-- Mathlib/MeasureTheory/Measure/Haar/Quotient.lean
theorem IsFundamentalDomain.QuotientMeasureEqMeasurePreimage_HaarMeasure {𝓕 : Set G}
    (h𝓕 : IsFundamentalDomain Γ.op 𝓕 ν) [IsMulLeftInvariant μ] [SigmaFinite μ]
    {V : Set (G ⧸ Γ)} (hV : (interior V).Nonempty) (meas_V : MeasurableSet V)
    (hμK : μ V = ν ((π ⁻¹' V) ∩ 𝓕)) (neTopV : μ V ≠ ⊤) :
    QuotientMeasureEqMeasurePreimage ν μ

Proof state:
case neZeroV.a
G : Type u_1
inst✝¹⁴ : Group G
inst✝¹³ : MeasurableSpace G
inst✝¹² : TopologicalSpace G
inst✝¹¹ : IsTopologicalGroup G
inst✝¹⁰ : BorelSpace G
inst✝⁹ : PolishSpace G
Γ : Subgroup G
inst✝⁸ : Γ.Normal
inst✝⁷ : T2Space (G ⧸ Γ)
inst✝⁶ : SecondCountableTopology (G ⧸ Γ)
μ : Measure (G ⧸ Γ)
inst✝⁵ : Countable ↥Γ
ν : Measure G
inst✝⁴ : ν.IsHaarMeasure
inst✝³ : ν.IsMulRightInvariant
inst✝² : SigmaFinite ν
𝓕 : Set G
h𝓕 : IsFundamentalDomain (↥Γ.op) 𝓕 ν
inst✝¹ : μ.IsMulLeftInvariant
inst✝ : SigmaFinite μ
V : Set (G ⧸ Γ)
hV : (interior V).Nonempty
meas_V : MeasurableSet V
hμK : μ V = ν (QuotientGroup.mk ⁻¹' V ∩ 𝓕)
neTopV : μ V ≠ ⊤
c_eq_zero : ν (QuotientGroup.mk ⁻¹' V ∩ 𝓕) = 0
⊢ IsOpen (interior (QuotientGroup.mk ⁻¹' V))

Tactic:
simp

Result:
no goals

Proof state:
case neZeroV
G : Type u_1
inst✝¹⁴ : Group G
inst✝¹³ : MeasurableSpace G
inst✝¹² : TopologicalSpace G
inst✝¹¹ : IsTopologicalGroup G
inst✝¹⁰ : BorelSpace G
inst✝⁹ : PolishSpace G
Γ : Subgroup G
inst✝⁸ : Γ.Normal
inst✝⁷ : T2Space (G ⧸ Γ)
inst✝⁶ : SecondCountableTopology (G ⧸ Γ)
μ : Measure (G ⧸ Γ)
inst✝⁵ : Countable ↥Γ
ν : Measure G
inst✝⁴ : ν.IsHaarMeasure
inst✝³ : ν.IsMulRightInvariant
inst✝² : SigmaFinite ν
𝓕 : Set G
h𝓕 : IsFundamentalDomain (↥Γ.op) 𝓕 ν
inst✝¹ : μ.IsMulLeftInvariant
inst✝ : SigmaFinite μ
V : Set (G ⧸ Γ)
hV : (interior V).Nonempty
meas_V : MeasurableSet V
hμK : μ V = ν (QuotientGroup.mk ⁻¹' V ∩ 𝓕)
neTopV : μ V ≠ ⊤
⊢ ν (QuotientGroup.mk ⁻¹' V ∩ 𝓕) ≠ 0

Tactic:
intro c_eq_zero

Result:
case neZeroV
G : Type u_1
inst✝¹⁴ : Group G
inst✝¹³ : MeasurableSpace G
inst✝¹² : TopologicalSpace G
inst✝¹¹ : IsTopologicalGroup G
inst✝¹⁰ : BorelSpace G
inst✝⁹ : PolishSpace G
Γ : Subgroup G
inst✝⁸ : Γ.Normal
inst✝⁷ : T2Space (G ⧸ Γ)
inst✝⁶ : SecondCountableTopology (G ⧸ Γ)
μ : Measure (G ⧸ Γ)
inst✝⁵ : Countable ↥Γ
ν : Measure G
inst✝⁴ : ν.IsHaarMeasure
inst✝³ : ν.IsMulRightInvariant
inst✝² : SigmaFinite ν
𝓕 : Set G
h𝓕 : IsFundamentalDomain (↥Γ.op) 𝓕 ν
inst✝¹ : μ.IsMulLeftInvariant
inst✝ : SigmaFinite μ
V : Set (G ⧸ Γ)
hV : (interior V).Nonempty
meas_V : MeasurableSet V
hμK : μ V = ν (QuotientGroup.mk ⁻¹' V ∩ 𝓕)
neTopV : μ V ≠ ⊤
c_eq_zero : ν (QuotientGroup.mk ⁻¹' V ∩ 𝓕) = 0
⊢ False

