-- Mathlib/Probability/Kernel/Disintegration/Unique.lean
theorem eq_condKernel_of_measure_eq_compProd (κ : Kernel α Ω) [IsFiniteKernel κ]
    (hκ : ρ = ρ.fst ⊗ₘ κ) :
    ∀ᵐ x ∂ρ.fst, κ x = ρ.condKernel x

Proof state:
α : Type u_1
Ω : Type u_3
mα : MeasurableSpace α
inst✝⁴ : MeasurableSpace Ω
inst✝³ : StandardBorelSpace Ω
inst✝² : Nonempty Ω
ρ : Measure (α × Ω)
inst✝¹ : IsFiniteMeasure ρ
κ : Kernel α Ω
inst✝ : IsFiniteKernel κ
hκ : ρ = ρ.fst ⊗ₘ κ
f : Ω → ℝ := embeddingReal Ω
hf : MeasurableEmbedding (embeddingReal Ω)
ρ' : Measure (α × ℝ) := Measure.map (Prod.map id f) ρ
hρ'def : ρ' = Measure.map (Prod.map id f) ρ
hρ' : ρ'.fst = ρ.fst
hρ'' : ∀ᵐ (x : α) ∂ρ.fst, (κ.map f) x = ρ'.condKernel x
⊢ ∀ᵐ (x : α) ∂ρ.fst, κ x = ρ.condKernel x

Tactic:
suffices ∀ᵐ x ∂ρ.fst, ∀ s, MeasurableSet s → ρ'.condKernel x s = ρ.condKernel x (f ⁻¹' s) by
  filter_upwards [hρ'', this] with x hx h
  rw [Kernel.map_apply _ hf.measurable] at hx
  ext s hs
  rw [← Set.preimage_image_eq s hf.injective,
    ← Measure.map_apply hf.measurable <| hf.measurableSet_image.2 hs, hx,
    h _ <| hf.measurableSet_image.2 hs]

Result:
α : Type u_1
Ω : Type u_3
mα : MeasurableSpace α
inst✝⁴ : MeasurableSpace Ω
inst✝³ : StandardBorelSpace Ω
inst✝² : Nonempty Ω
ρ : Measure (α × Ω)
inst✝¹ : IsFiniteMeasure ρ
κ : Kernel α Ω
inst✝ : IsFiniteKernel κ
hκ : ρ = ρ.fst ⊗ₘ κ
f : Ω → ℝ := embeddingReal Ω
hf : MeasurableEmbedding (embeddingReal Ω)
ρ' : Measure (α × ℝ) := Measure.map (Prod.map id f) ρ
hρ'def : ρ' = Measure.map (Prod.map id f) ρ
hρ' : ρ'.fst = ρ.fst
hρ'' : ∀ᵐ (x : α) ∂ρ.fst, (κ.map f) x = ρ'.condKernel x
⊢ ∀ᵐ (x : α) ∂ρ.fst, ∀ (s : Set ℝ), MeasurableSet s → (ρ'.condKernel x) s = (ρ.condKernel x) (f ⁻¹' s)

Proof state:
case h
α : Type u_1
Ω : Type u_3
mα : MeasurableSpace α
inst✝⁴ : MeasurableSpace Ω
inst✝³ : StandardBorelSpace Ω
inst✝² : Nonempty Ω
ρ : Measure (α × Ω)
inst✝¹ : IsFiniteMeasure ρ
κ : Kernel α Ω
inst✝ : IsFiniteKernel κ
hκ : ρ = ρ.fst ⊗ₘ κ
f : Ω → ℝ := embeddingReal Ω
hf : MeasurableEmbedding (embeddingReal Ω)
ρ' : Measure (α × ℝ) := Measure.map (Prod.map id f) ρ
hρ'def : ρ' = Measure.map (Prod.map id f) ρ
hρ' : ρ'.fst = ρ.fst
hρ'' : ∀ᵐ (x : α) ∂ρ.fst, (κ.map f) x = ρ'.condKernel x
s : Set (α × ℝ)
hs : MeasurableSet s
⊢ MeasurableSet (Prod.map id (embeddingReal Ω) ⁻¹' s)

