ E : Set α, MeasurableSet E ∧ 0 < ν E
        ∧ ∀ A, MeasurableSet A → ε * ν (A ∩ E) ≤ μ (A ∩ E)

Proof state:
case neg
α : Type u_1
m : MeasurableSpace α
μ ν : Measure α
inst✝¹ : IsFiniteMeasure μ
inst✝ : IsFiniteMeasure ν
h : ¬μ ⟂ₘ ν
f : ℕ → Set α
hf₁ : ∀ (n : ℕ), MeasurableSet (f n)
hf₂ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → ((1 / (↑n + 1)) • ν) (t ∩ f n) ≤ μ (t ∩ f n)
hf₃ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → μ (t ∩ (f n)ᶜ) ≤ ((1 / (↑n + 1)) • ν) (t ∩ (f n)ᶜ)
A : Set α := ⋂ n, (f n)ᶜ
hAmeas : MeasurableSet A
hA₂ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → μ (t ∩ A) ≤ ((1 / (↑n + 1)) • ν) (t ∩ A)
μA : ℝ≥0
hA₃✝¹ : ∀ (n : ℕ), ↑μA ≤ ↑(1 / (↑n + 1)) * ν A
νA : ℝ≥0
hA₃✝ hA₃ : ∀ (n : ℕ), ↑μA ≤ ↑(1 / (↑n + 1)) * ↑νA
hb : ¬0 < νA
⊢ μA = 0

Tactic:
rw [not_lt, le_zero_iff] at hb

Result:
case neg
α : Type u_1
m : MeasurableSpace α
μ ν : Measure α
inst✝¹ : IsFiniteMeasure μ
inst✝ : IsFiniteMeasure ν
h : ¬μ ⟂ₘ ν
f : ℕ → Set α
hf₁ : ∀ (n : ℕ), MeasurableSet (f n)
hf₂ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → ((1 / (↑n + 1)) • ν) (t ∩ f n) ≤ μ (t ∩ f n)
hf₃ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → μ (t ∩ (f n)ᶜ) ≤ ((1 / (↑n + 1)) • ν) (t ∩ (f n)ᶜ)
A : Set α := ⋂ n, (f n)ᶜ
hAmeas : MeasurableSet A
hA₂ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → μ (t ∩ A) ≤ ((1 / (↑n + 1)) • ν) (t ∩ A)
μA : ℝ≥0
hA₃✝¹ : ∀ (n : ℕ), ↑μA ≤ ↑(1 / (↑n + 1)) * ν A
νA : ℝ≥0
hA₃✝ hA₃ : ∀ (n : ℕ), ↑μA ≤ ↑(1 / (↑n + 1)) * ↑νA
hb : νA = 0
⊢ μA = 0

Proof state:
α : Type u_1
m : MeasurableSpace α
μ ν : Measure α
inst✝¹ : IsFiniteMeasure μ
inst✝ : IsFiniteMeasure ν
h : ¬μ ⟂ₘ ν
f : ℕ → Set α
hf₁ : ∀ (n : ℕ), MeasurableSet (f n)
hf₂ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → ((1 / (↑n + 1)) • ν) (t ∩ f n) ≤ μ (t ∩ f n)
hf₃ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → μ (t ∩ (f n)ᶜ) ≤ ((1 / (↑n + 1)) • ν) (t ∩ (f n)ᶜ)
A : Set α := ⋂ n, (f n)ᶜ
hAmeas : MeasurableSet A
hA₂ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → μ (t ∩ A) ≤ ((1 / (↑n + 1)) • ν) (t ∩ A)
hA₃ : ∀ (n : ℕ), μ A ≤ ↑(1 / (↑n + 1)) * ν A
hμ : μ A = 0
⊢ ∃ ε, 0 < ε ∧ ∃ E, MeasurableSet E ∧ 0 < ν E ∧ ∀ (A : Set α), MeasurableSet A → ↑ε * ν (A ∩ E) ≤ μ (A ∩ E)

