-- Mathlib/MeasureTheory/Decomposition/SignedHahn.lean
theorem exists_compl_positive_negative (s : SignedMeasure α) :
    ∃ i : Set α, MeasurableSet i ∧ 0 ≤[i] s ∧ s ≤[iᶜ] 0

Proof state:
case intro.intro.intro.intro.intro.intro.intro
α : Type u_1
inst✝ : MeasurableSpace α
s : SignedMeasure α
f : ℕ → ℝ
left✝ : Antitone f
hf₂ : Tendsto f atTop (nhds (sInf s.measureOfNegatives))
B : ℕ → Set α
hB : ∀ (n : ℕ), B n ∈ {B | MeasurableSet B ∧ s ≤[B] 0} ∧ ↑s (B n) = f n
hB₁ : ∀ (n : ℕ), MeasurableSet (B n)
hB₂ : ∀ (n : ℕ), s ≤[B n] 0
A : Set α := ⋃ n, B n
hA : A = ⋃ n, B n
hA₁ : MeasurableSet A
hA₂ : s ≤[A] 0
hA₃ : ↑s A = sInf s.measureOfNegatives
C : Set α
a✝ : MeasurableSet C
hC₁ : C ⊆ Aᶜ
hC₂ : ↑s C < ↑0 C
D : Set α
hD₁ : MeasurableSet D
hD : D ⊆ C
hD₂ : s ≤[D] 0
hD₃ : ↑s D < 0
⊢ False

Tactic:
have : s (A ∪ D) < sInf s.measureOfNegatives := by
  rw [← hA₃,
    of_union (Set.disjoint_of_subset_right (Set.Subset.trans hD hC₁) disjoint_compl_right) hA₁
      hD₁]
  linarith

Result:
case intro.intro.intro.intro.intro.intro.intro
α : Type u_1
inst✝ : MeasurableSpace α
s : SignedMeasure α
f : ℕ → ℝ
left✝ : Antitone f
hf₂ : Tendsto f atTop (nhds (sInf s.measureOfNegatives))
B : ℕ → Set α
hB : ∀ (n : ℕ), B n ∈ {B | MeasurableSet B ∧ s ≤[B] 0} ∧ ↑s (B n) = f n
hB₁ : ∀ (n : ℕ), MeasurableSet (B n)
hB₂ : ∀ (n : ℕ), s ≤[B n] 0
A : Set α := ⋃ n, B n
hA : A = ⋃ n, B n
hA₁ : MeasurableSet A
hA₂ : s ≤[A] 0
hA₃ : ↑s A = sInf s.measureOfNegatives
C : Set α
a✝ : MeasurableSet C
hC₁ : C ⊆ Aᶜ
hC₂ : ↑s C < ↑0 C
D : Set α
hD₁ : MeasurableSet D
hD : D ⊆ C
hD₂ : s ≤[D] 0
hD₃ : ↑s D < 0
this : ↑s (A ∪ D) < sInf s.measureOfNegatives
⊢ False

Proof state:
case intro.intro.intro.intro.intro.intro.intro
α : Type u_1
inst✝ : MeasurableSpace α
s : SignedMeasure α
f : ℕ → ℝ
left✝ : Antitone f
hf₂ : Tendsto f atTop (nhds (sInf s.measureOfNegatives))
B : ℕ → Set α
hB : ∀ (n : ℕ), B n ∈ {B | MeasurableSet B ∧ s ≤[B] 0} ∧ ↑s (B n) = f n
hB₁ : ∀ (n : ℕ), MeasurableSet (B n)
hB₂ : ∀ (n : ℕ), s ≤[B n] 0
A : Set α := ⋃ n, B n
hA : A = ⋃ n, B n
hA₁ : MeasurableSet A
hA₂ : s ≤[A] 0
hA₃ : ↑s A = sInf s.measureOfNegatives
C : Set α
a✝ : MeasurableSet C
hC₁ : C ⊆ Aᶜ
hC₂ : ↑s C < ↑0 C
D : Set α
hD₁ : MeasurableSet D
hD : D ⊆ C
hD₂ : s ≤[D] 0
hD₃ : ↑s D < 0
this : ↑s (A ∪ D) < sInf s.measureOfNegatives
⊢ False

