-- Mathlib/MeasureTheory/Integral/IntegrableOn.lean
theorem ContinuousOn.aemeasurable [TopologicalSpace α] [OpensMeasurableSpace α] [MeasurableSpace β]
    [TopologicalSpace β] [BorelSpace β] {f : α → β} {s : Set α} {μ : Measure α}
    (hf : ContinuousOn f s) (hs : MeasurableSet s) : AEMeasurable f (μ.restrict s)

Proof state:
α : Type u_1
β : Type u_2
inst✝⁵ : MeasurableSpace α
inst✝⁴ : TopologicalSpace α
inst✝³ : OpensMeasurableSpace α
inst✝² : MeasurableSpace β
inst✝¹ : TopologicalSpace β
inst✝ : BorelSpace β
f : α → β
s : Set α
μ : Measure α
hf : ContinuousOn f s
hs : MeasurableSet s
a✝ : Nontrivial α
inhabited_h : Inhabited α
this : (s.piecewise f fun x => f default) =ᶠ[ae (μ.restrict s)] f
⊢ AEMeasurable f (μ.restrict s)

Tactic:
refine ⟨Set.piecewise s f fun _ => f default, ?_, this.symm⟩

Result:
α : Type u_1
β : Type u_2
inst✝⁵ : MeasurableSpace α
inst✝⁴ : TopologicalSpace α
inst✝³ : OpensMeasurableSpace α
inst✝² : MeasurableSpace β
inst✝¹ : TopologicalSpace β
inst✝ : BorelSpace β
f : α → β
s : Set α
μ : Measure α
hf : ContinuousOn f s
hs : MeasurableSet s
a✝ : Nontrivial α
inhabited_h : Inhabited α
this : (s.piecewise f fun x => f default) =ᶠ[ae (μ.restrict s)] f
⊢ Measurable (s.piecewise f fun x => f default)

Proof state:
α : Type u_1
β : Type u_2
inst✝⁵ : MeasurableSpace α
inst✝⁴ : TopologicalSpace α
inst✝³ : OpensMeasurableSpace α
inst✝² : MeasurableSpace β
inst✝¹ : TopologicalSpace β
inst✝ : BorelSpace β
f : α → β
s : Set α
μ : Measure α
hf : ContinuousOn f s
hs : MeasurableSet s
a✝ : Nontrivial α
⊢ AEMeasurable f (μ.restrict s)

Tactic:
inhabit α

Result:
α : Type u_1
β : Type u_2
inst✝⁵ : MeasurableSpace α
inst✝⁴ : TopologicalSpace α
inst✝³ : OpensMeasurableSpace α
inst✝² : MeasurableSpace β
inst✝¹ : TopologicalSpace β
inst✝ : BorelSpace β
f : α → β
s : Set α
μ : Measure α
hf : ContinuousOn f s
hs : MeasurableSet s
a✝ : Nontrivial α
inhabited_h : Inhabited α
⊢ AEMeasurable f (μ.restrict s)

Proof state:
α : Type u_1
β : Type u_2
inst✝⁵ : MeasurableSpace α
inst✝⁴ : TopologicalSpace α
inst✝³ : OpensMeasurableSpace α
inst✝² : MeasurableSpace β
inst✝¹ : TopologicalSpace β
inst✝ : BorelSpace β
f : α → β
s : Set α
μ : Measure α
hf : ContinuousOn f s
hs : MeasurableSet s
⊢ AEMeasurable f (μ.restrict s)

Tactic:
classical
nontriviality α; inhabit α
have : (Set.piecewise s f fun _ => f default) =ᵐ[μ.restrict s] f := piecewise_ae_eq_restrict hs
refine ⟨Set.piecewise s f fun _ => f default, ?_, this.symm⟩
apply measurable_of_isOpen
intro t ht
obtain ⟨u, u_open, hu⟩ : ∃ u : Set α, IsOpen u ∧ f ⁻¹' t ∩ s = u ∩ s :=
  _root_.continuousOn_iff'.1 hf t ht
rw [piecewise_preimage, Set.ite, hu]
exact (u_open.measurableSet.inter hs).union ((measurable_const ht.measurableSet).diff hs)

Result:
no goals

Proof state:
case hf.intro.intro
α : Type u_1
β : Type u_2
inst✝⁵ : MeasurableSpace α
inst✝⁴ : TopologicalSpace α
inst✝³ : OpensMeasurableSpace α
inst✝² : MeasurableSpace β
inst✝¹ : TopologicalSpace β
inst✝ : BorelSpace β
f : α → β
s : Set α
μ : Measure α
hf : ContinuousOn f s
hs : MeasurableSet s
a✝ : Nontrivial α
inhabited_h : Inhabited α
this : (s.piecewise f fun x => f default) =ᶠ[ae (μ.restrict s)] f
t : Set β
ht : IsOpen t
u : Set α
u_open : IsOpen u
hu : f ⁻¹' t ∩ s = u ∩ s
⊢ MeasurableSet (u ∩ s ∪ (fun x => f default) ⁻¹' t \ s)

Tactic:
exact (u_open.measurableSet.inter hs).union ((measurable_const ht.measurableSet).diff hs)

Result:
no goals

Proof state:
α : Type u_1
β : Type u_2
inst✝⁵ : MeasurableSpace α
inst✝⁴ : TopologicalSpace α
inst✝³ : OpensMeasurableSpace α
inst✝² : MeasurableSpace β
inst✝¹ : TopologicalSpace β
inst✝ : BorelSpace β
f : α → β
s : Set α
μ : Measure α
hf : ContinuousOn f s
hs : MeasurableSet s
a✝ : Nontrivial α
inhabited_h : Inhabited α
this : (s.piecewise f fun x => f default) =ᶠ[ae (μ.restrict s)] f
⊢ Measurable (s.piecewise f fun x => f default)

