-- Mathlib/MeasureTheory/Measure/Haar/Unique.lean
@[to_additive addHaarScalarFactor_eq_mul]
lemma haarScalarFactor_eq_mul (μ' μ ν : Measure G)
    [IsHaarMeasure μ] [IsHaarMeasure ν] [IsFiniteMeasureOnCompacts μ'] [IsMulLeftInvariant μ'] :
    haarScalarFactor μ' ν = haarScalarFactor μ' μ * haarScalarFactor μ ν

Proof state:
case pos.intro.mk.intro.intro
G : Type u_1
inst✝⁸ : TopologicalSpace G
inst✝⁷ : Group G
inst✝⁶ : IsTopologicalGroup G
inst✝⁵ : MeasurableSpace G
inst✝⁴ : BorelSpace G
μ' μ ν : Measure G
inst✝³ : μ.IsHaarMeasure
inst✝² : ν.IsHaarMeasure
inst✝¹ : IsFiniteMeasureOnCompacts μ'
inst✝ : μ'.IsMulLeftInvariant
hG : LocallyCompactSpace G
g : G → ℝ
g_cont : Continuous g
g_comp : HasCompactSupport ⇑{ toFun := g, continuous_toFun := g_cont }
g_nonneg : 0 ≤ { toFun := g, continuous_toFun := g_cont }
g_one : { toFun := g, continuous_toFun := g_cont } 1 ≠ 0
Z : μ'.haarScalarFactor ν • ∫ (x : G), g x ∂ν = (μ'.haarScalarFactor μ * μ.haarScalarFactor ν) • ∫ (x : G), g x ∂ν
int_g_pos : 0 < ∫ (x : G), g x ∂ν
⊢ μ'.haarScalarFactor ν = μ'.haarScalarFactor μ * μ.haarScalarFactor ν

Tactic:
change (haarScalarFactor μ' ν : ℝ) * ∫ (x : G), g x ∂ν =
  (haarScalarFactor μ' μ * haarScalarFactor μ ν : ℝ≥0) * ∫ (x : G), g x ∂ν at Z

Result:
case pos.intro.mk.intro.intro
G : Type u_1
inst✝⁸ : TopologicalSpace G
inst✝⁷ : Group G
inst✝⁶ : IsTopologicalGroup G
inst✝⁵ : MeasurableSpace G
inst✝⁴ : BorelSpace G
μ' μ ν : Measure G
inst✝³ : μ.IsHaarMeasure
inst✝² : ν.IsHaarMeasure
inst✝¹ : IsFiniteMeasureOnCompacts μ'
inst✝ : μ'.IsMulLeftInvariant
hG : LocallyCompactSpace G
g : G → ℝ
g_cont : Continuous g
g_comp : HasCompactSupport ⇑{ toFun := g, continuous_toFun := g_cont }
g_nonneg : 0 ≤ { toFun := g, continuous_toFun := g_cont }
g_one : { toFun := g, continuous_toFun := g_cont } 1 ≠ 0
int_g_pos : 0 < ∫ (x : G), g x ∂ν
Z : ↑(μ'.haarScalarFactor ν) * ∫ (x : G), g x ∂ν = ↑(μ'.haarScalarFactor μ * μ.haarScalarFactor ν) * ∫ (x : G), g x ∂ν
⊢ μ'.haarScalarFactor ν = μ'.haarScalarFactor μ * μ.haarScalarFactor ν