Tactic:
refine not_le.2 this ?_

Result:
case intro.intro.intro.intro.intro.intro.intro
α : Type u_1
inst✝ : MeasurableSpace α
s : SignedMeasure α
f : ℕ → ℝ
left✝ : Antitone f
hf₂ : Tendsto f atTop (nhds (sInf s.measureOfNegatives))
B : ℕ → Set α
hB : ∀ (n : ℕ), B n ∈ {B | MeasurableSet B ∧ s ≤[B] 0} ∧ ↑s (B n) = f n
hB₁ : ∀ (n : ℕ), MeasurableSet (B n)
hB₂ : ∀ (n : ℕ), s ≤[B n] 0
A : Set α := ⋃ n, B n
hA : A = ⋃ n, B n
hA₁ : MeasurableSet A
hA₂ : s ≤[A] 0
hA₃ : ↑s A = sInf s.measureOfNegatives
C : Set α
a✝ : MeasurableSet C
hC₁ : C ⊆ Aᶜ
hC₂ : ↑s C < ↑0 C
D : Set α
hD₁ : MeasurableSet D
hD : D ⊆ C
hD₂ : s ≤[D] 0
hD₃ : ↑s D < 0
this : ↑s (A ∪ D) < sInf s.measureOfNegatives
⊢ sInf s.measureOfNegatives ≤ ↑s (A ∪ D)

Proof state:
case a
α : Type u_1
inst✝ : MeasurableSpace α
s : SignedMeasure α
f : ℕ → ℝ
left✝ : Antitone f
hf₂ : Tendsto f atTop (nhds (sInf s.measureOfNegatives))
B : ℕ → Set α
hB : ∀ (n : ℕ), B n ∈ {B | MeasurableSet B ∧ s ≤[B] 0} ∧ ↑s (B n) = f n
hB₁ : ∀ (n : ℕ), MeasurableSet (B n)
hB₂ : ∀ (n : ℕ), s ≤[B n] 0
A : Set α := ⋃ n, B n
hA : A = ⋃ n, B n
hA₁ : MeasurableSet A
hA₂ : s ≤[A] 0
n : ℕ
⊢ (fun x => ↑s A) n ≤ f n

Tactic:
rw [← (hB n).2, hA, ← Set.diff_union_of_subset (Set.subset_iUnion _ n),
  of_union Set.disjoint_sdiff_left _ (hB₁ n)]

Result:
case a
α : Type u_1
inst✝ : MeasurableSpace α
s : SignedMeasure α
f : ℕ → ℝ
left✝ : Antitone f
hf₂ : Tendsto f atTop (nhds (sInf s.measureOfNegatives))
B : ℕ → Set α
hB : ∀ (n : ℕ), B n ∈ {B | MeasurableSet B ∧ s ≤[B] 0} ∧ ↑s (B n) = f n
hB₁ : ∀ (n : ℕ), MeasurableSet (B n)
hB₂ : ∀ (n : ℕ), s ≤[B n] 0
A : Set α := ⋃ n, B n
hA : A = ⋃ n, B n
hA₁ : MeasurableSet A
hA₂ : s ≤[A] 0
n : ℕ
⊢ (fun x => ↑s ((⋃ i, B i) \ B n) + ↑s (B n)) n ≤ ↑s (B n)

α : Type u_1
inst✝ : MeasurableSpace α
s : SignedMeasure α
f : ℕ → ℝ
left✝ : Antitone f
hf₂ : Tendsto f atTop (nhds (sInf s.measureOfNegatives))
B : ℕ → Set α
hB : ∀ (n : ℕ), B n ∈ {B | MeasurableSet B ∧ s ≤[B] 0} ∧ ↑s (B n) = f n
hB₁ : ∀ (n : ℕ), MeasurableSet (B n)
hB₂ : ∀ (n : ℕ), s ≤[B n] 0
A : Set α := ⋃ n, B n
hA : A = ⋃ n, B n
hA₁ : MeasurableSet A
hA₂ : s ≤[A] 0
n : ℕ
⊢ MeasurableSet ((⋃ i, B i) \ B n)

Proof state:
α : Type u_1
inst✝ : MeasurableSpace α
s : SignedMeasure α
f : ℕ → ℝ
left✝ : Antitone f
hf₂ : Tendsto f atTop (nhds (sInf s.measureOfNegatives))
B : ℕ → Set α
hB : ∀ (n : ℕ), B n ∈ {B | MeasurableSet B ∧ s ≤[B] 0} ∧ ↑s (B n) = f n
hB₁ : ∀ (n : ℕ), MeasurableSet (B n)
hB₂ : ∀ (n : ℕ), s ≤[B n] 0
A : Set α := ⋃ n, B n
hA : A = ⋃ n, B n
hA₁ : MeasurableSet A
hA₂ : s ≤[A] 0
n : ℕ
⊢ MeasurableSet ((⋃ i, B i) \ B n)

Tactic:
exact (MeasurableSet.iUnion hB₁).diff (hB₁ n)

