-- Mathlib/MeasureTheory/Measure/Haar/Quotient.lean
/-- This is the **Unfolding Trick**: Given a subgroup `Γ` of a group `G`, the integral of a
  function `f` on `G` times the lift to `G` of a function `g` on the quotient `G ⧸ Γ` with respect
  to a right-invariant measure `μ` on `G`, is equal to the integral over the quotient of the
  automorphization of `f` times `g`. -/
lemma QuotientGroup.integral_mul_eq_integral_automorphize_mul {K : Type*} [NormedField K]
    [NormedSpace ℝ K] [μ.IsMulRightInvariant] {f : G → K}
    (f_ℒ_1 : Integrable f μ) {g : G ⧸ Γ → K} (hg : AEStronglyMeasurable g μ_𝓕)
    (g_ℒ_infinity : essSup (fun x ↦ ↑‖g x‖ₑ) μ_𝓕 ≠ ∞)
    (F_ae_measurable : AEStronglyMeasurable (QuotientGroup.automorphize f) μ_𝓕) :
    ∫ x : G, g (x : G ⧸ Γ) * (f x) ∂μ
      = ∫ x : G ⧸ Γ, g x * (QuotientGroup.automorphize f x) ∂μ_𝓕

Proof state:
G : Type u_1
inst✝¹⁰ : Group G
inst✝⁹ : MeasurableSpace G
inst✝⁸ : TopologicalSpace G
inst✝⁷ : IsTopologicalGroup G
inst✝⁶ : BorelSpace G
μ : Measure G
Γ : Subgroup G
𝓕 : Set G
h𝓕 : IsFundamentalDomain (↥Γ.op) 𝓕 μ
inst✝⁵ : Countable ↥Γ
inst✝⁴ : MeasurableSpace (G ⧸ Γ)
inst✝³ : BorelSpace (G ⧸ Γ)
K : Type u_2
inst✝² : NormedField K
inst✝¹ : NormedSpace ℝ K
inst✝ : μ.IsMulRightInvariant
f : G → K
f_ℒ_1 : Integrable f μ
g : G ⧸ Γ → K
hg : AEStronglyMeasurable g (Measure.map mk (μ.restrict 𝓕))
g_ℒ_infinity : essSup (fun x => ‖g x‖ₑ) (Measure.map mk (μ.restrict 𝓕)) ≠ ⊤
F_ae_measurable : AEStronglyMeasurable (automorphize f) (Measure.map mk (μ.restrict 𝓕))
π : G → G ⧸ Γ := mk
meas_π : Measurable π
H₀ : automorphize (g ∘ π * f) = g * automorphize f
H₁ : Integrable (g ∘ π * f) μ
⊢ ∫ (x : G), g (π x) * f x ∂μ = ∫ (x : G ⧸ Γ), automorphize (g ∘ π * f) x ∂Measure.map mk (μ.restrict 𝓕)

Tactic:
have H₂ : AEStronglyMeasurable (QuotientGroup.automorphize ((g ∘ π) * f)) μ_𝓕 := by
  simp_rw [H₀]
  exact hg.mul F_ae_measurable

Result:
G : Type u_1
inst✝¹⁰ : Group G
inst✝⁹ : MeasurableSpace G
inst✝⁸ : TopologicalSpace G
inst✝⁷ : IsTopologicalGroup G
inst✝⁶ : BorelSpace G
μ : Measure G
Γ : Subgroup G
𝓕 : Set G
h𝓕 : IsFundamentalDomain (↥Γ.op) 𝓕 μ
inst✝⁵ : Countable ↥Γ
inst✝⁴ : MeasurableSpace (G ⧸ Γ)
inst✝³ : BorelSpace (G ⧸ Γ)
K : Type u_2
inst✝² : NormedField K
inst✝¹ : NormedSpace ℝ K
inst✝ : μ.IsMulRightInvariant
f : G → K
f_ℒ_1 : Integrable f μ
g : G ⧸ Γ → K
hg : AEStronglyMeasurable g (Measure.map mk (μ.restrict 𝓕))
g_ℒ_infinity : essSup (fun x => ‖g x‖ₑ) (Measure.map mk (μ.restrict 𝓕)) ≠ ⊤
F_ae_measurable : AEStronglyMeasurable (automorphize f) (Measure.map mk (μ.restrict 𝓕))
π : G → G ⧸ Γ := mk
meas_π : Measurable π
H₀ : automorphize (g ∘ π * f) = g * automorphize f
H₁ : Integrable (g ∘ π * f) μ
H₂ : AEStronglyMeasurable (automorphize (g ∘ π * f)) (Measure.map mk (μ.restrict 𝓕))
⊢ ∫ (x : G), g (π x) * f x ∂μ = ∫ (x : G ⧸ Γ), automorphize (g ∘ π * f) x ∂Measure.map mk (μ.restrict 𝓕)