Tactic:
exact measurable_id.prod_map hf.measurable hs

Result:
no goals

Proof state:
case h
α : Type u_1
Ω : Type u_3
mα : MeasurableSpace α
inst✝⁴ : MeasurableSpace Ω
inst✝³ : StandardBorelSpace Ω
inst✝² : Nonempty Ω
ρ : Measure (α × Ω)
inst✝¹ : IsFiniteMeasure ρ
κ : Kernel α Ω
inst✝ : IsFiniteKernel κ
hκ : ρ = ρ.fst ⊗ₘ κ
f : Ω → ℝ := embeddingReal Ω
hf : MeasurableEmbedding (embeddingReal Ω)
ρ' : Measure (α × ℝ) := Measure.map (Prod.map id f) ρ
hρ'def : ρ' = Measure.map (Prod.map id f) ρ
hρ' : ρ'.fst = ρ.fst
s : Set (α × ℝ)
hs : MeasurableSet s
⊢ ∫⁻ (a : α), (κ a) (Prod.mk a ⁻¹' (Prod.map id (embeddingReal Ω) ⁻¹' s)) ∂ρ.fst =
    ∫⁻ (a : α), ((κ.map f) a) (Prod.mk a ⁻¹' s) ∂ρ.fst

Tactic:
congr with a

Result:
case h.e_f.h
α : Type u_1
Ω : Type u_3
mα : MeasurableSpace α
inst✝⁴ : MeasurableSpace Ω
inst✝³ : StandardBorelSpace Ω
inst✝² : Nonempty Ω
ρ : Measure (α × Ω)
inst✝¹ : IsFiniteMeasure ρ
κ : Kernel α Ω
inst✝ : IsFiniteKernel κ
hκ : ρ = ρ.fst ⊗ₘ κ
f : Ω → ℝ := embeddingReal Ω
hf : MeasurableEmbedding (embeddingReal Ω)
ρ' : Measure (α × ℝ) := Measure.map (Prod.map id f) ρ
hρ'def : ρ' = Measure.map (Prod.map id f) ρ
hρ' : ρ'.fst = ρ.fst
s : Set (α × ℝ)
hs : MeasurableSet s
a : α
⊢ (κ a) (Prod.mk a ⁻¹' (Prod.map id (embeddingReal Ω) ⁻¹' s)) = ((κ.map f) a) (Prod.mk a ⁻¹' s)

Proof state:
case h.e_f.h
α : Type u_1
Ω : Type u_3
mα : MeasurableSpace α
inst✝⁴ : MeasurableSpace Ω
inst✝³ : StandardBorelSpace Ω
inst✝² : Nonempty Ω
ρ : Measure (α × Ω)
inst✝¹ : IsFiniteMeasure ρ
κ : Kernel α Ω
inst✝ : IsFiniteKernel κ
hκ : ρ = ρ.fst ⊗ₘ κ
f : Ω → ℝ := embeddingReal Ω
hf : MeasurableEmbedding (embeddingReal Ω)
ρ' : Measure (α × ℝ) := Measure.map (Prod.map id f) ρ
hρ'def : ρ' = Measure.map (Prod.map id f) ρ
hρ' : ρ'.fst = ρ.fst
s : Set (α × ℝ)
hs : MeasurableSet s
a : α
⊢ (κ a) (Prod.mk a ⁻¹' (Prod.map id (embeddingReal Ω) ⁻¹' s)) = (κ a) (embeddingReal Ω ⁻¹' (Prod.mk a ⁻¹' s))

