-- Mathlib/MeasureTheory/Decomposition/RadonNikodym.lean
/-- Auxiliary lemma for `rnDeriv_add_right_of_mutuallySingular`. -/
lemma rnDeriv_add_right_of_mutuallySingular' {ν' : Measure α}
    [SigmaFinite μ] [SigmaFinite ν] [SigmaFinite ν']
    (hμν' : μ ⟂ₘ ν') (hνν' : ν ⟂ₘ ν') :
    μ.rnDeriv (ν + ν') =ᵐ[ν] μ.rnDeriv ν

Proof state:
α : Type u_1
m : MeasurableSpace α
μ ν ν' : Measure α
inst✝² : SigmaFinite μ
inst✝¹ : SigmaFinite ν
inst✝ : SigmaFinite ν'
hμν' : μ ⟂ₘ ν'
hνν' : ν ⟂ₘ ν'
h_ac : ν ≪ ν + ν'
h₁ :
  (μ.singularPart ν + ν.withDensity (μ.rnDeriv ν)).rnDeriv (ν + ν') =ᶠ[ae (ν + ν')]
    (μ.singularPart ν).rnDeriv (ν + ν') + (ν.withDensity (μ.rnDeriv ν)).rnDeriv (ν + ν')
h₂ :
  (μ.singularPart ν + ν.withDensity (μ.rnDeriv ν)).rnDeriv ν =ᶠ[ae ν]
    (μ.singularPart ν).rnDeriv ν + (ν.withDensity (μ.rnDeriv ν)).rnDeriv ν
h₃ : (ν.withDensity (μ.rnDeriv ν)).rnDeriv (ν + ν') =ᶠ[ae ν] (ν.withDensity (μ.rnDeriv ν)).rnDeriv ν
⊢ (μ.singularPart ν).rnDeriv (ν + ν') + (ν.withDensity (μ.rnDeriv ν)).rnDeriv (ν + ν') =ᶠ[ae ν]
    (μ.singularPart ν).rnDeriv ν + (ν.withDensity (μ.rnDeriv ν)).rnDeriv ν

Tactic:
have h₄ : (μ.singularPart ν).rnDeriv (ν + ν') =ᵐ[ν] 0 := by
  refine h_ac.ae_eq ?_
  simp only [rnDeriv_eq_zero, MutuallySingular.add_right_iff]
  exact ⟨mutuallySingular_singularPart μ ν, hμν'.singularPart ν⟩

Result:
α : Type u_1
m : MeasurableSpace α
μ ν ν' : Measure α
inst✝² : SigmaFinite μ
inst✝¹ : SigmaFinite ν
inst✝ : SigmaFinite ν'
hμν' : μ ⟂ₘ ν'
hνν' : ν ⟂ₘ ν'
h_ac : ν ≪ ν + ν'
h₁ :
  (μ.singularPart ν + ν.withDensity (μ.rnDeriv ν)).rnDeriv (ν + ν') =ᶠ[ae (ν + ν')]
    (μ.singularPart ν).rnDeriv (ν + ν') + (ν.withDensity (μ.rnDeriv ν)).rnDeriv (ν + ν')
h₂ :
  (μ.singularPart ν + ν.withDensity (μ.rnDeriv ν)).rnDeriv ν =ᶠ[ae ν]
    (μ.singularPart ν).rnDeriv ν + (ν.withDensity (μ.rnDeriv ν)).rnDeriv ν
h₃ : (ν.withDensity (μ.rnDeriv ν)).rnDeriv (ν + ν') =ᶠ[ae ν] (ν.withDensity (μ.rnDeriv ν)).rnDeriv ν
h₄ : (μ.singularPart ν).rnDeriv (ν + ν') =ᶠ[ae ν] 0
⊢ (μ.singularPart ν).rnDeriv (ν + ν') + (ν.withDensity (μ.rnDeriv ν)).rnDeriv (ν + ν') =ᶠ[ae ν]
    (μ.singularPart ν).rnDeriv ν + (ν.withDensity (μ.rnDeriv ν)).rnDeriv ν