Tactic:
rw [MutuallySingular] at h

Result:
α : Type u_1
m : MeasurableSpace α
μ ν : Measure α
inst✝¹ : IsFiniteMeasure μ
inst✝ : IsFiniteMeasure ν
h : ¬∃ s, MeasurableSet s ∧ μ s = 0 ∧ ν sᶜ = 0
f : ℕ → Set α
hf₁ : ∀ (n : ℕ), MeasurableSet (f n)
hf₂ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → ((1 / (↑n + 1)) • ν) (t ∩ f n) ≤ μ (t ∩ f n)
hf₃ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → μ (t ∩ (f n)ᶜ) ≤ ((1 / (↑n + 1)) • ν) (t ∩ (f n)ᶜ)
A : Set α := ⋂ n, (f n)ᶜ
hAmeas : MeasurableSet A
hA₂ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → μ (t ∩ A) ≤ ((1 / (↑n + 1)) • ν) (t ∩ A)
hA₃ : ∀ (n : ℕ), μ A ≤ ↑(1 / (↑n + 1)) * ν A
hμ : μ A = 0
⊢ ∃ ε, 0 < ε ∧ ∃ E, MeasurableSet E ∧ 0 < ν E ∧ ∀ (A : Set α), MeasurableSet A → ↑ε * ν (A ∩ E) ≤ μ (A ∩ E)

Proof state:
α : Type u_1
m : MeasurableSpace α
μ ν : Measure α
inst✝¹ : IsFiniteMeasure μ
inst✝ : IsFiniteMeasure ν
h : ¬μ ⟂ₘ ν
f : ℕ → Set α
hf₁ : ∀ (n : ℕ), MeasurableSet (f n)
hf₂ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → ((1 / (↑n + 1)) • ν) (t ∩ f n) ≤ μ (t ∩ f n)
hf₃ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → μ (t ∩ (f n)ᶜ) ≤ ((1 / (↑n + 1)) • ν) (t ∩ (f n)ᶜ)
A : Set α := ⋂ n, (f n)ᶜ
hAmeas : MeasurableSet A
hA₂ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → μ (t ∩ A) ≤ ((1 / (↑n + 1)) • ν) (t ∩ A)
μA : ℝ≥0
hA₃✝¹ : ∀ (n : ℕ), ↑μA ≤ ↑(1 / (↑n + 1)) * ν A
νA : ℝ≥0
hA₃✝ hA₃ : ∀ (n : ℕ), ↑μA ≤ ↑(1 / (↑n + 1)) * ↑νA
hb : 0 < νA
c : ℝ≥0
hc : 0 < c
⊢ 0 < ↑c * (↑νA)⁻¹

Tactic:
positivity

Result:
no goals

Proof state:
α : Type u_1
m : MeasurableSpace α
μ ν : Measure α
inst✝¹ : IsFiniteMeasure μ
inst✝ : IsFiniteMeasure ν
f : ℕ → Set α
hf₁ : ∀ (n : ℕ), MeasurableSet (f n)
hf₂ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → ((1 / (↑n + 1)) • ν) (t ∩ f n) ≤ μ (t ∩ f n)
hf₃ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → μ (t ∩ (f n)ᶜ) ≤ ((1 / (↑n + 1)) • ν) (t ∩ (f n)ᶜ)
A : Set α := ⋂ n, (f n)ᶜ
hAmeas : MeasurableSet A
hA₂ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → μ (t ∩ A) ≤ ((1 / (↑n + 1)) • ν) (t ∩ A)
hA₃ : ∀ (n : ℕ), μ A ≤ ↑(1 / (↑n + 1)) * ν A
hμ : μ A = 0
h : ∀ (s : Set α), MeasurableSet s → μ s = 0 → ν sᶜ ≠ 0
this : ν Aᶜ ≠ 0
⊢ ∃ ε, 0 < ε ∧ ∃ E, MeasurableSet E ∧ 0 < ν E ∧ ∀ (A : Set α), MeasurableSet A → ↑ε * ν (A ∩ E) ≤ μ (A ∩ E)

