-- 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
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
⊢ False

Tactic:
rcases exists_subset_restrict_nonpos hC₂ with ⟨D, hD₁, hD, hD₂, hD₃⟩

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
⊢ False

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
⊢ sInf s.measureOfNegatives ≤ ↑s A

Tactic:
exact csInf_le bddBelow_measureOfNegatives ⟨A, ⟨hA₁, hA₂⟩, rfl⟩

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
hA₂ : s ≤[A] 0
hA₃ : ↑s A = sInf s.measureOfNegatives
C : Set α
a✝ : MeasurableSet C
hC₁ : C ⊆ Aᶜ
⊢ ↑0 C ≤ ↑s C

Tactic:
by_contra! hC₂

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
hA₃ : ↑s A = sInf s.measureOfNegatives
C : Set α
a✝ : MeasurableSet C
hC₁ : C ⊆ Aᶜ
hC₂ : ↑s C < ↑0 C
⊢ False

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
⊢ ↑s A = sInf s.measureOfNegatives

Tactic:
apply le_antisymm

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
⊢ ↑s A ≤ sInf s.measureOfNegatives

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
⊢ sInf s.measureOfNegatives ≤ ↑s A

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
hA₂ : s ≤[A] 0
hA₃ : ↑s A = sInf s.measureOfNegatives
⊢ 0 ≤[Aᶜ] s

Tactic:
rw [restrict_le_restrict_iff _ _ hA₁.compl]

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
hA₃ : ↑s A = sInf s.measureOfNegatives
⊢ ∀ ⦃j : Set α⦄, MeasurableSet j → j ⊆ Aᶜ → ↑0 j ≤ ↑s j