Tactic:
apply measurable_of_isOpen

Result:
case hf
α : Type u_1
β : Type u_2
inst✝⁵ : MeasurableSpace α
inst✝⁴ : TopologicalSpace α
inst✝³ : OpensMeasurableSpace α
inst✝² : MeasurableSpace β
inst✝¹ : TopologicalSpace β
inst✝ : BorelSpace β
f : α → β
s : Set α
μ : Measure α
hf : ContinuousOn f s
hs : MeasurableSet s
a✝ : Nontrivial α
inhabited_h : Inhabited α
this : (s.piecewise f fun x => f default) =ᶠ[ae (μ.restrict s)] f
⊢ ∀ (s_1 : Set β), IsOpen s_1 → MeasurableSet ((s.piecewise f fun x => f default) ⁻¹' s_1)

Proof state:
α : Type u_1
β : Type u_2
inst✝⁵ : MeasurableSpace α
inst✝⁴ : TopologicalSpace α
inst✝³ : OpensMeasurableSpace α
inst✝² : MeasurableSpace β
inst✝¹ : TopologicalSpace β
inst✝ : BorelSpace β
f : α → β
s : Set α
μ : Measure α
hf : ContinuousOn f s
hs : MeasurableSet s
⊢ AEMeasurable f (μ.restrict s)

Tactic:
nontriviality α

Result:
α : Type u_1
β : Type u_2
inst✝⁵ : MeasurableSpace α
inst✝⁴ : TopologicalSpace α
inst✝³ : OpensMeasurableSpace α
inst✝² : MeasurableSpace β
inst✝¹ : TopologicalSpace β
inst✝ : BorelSpace β
f : α → β
s : Set α
μ : Measure α
hf : ContinuousOn f s
hs : MeasurableSet s
a✝ : Nontrivial α
⊢ AEMeasurable f (μ.restrict s)

Proof state:
case hf.intro.intro
α : Type u_1
β : Type u_2
inst✝⁵ : MeasurableSpace α
inst✝⁴ : TopologicalSpace α
inst✝³ : OpensMeasurableSpace α
inst✝² : MeasurableSpace β
inst✝¹ : TopologicalSpace β
inst✝ : BorelSpace β
f : α → β
s : Set α
μ : Measure α
hf : ContinuousOn f s
hs : MeasurableSet s
a✝ : Nontrivial α
inhabited_h : Inhabited α
this : (s.piecewise f fun x => f default) =ᶠ[ae (μ.restrict s)] f
t : Set β
ht : IsOpen t
u : Set α
u_open : IsOpen u
hu : f ⁻¹' t ∩ s = u ∩ s
⊢ MeasurableSet ((s.piecewise f fun x => f default) ⁻¹' t)

Tactic:
rw [piecewise_preimage, Set.ite, hu]

Result:
case hf.intro.intro
α : Type u_1
β : Type u_2
inst✝⁵ : MeasurableSpace α
inst✝⁴ : TopologicalSpace α
inst✝³ : OpensMeasurableSpace α
inst✝² : MeasurableSpace β
inst✝¹ : TopologicalSpace β
inst✝ : BorelSpace β
f : α → β
s : Set α
μ : Measure α
hf : ContinuousOn f s
hs : MeasurableSet s
a✝ : Nontrivial α
inhabited_h : Inhabited α
this : (s.piecewise f fun x => f default) =ᶠ[ae (μ.restrict s)] f
t : Set β
ht : IsOpen t
u : Set α
u_open : IsOpen u
hu : f ⁻¹' t ∩ s = u ∩ s
⊢ MeasurableSet (u ∩ s ∪ (fun x => f default) ⁻¹' t \ s)

Proof state:
case hf
α : Type u_1
β : Type u_2
inst✝⁵ : MeasurableSpace α
inst✝⁴ : TopologicalSpace α
inst✝³ : OpensMeasurableSpace α
inst✝² : MeasurableSpace β
inst✝¹ : TopologicalSpace β
inst✝ : BorelSpace β
f : α → β
s : Set α
μ : Measure α
hf : ContinuousOn f s
hs : MeasurableSet s
a✝ : Nontrivial α
inhabited_h : Inhabited α
this : (s.piecewise f fun x => f default) =ᶠ[ae (μ.restrict s)] f
⊢ ∀ (s_1 : Set β), IsOpen s_1 → MeasurableSet ((s.piecewise f fun x => f default) ⁻¹' s_1)

Tactic:
intro t ht

Result:
case hf
α : Type u_1
β : Type u_2
inst✝⁵ : MeasurableSpace α
inst✝⁴ : TopologicalSpace α
inst✝³ : OpensMeasurableSpace α
inst✝² : MeasurableSpace β
inst✝¹ : TopologicalSpace β
inst✝ : BorelSpace β
f : α → β
s : Set α
μ : Measure α
hf : ContinuousOn f s
hs : MeasurableSet s
a✝ : Nontrivial α
inhabited_h : Inhabited α
this : (s.piecewise f fun x => f default) =ᶠ[ae (μ.restrict s)] f
t : Set β
ht : IsOpen t
⊢ MeasurableSet ((s.piecewise f fun x => f default) ⁻¹' t)