Proof state:
α : Type u_1
m : MeasurableSpace α
μ ν ν' : Measure α
inst✝² : SigmaFinite μ
inst✝¹ : SigmaFinite ν
inst✝ : SigmaFinite ν'
hμν' : μ ⟂ₘ ν'
hνν' : ν ⟂ₘ ν'
h_ac : ν ≪ ν + ν'
h₁ :
  (μ.singularPart ν + ν.withDensity (μ.rnDeriv ν)).rnDeriv (ν + ν') =ᶠ[ae (ν + ν')]
    (μ.singularPart ν).rnDeriv (ν + ν') + (ν.withDensity (μ.rnDeriv ν)).rnDeriv (ν + ν')
⊢ (μ.singularPart ν + ν.withDensity (μ.rnDeriv ν)).rnDeriv (ν + ν') =ᶠ[ae ν]
    (μ.singularPart ν + ν.withDensity (μ.rnDeriv ν)).rnDeriv ν

Tactic:
have h₂ := rnDeriv_add' (μ.singularPart ν) (ν.withDensity (μ.rnDeriv ν)) ν

Result:
α : Type u_1
m : MeasurableSpace α
μ ν ν' : Measure α
inst✝² : SigmaFinite μ
inst✝¹ : SigmaFinite ν
inst✝ : SigmaFinite ν'
hμν' : μ ⟂ₘ ν'
hνν' : ν ⟂ₘ ν'
h_ac : ν ≪ ν + ν'
h₁ :
  (μ.singularPart ν + ν.withDensity (μ.rnDeriv ν)).rnDeriv (ν + ν') =ᶠ[ae (ν + ν')]
    (μ.singularPart ν).rnDeriv (ν + ν') + (ν.withDensity (μ.rnDeriv ν)).rnDeriv (ν + ν')
h₂ :
  (μ.singularPart ν + ν.withDensity (μ.rnDeriv ν)).rnDeriv ν =ᶠ[ae ν]
    (μ.singularPart ν).rnDeriv ν + (ν.withDensity (μ.rnDeriv ν)).rnDeriv ν
⊢ (μ.singularPart ν + ν.withDensity (μ.rnDeriv ν)).rnDeriv (ν + ν') =ᶠ[ae ν]
    (μ.singularPart ν + ν.withDensity (μ.rnDeriv ν)).rnDeriv ν

Proof state:
α : Type u_1
m : MeasurableSpace α
μ ν ν' : Measure α
inst✝² : SigmaFinite μ
inst✝¹ : SigmaFinite ν
inst✝ : SigmaFinite ν'
hμν' : μ ⟂ₘ ν'
hνν' : ν ⟂ₘ ν'
h_ac : ν ≪ ν + ν'
h₁ :
  (μ.singularPart ν + ν.withDensity (μ.rnDeriv ν)).rnDeriv (ν + ν') =ᶠ[ae (ν + ν')]
    (μ.singularPart ν).rnDeriv (ν + ν') + (ν.withDensity (μ.rnDeriv ν)).rnDeriv (ν + ν')
h₂ :
  (μ.singularPart ν + ν.withDensity (μ.rnDeriv ν)).rnDeriv ν =ᶠ[ae ν]
    (μ.singularPart ν).rnDeriv ν + (ν.withDensity (μ.rnDeriv ν)).rnDeriv ν
h₃ : (ν.withDensity (μ.rnDeriv ν)).rnDeriv (ν + ν') =ᶠ[ae ν] (ν.withDensity (μ.rnDeriv ν)).rnDeriv ν
⊢ μ.singularPart ν ⟂ₘ ν ∧ μ.singularPart ν ⟂ₘ ν'