Tactic:
simp_rw [A, compl_iInter, compl_compl] at this

Result:
α : Type u_1
m : MeasurableSpace α
μ ν : Measure α
inst✝¹ : IsFiniteMeasure μ
inst✝ : IsFiniteMeasure ν
f : ℕ → Set α
hf₁ : ∀ (n : ℕ), MeasurableSet (f n)
hf₂ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → ((1 / (↑n + 1)) • ν) (t ∩ f n) ≤ μ (t ∩ f n)
hf₃ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → μ (t ∩ (f n)ᶜ) ≤ ((1 / (↑n + 1)) • ν) (t ∩ (f n)ᶜ)
A : Set α := ⋂ n, (f n)ᶜ
hAmeas : MeasurableSet A
hA₂ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → μ (t ∩ A) ≤ ((1 / (↑n + 1)) • ν) (t ∩ A)
hA₃ : ∀ (n : ℕ), μ A ≤ ↑(1 / (↑n + 1)) * ν A
hμ : μ A = 0
h : ∀ (s : Set α), MeasurableSet s → μ s = 0 → ν sᶜ ≠ 0
this : ν (⋃ i, f i) ≠ 0
⊢ ∃ ε, 0 < ε ∧ ∃ E, MeasurableSet E ∧ 0 < ν E ∧ ∀ (A : Set α), MeasurableSet A → ↑ε * ν (A ∩ E) ≤ μ (A ∩ E)

Proof state:
α : Type u_1
m : MeasurableSpace α
μ ν : Measure α
inst✝¹ : IsFiniteMeasure μ
inst✝ : IsFiniteMeasure ν
f : ℕ → Set α
hf₁ : ∀ (n : ℕ), MeasurableSet (f n)
hf₂ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → ((1 / (↑n + 1)) • ν) (t ∩ f n) ≤ μ (t ∩ f n)
hf₃ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → μ (t ∩ (f n)ᶜ) ≤ ((1 / (↑n + 1)) • ν) (t ∩ (f n)ᶜ)
A : Set α := ⋂ n, (f n)ᶜ
hAmeas : MeasurableSet A
hA₂ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → μ (t ∩ A) ≤ ((1 / (↑n + 1)) • ν) (t ∩ A)
hA₃ : ∀ (n : ℕ), μ A ≤ ↑(1 / (↑n + 1)) * ν A
hμ : μ A = 0
h : ∀ (s : Set α), MeasurableSet s → μ s = 0 → ν sᶜ ≠ 0
this : ν (⋃ i, f i) ≠ 0
⊢ ∃ ε, 0 < ε ∧ ∃ E, MeasurableSet E ∧ 0 < ν E ∧ ∀ (A : Set α), MeasurableSet A → ↑ε * ν (A ∩ E) ≤ μ (A ∩ E)

Tactic:
obtain ⟨n, hn⟩ := exists_measure_pos_of_not_measure_iUnion_null this

Result:
case intro
α : Type u_1
m : MeasurableSpace α
μ ν : Measure α
inst✝¹ : IsFiniteMeasure μ
inst✝ : IsFiniteMeasure ν
f : ℕ → Set α
hf₁ : ∀ (n : ℕ), MeasurableSet (f n)
hf₂ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → ((1 / (↑n + 1)) • ν) (t ∩ f n) ≤ μ (t ∩ f n)
hf₃ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → μ (t ∩ (f n)ᶜ) ≤ ((1 / (↑n + 1)) • ν) (t ∩ (f n)ᶜ)
A : Set α := ⋂ n, (f n)ᶜ
hAmeas : MeasurableSet A
hA₂ : ∀ (n : ℕ) (t : Set α), MeasurableSet t → μ (t ∩ A) ≤ ((1 / (↑n + 1)) • ν) (t ∩ A)
hA₃ : ∀ (n : ℕ), μ A ≤ ↑(1 / (↑n + 1)) * ν A
hμ : μ A = 0
h : ∀ (s : Set α), MeasurableSet s → μ s = 0 → ν sᶜ ≠ 0
this : ν (⋃ i, f i) ≠ 0
n : ℕ
hn : 0 < ν (f n)
⊢ ∃ ε, 0 < ε ∧ ∃ E, MeasurableSet E ∧ 0 < ν E ∧ ∀ (A : Set α), MeasurableSet A → ↑ε * ν (A ∩ E) ≤ μ (A ∩ E)