Result:
no goals

Proof state:
case intro.intro.intro.intro.intro.intro.intro.refine_1
α : Type u_1
inst✝ : MeasurableSpace α
s : SignedMeasure α
f : ℕ → ℝ
left✝ : Antitone f
hf₂ : Tendsto f atTop (nhds (sInf s.measureOfNegatives))
B : ℕ → Set α
hB : ∀ (n : ℕ), B n ∈ {B | MeasurableSet B ∧ s ≤[B] 0} ∧ ↑s (B n) = f n
hB₁ : ∀ (n : ℕ), MeasurableSet (B n)
hB₂ : ∀ (n : ℕ), s ≤[B n] 0
A : Set α := ⋃ n, B n
hA : A = ⋃ n, B n
hA₁ : MeasurableSet A
hA₂ : s ≤[A] 0
hA₃ : ↑s A = sInf s.measureOfNegatives
C : Set α
a✝ : MeasurableSet C
hC₁ : C ⊆ Aᶜ
hC₂ : ↑s C < ↑0 C
D : Set α
hD₁ : MeasurableSet D
hD : D ⊆ C
hD₂ : s ≤[D] 0
hD₃ : ↑s D < 0
this : ↑s (A ∪ D) < sInf s.measureOfNegatives
⊢ MeasurableSet (A ∪ D)

Tactic:
exact hA₁.union hD₁

Result:
no goals

Proof state:
case intro.intro.intro
α : Type u_1
inst✝ : MeasurableSpace α
s : SignedMeasure α
f : ℕ → ℝ
left✝ : Antitone f
hf₂ : Tendsto f atTop (nhds (sInf s.measureOfNegatives))
B : ℕ → Set α
hB : ∀ (n : ℕ), B n ∈ {B | MeasurableSet B ∧ s ≤[B] 0} ∧ ↑s (B n) = f n
hB₁ : ∀ (n : ℕ), MeasurableSet (B n)
hB₂ : ∀ (n : ℕ), s ≤[B n] 0
A : Set α := ⋃ n, B n
hA : A = ⋃ n, B n
hA₁ : MeasurableSet A
⊢ ∃ i, MeasurableSet i ∧ 0 ≤[i] s ∧ s ≤[iᶜ] 0

Tactic:
have hA₂ : s ≤[A] 0 := restrict_le_restrict_iUnion _ _ hB₁ hB₂

Result:
case intro.intro.intro
α : Type u_1
inst✝ : MeasurableSpace α
s : SignedMeasure α
f : ℕ → ℝ
left✝ : Antitone f
hf₂ : Tendsto f atTop (nhds (sInf s.measureOfNegatives))
B : ℕ → Set α
hB : ∀ (n : ℕ), B n ∈ {B | MeasurableSet B ∧ s ≤[B] 0} ∧ ↑s (B n) = f n
hB₁ : ∀ (n : ℕ), MeasurableSet (B n)
hB₂ : ∀ (n : ℕ), s ≤[B n] 0
A : Set α := ⋃ n, B n
hA : A = ⋃ n, B n
hA₁ : MeasurableSet A
hA₂ : s ≤[A] 0
⊢ ∃ i, MeasurableSet i ∧ 0 ≤[i] s ∧ s ≤[iᶜ] 0

Proof state:
case intro.intro.intro
α : Type u_1
inst✝ : MeasurableSpace α
s : SignedMeasure α
f : ℕ → ℝ
left✝ : Antitone f
hf₂ : Tendsto f atTop (nhds (sInf s.measureOfNegatives))
B : ℕ → Set α
hB : ∀ (n : ℕ), B n ∈ {B | MeasurableSet B ∧ s ≤[B] 0} ∧ ↑s (B n) = f n
⊢ ∃ i, MeasurableSet i ∧ 0 ≤[i] s ∧ s ≤[iᶜ] 0

Tactic:
have hB₁ : ∀ n, MeasurableSet (B n) := fun n => (hB n).1.1

Result:
case intro.intro.intro
α : Type u_1
inst✝ : MeasurableSpace α
s : SignedMeasure α
f : ℕ → ℝ
left✝ : Antitone f
hf₂ : Tendsto f atTop (nhds (sInf s.measureOfNegatives))
B : ℕ → Set α
hB : ∀ (n : ℕ), B n ∈ {B | MeasurableSet B ∧ s ≤[B] 0} ∧ ↑s (B n) = f n
hB₁ : ∀ (n : ℕ), MeasurableSet (B n)
⊢ ∃ i, MeasurableSet i ∧ 0 ≤[i] s ∧ s ≤[iᶜ] 0