Tactic:
exact ⟨mutuallySingular_singularPart μ ν, hμν'.singularPart ν⟩

Result:
no goals

Proof state:
α : Type u_1
m : MeasurableSpace α
μ ν ν' : Measure α
inst✝² : SigmaFinite μ
inst✝¹ : SigmaFinite ν
inst✝ : SigmaFinite ν'
hμν' : μ ⟂ₘ ν'
hνν' : ν ⟂ₘ ν'
h_ac : ν ≪ ν + ν'
⊢ μ.rnDeriv (ν + ν') =ᶠ[ae ν] μ.rnDeriv ν

Tactic:
rw [haveLebesgueDecomposition_add μ ν]

Result:
α : Type u_1
m : MeasurableSpace α
μ ν ν' : Measure α
inst✝² : SigmaFinite μ
inst✝¹ : SigmaFinite ν
inst✝ : SigmaFinite ν'
hμν' : μ ⟂ₘ ν'
hνν' : ν ⟂ₘ ν'
h_ac : ν ≪ ν + ν'
⊢ (μ.singularPart ν + ν.withDensity (μ.rnDeriv ν)).rnDeriv (ν + ν') =ᶠ[ae ν]
    (μ.singularPart ν + ν.withDensity (μ.rnDeriv ν)).rnDeriv ν

Proof state:
α : Type u_1
m : MeasurableSpace α
μ ν ν' : Measure α
inst✝² : SigmaFinite μ
inst✝¹ : SigmaFinite ν
inst✝ : SigmaFinite ν'
hμν' : μ ⟂ₘ ν'
hνν' : ν ⟂ₘ ν'
h_ac : ν ≪ ν + ν'
h₁ :
  (μ.singularPart ν + ν.withDensity (μ.rnDeriv ν)).rnDeriv (ν + ν') =ᶠ[ae (ν + ν')]
    (μ.singularPart ν).rnDeriv (ν + ν') + (ν.withDensity (μ.rnDeriv ν)).rnDeriv (ν + ν')
h₂ :
  (μ.singularPart ν + ν.withDensity (μ.rnDeriv ν)).rnDeriv ν =ᶠ[ae ν]
    (μ.singularPart ν).rnDeriv ν + (ν.withDensity (μ.rnDeriv ν)).rnDeriv ν
h₃ : (ν.withDensity (μ.rnDeriv ν)).rnDeriv (ν + ν') =ᶠ[ae ν] (ν.withDensity (μ.rnDeriv ν)).rnDeriv ν
⊢ (μ.singularPart ν).rnDeriv (ν + ν') =ᶠ[ae (ν + ν')] 0

Tactic:
simp only [rnDeriv_eq_zero, MutuallySingular.add_right_iff]

Result:
α : Type u_1
m : MeasurableSpace α
μ ν ν' : Measure α
inst✝² : SigmaFinite μ
inst✝¹ : SigmaFinite ν
inst✝ : SigmaFinite ν'
hμν' : μ ⟂ₘ ν'
hνν' : ν ⟂ₘ ν'
h_ac : ν ≪ ν + ν'
h₁ :
  (μ.singularPart ν + ν.withDensity (μ.rnDeriv ν)).rnDeriv (ν + ν') =ᶠ[ae (ν + ν')]
    (μ.singularPart ν).rnDeriv (ν + ν') + (ν.withDensity (μ.rnDeriv ν)).rnDeriv (ν + ν')
h₂ :
  (μ.singularPart ν + ν.withDensity (μ.rnDeriv ν)).rnDeriv ν =ᶠ[ae ν]
    (μ.singularPart ν).rnDeriv ν + (ν.withDensity (μ.rnDeriv ν)).rnDeriv ν
h₃ : (ν.withDensity (μ.rnDeriv ν)).rnDeriv (ν + ν') =ᶠ[ae ν] (ν.withDensity (μ.rnDeriv ν)).rnDeriv ν
⊢ μ.singularPart ν ⟂ₘ ν ∧ μ.singularPart ν ⟂ₘ ν'