case h.e_f.h.hs
α : Type u_1
Ω : Type u_3
mα : MeasurableSpace α
inst✝⁴ : MeasurableSpace Ω
inst✝³ : StandardBorelSpace Ω
inst✝² : Nonempty Ω
ρ : Measure (α × Ω)
inst✝¹ : IsFiniteMeasure ρ
κ : Kernel α Ω
inst✝ : IsFiniteKernel κ
hκ : ρ = ρ.fst ⊗ₘ κ
f : Ω → ℝ := embeddingReal Ω
hf : MeasurableEmbedding (embeddingReal Ω)
ρ' : Measure (α × ℝ) := Measure.map (Prod.map id f) ρ
hρ'def : ρ' = Measure.map (Prod.map id f) ρ
hρ' : ρ'.fst = ρ.fst
s : Set (α × ℝ)
hs : MeasurableSet s
a : α
⊢ MeasurableSet (Prod.mk a ⁻¹' s)

Tactic:
exacts [rfl, measurable_prod_mk_left hs]

Result:
no goals

Proof state:
α : Type u_1
Ω : Type u_3
mα : MeasurableSpace α
inst✝⁴ : MeasurableSpace Ω
inst✝³ : StandardBorelSpace Ω
inst✝² : Nonempty Ω
ρ : Measure (α × Ω)
inst✝¹ : IsFiniteMeasure ρ
κ : Kernel α Ω
inst✝ : IsFiniteKernel κ
hκ : ρ = ρ.fst ⊗ₘ κ
f : Ω → ℝ := embeddingReal Ω
hf : MeasurableEmbedding (embeddingReal Ω)
ρ' : Measure (α × ℝ) := Measure.map (Prod.map id f) ρ
hρ'def : ρ' = Measure.map (Prod.map id f) ρ
hρ' : ρ'.fst = ρ.fst
hρ'' : ∀ᵐ (x : α) ∂ρ.fst, (κ.map f) x = ρ'.condKernel x
this : Measure.map (Prod.map id f) ρ = ρ'.fst ⊗ₘ ρ.condKernel.map f
heq : ∀ᵐ (x : α) ∂ρ.fst, (ρ.condKernel.map f) x = (Measure.map (Prod.map id f) ρ).condKernel x
⊢ ∀ᵐ (x : α) ∂ρ.fst, ∀ (s : Set ℝ), MeasurableSet s → (ρ'.condKernel x) s = (ρ.condKernel x) (f ⁻¹' s)

Tactic:
filter_upwards [heq] with x hx s hs

Result:
case h
α : Type u_1
Ω : Type u_3
mα : MeasurableSpace α
inst✝⁴ : MeasurableSpace Ω
inst✝³ : StandardBorelSpace Ω
inst✝² : Nonempty Ω
ρ : Measure (α × Ω)
inst✝¹ : IsFiniteMeasure ρ
κ : Kernel α Ω
inst✝ : IsFiniteKernel κ
hκ : ρ = ρ.fst ⊗ₘ κ
f : Ω → ℝ := embeddingReal Ω
hf : MeasurableEmbedding (embeddingReal Ω)
ρ' : Measure (α × ℝ) := Measure.map (Prod.map id f) ρ
hρ'def : ρ' = Measure.map (Prod.map id f) ρ
hρ' : ρ'.fst = ρ.fst
hρ'' : ∀ᵐ (x : α) ∂ρ.fst, (κ.map f) x = ρ'.condKernel x
this : Measure.map (Prod.map id f) ρ = ρ'.fst ⊗ₘ ρ.condKernel.map f
heq : ∀ᵐ (x : α) ∂ρ.fst, (ρ.condKernel.map f) x = (Measure.map (Prod.map id f) ρ).condKernel x
x : α
hx : (ρ.condKernel.map f) x = (Measure.map (Prod.map id f) ρ).condKernel x
s : Set ℝ
hs : MeasurableSet s
⊢ (ρ'.condKernel x) s = (ρ.condKernel x) (f ⁻¹' s)