Theorem: ProbabilityTheory.Kernel.withDensity_rnDeriv_eq_zero_iff_measure_eq_zero
File: Mathlib/Probability/Kernel/RadonNikodym.lean

Proof steps:
state_before: α : Type u_1
γ : Type u_2
mα : MeasurableSpace α
mγ : MeasurableSpace γ
κ✝ η✝ : Kernel α γ
hαγ : MeasurableSpace.CountableOrCountablyGenerated α γ
κ η : Kernel α γ
inst✝¹ : IsFiniteKernel κ
inst✝ : IsFiniteKernel η
a : α
⊢ (η.withDensity (κ.rnDeriv η)) a = 0 ↔ (κ a) (κ.mutuallySingularSetSlice η a)ᶜ = 0
state_after: α : Type u_1
γ : Type u_2
mα : MeasurableSpace α
mγ : MeasurableSpace γ
κ✝ η✝ : Kernel α γ
hαγ : MeasurableSpace.CountableOrCountablyGenerated α γ
κ η : Kernel α γ
inst✝¹ : IsFiniteKernel κ
inst✝ : IsFiniteKernel η
a : α
h_eq_add : η.withDensity (κ.rnDeriv η) + κ.singularPart η = κ
⊢ (η.withDensity (κ.rnDeriv η)) a = 0 ↔ (κ a) (κ.mutuallySingularSetSlice η a)ᶜ = 0
tactic: have h_eq_add := rnDeriv_add_singularPart κ η
state_before: α : Type u_1
γ : Type u_2
mα : MeasurableSpace α
mγ : MeasurableSpace γ
κ✝ η✝ : Kernel α γ
hαγ : MeasurableSpace.CountableOrCountablyGenerated α γ
κ η : Kernel α γ
inst✝¹ : IsFiniteKernel κ
inst✝ : IsFiniteKernel η
a : α
h_eq_add : η.withDensity (κ.rnDeriv η) + κ.singularPart η = κ
⊢ (η.withDensity (κ.rnDeriv η)) a = 0 ↔ (κ a) (κ.mutuallySingularSetSlice η a)ᶜ = 0
state_after: α : Type u_1
γ : Type u_2
mα : MeasurableSpace α
mγ : MeasurableSpace γ
κ✝ η✝ : Kernel α γ
hαγ : MeasurableSpace.CountableOrCountablyGenerated α γ
κ η : Kernel α γ
inst✝¹ : IsFiniteKernel κ
inst✝ : IsFiniteKernel η
a : α
h_eq_add : ∀ (a : α) (s : Set γ), MeasurableSet s → ((η.withDensity (κ.rnDeriv η) + κ.singularPart η) a) s = (κ a) s
⊢ (η.withDensity (κ.rnDeriv η)) a = 0 ↔ (κ a) (κ.mutuallySingularSetSlice η a)ᶜ = 0
tactic: simp_rw [ext_iff, Measure.ext_iff] at h_eq_add
state_before: α : Type u_1
γ : Type u_2
mα : MeasurableSpace α
mγ : MeasurableSpace γ
κ✝ η✝ : Kernel α γ
hαγ : MeasurableSpace.CountableOrCountablyGenerated α γ
κ η : Kernel α γ
inst✝¹ : IsFiniteKernel κ
inst✝ : IsFiniteKernel η
a : α
h_eq_add : ∀ (a : α) (s : Set γ), MeasurableSet s → ((η.withDensity (κ.rnDeriv η) + κ.singularPart η) a) s = (κ a) s
⊢ (η.withDensity (κ.rnDeriv η)) a = 0 ↔ (κ a) (κ.mutuallySingularSetSlice η a)ᶜ = 0
state_after: α : Type u_1
γ : Type u_2
mα : MeasurableSpace α
mγ : MeasurableSpace γ
κ✝ η✝ : Kernel α γ
hαγ : MeasurableSpace.CountableOrCountablyGenerated α γ
κ η : Kernel α γ
inst✝¹ : IsFiniteKernel κ
inst✝ : IsFiniteKernel η
a : α
h_eq_add :
  ((η.withDensity (κ.rnDeriv η) + κ.singularPart η) a) (κ.mutuallySingularSetSlice η a)ᶜ =
    (κ a) (κ.mutuallySingularSetSlice η a)ᶜ
⊢ (η.withDensity (κ.rnDeriv η)) a = 0 ↔ (κ a) (κ.mutuallySingularSetSlice η a)ᶜ = 0