Proof state:
G : Type u_1
inst✝¹⁰ : Group G
inst✝⁹ : MeasurableSpace G
inst✝⁸ : TopologicalSpace G
inst✝⁷ : IsTopologicalGroup G
inst✝⁶ : BorelSpace G
μ : Measure G
Γ : Subgroup G
𝓕 : Set G
h𝓕 : IsFundamentalDomain (↥Γ.op) 𝓕 μ
inst✝⁵ : Countable ↥Γ
inst✝⁴ : MeasurableSpace (G ⧸ Γ)
inst✝³ : BorelSpace (G ⧸ Γ)
K : Type u_2
inst✝² : NormedField K
inst✝¹ : NormedSpace ℝ K
inst✝ : μ.IsMulRightInvariant
f : G → K
f_ℒ_1 : Integrable f μ
g : G ⧸ Γ → K
hg : AEStronglyMeasurable g (Measure.map mk (μ.restrict 𝓕))
g_ℒ_infinity : essSup (fun x => ‖g x‖ₑ) (Measure.map mk (μ.restrict 𝓕)) ≠ ⊤
F_ae_measurable : AEStronglyMeasurable (automorphize f) (Measure.map mk (μ.restrict 𝓕))
π : G → G ⧸ Γ := mk
meas_π : Measurable π
H₀ : automorphize (g ∘ π * f) = g * automorphize f
this : AEStronglyMeasurable (fun x => g ↑x) μ
⊢ Integrable (g ∘ π * f) μ

Tactic:
refine Integrable.essSup_smul f_ℒ_1 this ?_

Result:
G : Type u_1
inst✝¹⁰ : Group G
inst✝⁹ : MeasurableSpace G
inst✝⁸ : TopologicalSpace G
inst✝⁷ : IsTopologicalGroup G
inst✝⁶ : BorelSpace G
μ : Measure G
Γ : Subgroup G
𝓕 : Set G
h𝓕 : IsFundamentalDomain (↥Γ.op) 𝓕 μ
inst✝⁵ : Countable ↥Γ
inst✝⁴ : MeasurableSpace (G ⧸ Γ)
inst✝³ : BorelSpace (G ⧸ Γ)
K : Type u_2
inst✝² : NormedField K
inst✝¹ : NormedSpace ℝ K
inst✝ : μ.IsMulRightInvariant
f : G → K
f_ℒ_1 : Integrable f μ
g : G ⧸ Γ → K
hg : AEStronglyMeasurable g (Measure.map mk (μ.restrict 𝓕))
g_ℒ_infinity : essSup (fun x => ‖g x‖ₑ) (Measure.map mk (μ.restrict 𝓕)) ≠ ⊤
F_ae_measurable : AEStronglyMeasurable (automorphize f) (Measure.map mk (μ.restrict 𝓕))
π : G → G ⧸ Γ := mk
meas_π : Measurable π
H₀ : automorphize (g ∘ π * f) = g * automorphize f
this : AEStronglyMeasurable (fun x => g ↑x) μ
⊢ essSup (fun x => ‖g ↑x‖ₑ) μ ≠ ⊤

Proof state:
G : Type u_1
inst✝¹⁰ : Group G
inst✝⁹ : MeasurableSpace G
inst✝⁸ : TopologicalSpace G
inst✝⁷ : IsTopologicalGroup G
inst✝⁶ : BorelSpace G
μ : Measure G
Γ : Subgroup G
𝓕 : Set G
h𝓕 : IsFundamentalDomain (↥Γ.op) 𝓕 μ
inst✝⁵ : Countable ↥Γ
inst✝⁴ : MeasurableSpace (G ⧸ Γ)
inst✝³ : BorelSpace (G ⧸ Γ)
K : Type u_2
inst✝² : NormedField K
inst✝¹ : NormedSpace ℝ K
inst✝ : μ.IsMulRightInvariant
f : G → K
f_ℒ_1 : Integrable f μ
g : G ⧸ Γ → K
hg : AEStronglyMeasurable g (Measure.map mk (μ.restrict 𝓕))
g_ℒ_infinity : essSup (fun x => ‖g x‖ₑ) (Measure.map mk (μ.restrict 𝓕)) ≠ ⊤
F_ae_measurable : AEStronglyMeasurable (automorphize f) (Measure.map mk (μ.restrict 𝓕))
π : G → G ⧸ Γ := mk
meas_π : Measurable π
H₀ : automorphize (g ∘ π * f) = g * automorphize f
H₁ : Integrable (g ∘ π * f) μ
⊢ AEStronglyMeasurable (automorphize (g ∘ π * f)) (Measure.map mk (μ.restrict 𝓕))

Tactic:
simp_rw [H₀]

Result:
G : Type u_1
inst✝¹⁰ : Group G
inst✝⁹ : MeasurableSpace G
inst✝⁸ : TopologicalSpace G
inst✝⁷ : IsTopologicalGroup G
inst✝⁶ : BorelSpace G
μ : Measure G
Γ : Subgroup G
𝓕 : Set G
h𝓕 : IsFundamentalDomain (↥Γ.op) 𝓕 μ
inst✝⁵ : Countable ↥Γ
inst✝⁴ : MeasurableSpace (G ⧸ Γ)
inst✝³ : BorelSpace (G ⧸ Γ)
K : Type u_2
inst✝² : NormedField K
inst✝¹ : NormedSpace ℝ K
inst✝ : μ.IsMulRightInvariant
f : G → K
f_ℒ_1 : Integrable f μ
g : G ⧸ Γ → K
hg : AEStronglyMeasurable g (Measure.map mk (μ.restrict 𝓕))
g_ℒ_infinity : essSup (fun x => ‖g x‖ₑ) (Measure.map mk (μ.restrict 𝓕)) ≠ ⊤
F_ae_measurable : AEStronglyMeasurable (automorphize f) (Measure.map mk (μ.restrict 𝓕))
π : G → G ⧸ Γ := mk
meas_π : Measurable π
H₀ : automorphize (g ∘ π * f) = g * automorphize f
H₁ : Integrable (g ∘ π * f) μ
⊢ AEStronglyMeasurable (g * automorphize f) (Measure.map mk (μ.restrict 𝓕))