Proof state:
case neZeroV.a
G : Type u_1
inst✝¹⁴ : Group G
inst✝¹³ : MeasurableSpace G
inst✝¹² : TopologicalSpace G
inst✝¹¹ : IsTopologicalGroup G
inst✝¹⁰ : BorelSpace G
inst✝⁹ : PolishSpace G
Γ : Subgroup G
inst✝⁸ : Γ.Normal
inst✝⁷ : T2Space (G ⧸ Γ)
inst✝⁶ : SecondCountableTopology (G ⧸ Γ)
μ : Measure (G ⧸ Γ)
inst✝⁵ : Countable ↥Γ
ν : Measure G
inst✝⁴ : ν.IsHaarMeasure
inst✝³ : ν.IsMulRightInvariant
inst✝² : SigmaFinite ν
𝓕 : Set G
h𝓕 : IsFundamentalDomain (↥Γ.op) 𝓕 ν
inst✝¹ : μ.IsMulLeftInvariant
inst✝ : SigmaFinite μ
V : Set (G ⧸ Γ)
hV : (interior V).Nonempty
meas_V : MeasurableSet V
hμK : μ V = ν (QuotientGroup.mk ⁻¹' V ∩ 𝓕)
neTopV : μ V ≠ ⊤
c_eq_zero : ν (QuotientGroup.mk ⁻¹' V ∩ 𝓕) = 0
⊢ ν (QuotientGroup.mk ⁻¹' V) = 0

Tactic:
apply h𝓕.measure_zero_of_invariant (ht := fun g ↦ QuotientGroup.sound _ _ g)

Result:
case neZeroV.a
G : Type u_1
inst✝¹⁴ : Group G
inst✝¹³ : MeasurableSpace G
inst✝¹² : TopologicalSpace G
inst✝¹¹ : IsTopologicalGroup G
inst✝¹⁰ : BorelSpace G
inst✝⁹ : PolishSpace G
Γ : Subgroup G
inst✝⁸ : Γ.Normal
inst✝⁷ : T2Space (G ⧸ Γ)
inst✝⁶ : SecondCountableTopology (G ⧸ Γ)
μ : Measure (G ⧸ Γ)
inst✝⁵ : Countable ↥Γ
ν : Measure G
inst✝⁴ : ν.IsHaarMeasure
inst✝³ : ν.IsMulRightInvariant
inst✝² : SigmaFinite ν
𝓕 : Set G
h𝓕 : IsFundamentalDomain (↥Γ.op) 𝓕 ν
inst✝¹ : μ.IsMulLeftInvariant
inst✝ : SigmaFinite μ
V : Set (G ⧸ Γ)
hV : (interior V).Nonempty
meas_V : MeasurableSet V
hμK : μ V = ν (QuotientGroup.mk ⁻¹' V ∩ 𝓕)
neTopV : μ V ≠ ⊤
c_eq_zero : ν (QuotientGroup.mk ⁻¹' V ∩ 𝓕) = 0
⊢ ν (⇑(QuotientGroup.mk' Γ) ⁻¹' V ∩ 𝓕) = 0

Proof state:
case neZeroV.a
G : Type u_1
inst✝¹⁴ : Group G
inst✝¹³ : MeasurableSpace G
inst✝¹² : TopologicalSpace G
inst✝¹¹ : IsTopologicalGroup G
inst✝¹⁰ : BorelSpace G
inst✝⁹ : PolishSpace G
Γ : Subgroup G
inst✝⁸ : Γ.Normal
inst✝⁷ : T2Space (G ⧸ Γ)
inst✝⁶ : SecondCountableTopology (G ⧸ Γ)
μ : Measure (G ⧸ Γ)
inst✝⁵ : Countable ↥Γ
ν : Measure G
inst✝⁴ : ν.IsHaarMeasure
inst✝³ : ν.IsMulRightInvariant
inst✝² : SigmaFinite ν
𝓕 : Set G
h𝓕 : IsFundamentalDomain (↥Γ.op) 𝓕 ν
inst✝¹ : μ.IsMulLeftInvariant
inst✝ : SigmaFinite μ
V : Set (G ⧸ Γ)
hV : (interior V).Nonempty
meas_V : MeasurableSet V
hμK : μ V = ν (QuotientGroup.mk ⁻¹' V ∩ 𝓕)
neTopV : μ V ≠ ⊤
c_eq_zero : ν (QuotientGroup.mk ⁻¹' V ∩ 𝓕) = 0
⊢ (QuotientGroup.mk ⁻¹' interior V).Nonempty

Tactic:
apply hV.preimage'

Result:
case neZeroV.a
G : Type u_1
inst✝¹⁴ : Group G
inst✝¹³ : MeasurableSpace G
inst✝¹² : TopologicalSpace G
inst✝¹¹ : IsTopologicalGroup G
inst✝¹⁰ : BorelSpace G
inst✝⁹ : PolishSpace G
Γ : Subgroup G
inst✝⁸ : Γ.Normal
inst✝⁷ : T2Space (G ⧸ Γ)
inst✝⁶ : SecondCountableTopology (G ⧸ Γ)
μ : Measure (G ⧸ Γ)
inst✝⁵ : Countable ↥Γ
ν : Measure G
inst✝⁴ : ν.IsHaarMeasure
inst✝³ : ν.IsMulRightInvariant
inst✝² : SigmaFinite ν
𝓕 : Set G
h𝓕 : IsFundamentalDomain (↥Γ.op) 𝓕 ν
inst✝¹ : μ.IsMulLeftInvariant
inst✝ : SigmaFinite μ
V : Set (G ⧸ Γ)
hV : (interior V).Nonempty
meas_V : MeasurableSet V
hμK : μ V = ν (QuotientGroup.mk ⁻¹' V ∩ 𝓕)
neTopV : μ V ≠ ⊤
c_eq_zero : ν (QuotientGroup.mk ⁻¹' V ∩ 𝓕) = 0
⊢ interior V ⊆ range QuotientGroup.mk