tactic: specialize h_eq_add a (mutuallySingularSetSlice κ η a)ᶜ
    (measurableSet_mutuallySingularSetSlice κ η a).compl
state_before: α : Type u_1
γ : Type u_2
mα : MeasurableSpace α
mγ : MeasurableSpace γ
κ✝ η✝ : Kernel α γ
hαγ : MeasurableSpace.CountableOrCountablyGenerated α γ
κ η : Kernel α γ
inst✝¹ : IsFiniteKernel κ
inst✝ : IsFiniteKernel η
a : α
h_eq_add :
  ((η.withDensity (κ.rnDeriv η) + κ.singularPart η) a) (κ.mutuallySingularSetSlice η a)ᶜ =
    (κ a) (κ.mutuallySingularSetSlice η a)ᶜ
⊢ (η.withDensity (κ.rnDeriv η)) a = 0 ↔ (κ a) (κ.mutuallySingularSetSlice η a)ᶜ = 0
state_after: α : Type u_1
γ : Type u_2
mα : MeasurableSpace α
mγ : MeasurableSpace γ
κ✝ η✝ : Kernel α γ
hαγ : MeasurableSpace.CountableOrCountablyGenerated α γ
κ η : Kernel α γ
inst✝¹ : IsFiniteKernel κ
inst✝ : IsFiniteKernel η
a : α
h_eq_add : ((η.withDensity (κ.rnDeriv η)) a) (κ.mutuallySingularSetSlice η a)ᶜ = (κ a) (κ.mutuallySingularSetSlice η a)ᶜ
⊢ (η.withDensity (κ.rnDeriv η)) a = 0 ↔ (κ a) (κ.mutuallySingularSetSlice η a)ᶜ = 0
tactic: simp only [coe_add, Pi.add_apply, Measure.coe_add,
    singularPart_compl_mutuallySingularSetSlice κ η, add_zero] at h_eq_add
state_before: α : Type u_1
γ : Type u_2
mα : MeasurableSpace α
mγ : MeasurableSpace γ
κ✝ η✝ : Kernel α γ
hαγ : MeasurableSpace.CountableOrCountablyGenerated α γ
κ η : Kernel α γ
inst✝¹ : IsFiniteKernel κ
inst✝ : IsFiniteKernel η
a : α
h_eq_add : ((η.withDensity (κ.rnDeriv η)) a) (κ.mutuallySingularSetSlice η a)ᶜ = (κ a) (κ.mutuallySingularSetSlice η a)ᶜ
⊢ (η.withDensity (κ.rnDeriv η)) a = 0 ↔ (κ a) (κ.mutuallySingularSetSlice η a)ᶜ = 0
state_after: α : Type u_1
γ : Type u_2
mα : MeasurableSpace α
mγ : MeasurableSpace γ
κ✝ η✝ : Kernel α γ
hαγ : MeasurableSpace.CountableOrCountablyGenerated α γ
κ η : Kernel α γ
inst✝¹ : IsFiniteKernel κ
inst✝ : IsFiniteKernel η
a : α
h_eq_add : ((η.withDensity (κ.rnDeriv η)) a) (κ.mutuallySingularSetSlice η a)ᶜ = (κ a) (κ.mutuallySingularSetSlice η a)ᶜ
⊢ (η.withDensity (κ.rnDeriv η)) a = 0 ↔ ((η.withDensity (κ.rnDeriv η)) a) (κ.mutuallySingularSetSlice η a)ᶜ = 0
tactic: rw [← h_eq_add]
state_before: α : Type u_1
γ : Type u_2
mα : MeasurableSpace α
mγ : MeasurableSpace γ
κ✝ η✝ : Kernel α γ
hαγ : MeasurableSpace.CountableOrCountablyGenerated α γ
κ η : Kernel α γ
inst✝¹ : IsFiniteKernel κ
inst✝ : IsFiniteKernel η
a : α
h_eq_add : ((η.withDensity (κ.rnDeriv η)) a) (κ.mutuallySingularSetSlice η a)ᶜ = (κ a) (κ.mutuallySingularSetSlice η a)ᶜ
⊢ (η.withDensity (κ.rnDeriv η)) a = 0 ↔ ((η.withDensity (κ.rnDeriv η)) a) (κ.mutuallySingularSetSlice η a)ᶜ = 0
state_after: no goals
tactic: exact withDensity_rnDeriv_eq_zero_iff_apply_eq_zero κ η a