-- Mathlib/MeasureTheory/Measure/Haar/Quotient.lean
/-- This is a simple version of the **Unfolding Trick**: Given a subgroup `Γ` of a group `G`, the
  integral of a function `f` on `G` with respect to a right-invariant measure `μ` is equal to the
  integral over the quotient `G ⧸ Γ` of the automorphization of `f`. -/
@[to_additive "This is a simple version of the **Unfolding Trick**: Given a subgroup `Γ` of an
  additive group `G`, the integral of a function `f` on `G` with respect to a right-invariant
  measure `μ` is equal to the integral over the quotient `G ⧸ Γ` of the automorphization of `f`."]
lemma QuotientGroup.integral_eq_integral_automorphize {E : Type*} [NormedAddCommGroup E]
    [NormedSpace ℝ E] [μ.IsMulRightInvariant] {f : G → E}
    (hf₁ : Integrable f μ) (hf₂ : AEStronglyMeasurable (automorphize f) μ_𝓕) :
    ∫ x : G, f x ∂μ = ∫ x : G ⧸ Γ, 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)
E : Type u_2
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : μ.IsMulRightInvariant
f : G → E
hf₁ : Integrable f μ
hf₂ : AEStronglyMeasurable (automorphize f) (Measure.map mk (μ.restrict 𝓕))
⊢ ∑' (γ : ↥Γ.op), ∫ (x : G) in 𝓕, f (γ • x) ∂μ = ∫ (x : G) in 𝓕, ∑' (γ : ↥Γ.op), f (γ • x) ∂μ

Tactic:
rw [integral_tsum]

Result:
case hf
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)
E : Type u_2
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : μ.IsMulRightInvariant
f : G → E
hf₁ : Integrable f μ
hf₂ : AEStronglyMeasurable (automorphize f) (Measure.map mk (μ.restrict 𝓕))
⊢ ∀ (i : ↥Γ.op), AEStronglyMeasurable (fun x => f (i • x)) (μ.restrict 𝓕)

case hf'
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)
E : Type u_2
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : μ.IsMulRightInvariant
f : G → E
hf₁ : Integrable f μ
hf₂ : AEStronglyMeasurable (automorphize f) (Measure.map mk (μ.restrict 𝓕))
⊢ ∑' (i : ↥Γ.op), ∫⁻ (a : G) in 𝓕, ‖f (i • a)‖ₑ ∂μ ≠ ⊤

Proof state:
case hf
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)
E : Type u_2
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : μ.IsMulRightInvariant
f : G → E
hf₁ : Integrable f μ
hf₂ : AEStronglyMeasurable (automorphize f) (Measure.map mk (μ.restrict 𝓕))
⊢ ∀ (i : ↥Γ.op), AEStronglyMeasurable (fun x => f (i • x)) (μ.restrict 𝓕)

Tactic:
exact fun i ↦ (hf₁.1.comp_quasiMeasurePreserving
  (measurePreserving_smul i μ).quasiMeasurePreserving).restrict

Result:
no goals

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)
E : Type u_2
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : μ.IsMulRightInvariant
f : G → E
hf₁ : Integrable f μ
hf₂ : AEStronglyMeasurable (automorphize f) (Measure.map mk (μ.restrict 𝓕))
⊢ ∫ (x : G), f x ∂μ = ∫ (x : G ⧸ Γ), automorphize f x ∂Measure.map mk (μ.restrict 𝓕)

Tactic:
calc ∫ x : G, f x ∂μ = ∑' γ : Γ.op, ∫ x in 𝓕, f (γ • x) ∂μ :=
  h𝓕.integral_eq_tsum'' f hf₁
  _ = ∫ x in 𝓕, ∑' γ : Γ.op, f (γ • x) ∂μ := ?_
  _ = ∫ x : G ⧸ Γ, automorphize f x ∂μ_𝓕 :=
    (integral_map continuous_quotient_mk'.aemeasurable hf₂).symm

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)
E : Type u_2
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : μ.IsMulRightInvariant
f : G → E
hf₁ : Integrable f μ
hf₂ : AEStronglyMeasurable (automorphize f) (Measure.map mk (μ.restrict 𝓕))
⊢ ∑' (γ : ↥Γ.op), ∫ (x : G) in 𝓕, f (γ • x) ∂μ = ∫ (x : G) in 𝓕, ∑' (γ : ↥Γ.op), f (γ • x) ∂μ

Proof state:
case hf'
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)
E : Type u_2
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : μ.IsMulRightInvariant
f : G → E
hf₁ : Integrable f μ
hf₂ : AEStronglyMeasurable (automorphize f) (Measure.map mk (μ.restrict 𝓕))
⊢ ∫⁻ (x : G), ‖f x‖ₑ ∂μ ≠ ⊤

Tactic:
exact ne_of_lt hf₁.2

Result:
no goals