Proof state:
case pos.intro.mk.intro.intro
G : Type u_1
inst✝⁸ : TopologicalSpace G
inst✝⁷ : Group G
inst✝⁶ : IsTopologicalGroup G
inst✝⁵ : MeasurableSpace G
inst✝⁴ : BorelSpace G
μ' μ ν : Measure G
inst✝³ : μ.IsHaarMeasure
inst✝² : ν.IsHaarMeasure
inst✝¹ : IsFiniteMeasureOnCompacts μ'
inst✝ : μ'.IsMulLeftInvariant
hG : LocallyCompactSpace G
g : G → ℝ
g_cont : Continuous g
g_comp : HasCompactSupport ⇑{ toFun := g, continuous_toFun := g_cont }
g_nonneg : 0 ≤ { toFun := g, continuous_toFun := g_cont }
g_one : { toFun := g, continuous_toFun := g_cont } 1 ≠ 0
Z : ∫ (x : G), g x ∂μ' = ∫ (x : G), g x ∂μ'.haarScalarFactor μ • μ
⊢ μ'.haarScalarFactor ν = μ'.haarScalarFactor μ * μ.haarScalarFactor ν

Tactic:
simp only [integral_smul_nnreal_measure, smul_smul,
  integral_isMulLeftInvariant_eq_smul_of_hasCompactSupport μ' ν g_cont g_comp,
  integral_isMulLeftInvariant_eq_smul_of_hasCompactSupport μ ν g_cont g_comp] at Z

Result:
case pos.intro.mk.intro.intro
G : Type u_1
inst✝⁸ : TopologicalSpace G
inst✝⁷ : Group G
inst✝⁶ : IsTopologicalGroup G
inst✝⁵ : MeasurableSpace G
inst✝⁴ : BorelSpace G
μ' μ ν : Measure G
inst✝³ : μ.IsHaarMeasure
inst✝² : ν.IsHaarMeasure
inst✝¹ : IsFiniteMeasureOnCompacts μ'
inst✝ : μ'.IsMulLeftInvariant
hG : LocallyCompactSpace G
g : G → ℝ
g_cont : Continuous g
g_comp : HasCompactSupport ⇑{ toFun := g, continuous_toFun := g_cont }
g_nonneg : 0 ≤ { toFun := g, continuous_toFun := g_cont }
g_one : { toFun := g, continuous_toFun := g_cont } 1 ≠ 0
Z : μ'.haarScalarFactor ν • ∫ (x : G), g x ∂ν = (μ'.haarScalarFactor μ * μ.haarScalarFactor ν) • ∫ (x : G), g x ∂ν
⊢ μ'.haarScalarFactor ν = μ'.haarScalarFactor μ * μ.haarScalarFactor ν

Proof state:
G : Type u_1
inst✝⁸ : TopologicalSpace G
inst✝⁷ : Group G
inst✝⁶ : IsTopologicalGroup G
inst✝⁵ : MeasurableSpace G
inst✝⁴ : BorelSpace G
μ' μ ν : Measure G
inst✝³ : μ.IsHaarMeasure
inst✝² : ν.IsHaarMeasure
inst✝¹ : IsFiniteMeasureOnCompacts μ'
inst✝ : μ'.IsMulLeftInvariant
hG : LocallyCompactSpace G
g : G → ℝ
g_cont : Continuous g
g_comp : HasCompactSupport ⇑{ toFun := g, continuous_toFun := g_cont }
g_nonneg : 0 ≤ { toFun := g, continuous_toFun := g_cont }
g_one : { toFun := g, continuous_toFun := g_cont } 1 ≠ 0
Z : μ'.haarScalarFactor ν • ∫ (x : G), g x ∂ν = (μ'.haarScalarFactor μ * μ.haarScalarFactor ν) • ∫ (x : G), g x ∂ν
⊢ Integrable ((fun f => f.toFun) { toFun := g, continuous_toFun := g_cont }) ν

Tactic:
exact g_cont.integrable_of_hasCompactSupport g_comp

Result:
no goals

Proof state:
case pos.intro.mk.intro.intro
G : Type u_1
inst✝⁸ : TopologicalSpace G
inst✝⁷ : Group G
inst✝⁶ : IsTopologicalGroup G
inst✝⁵ : MeasurableSpace G
inst✝⁴ : BorelSpace G
μ' μ ν : Measure G
inst✝³ : μ.IsHaarMeasure
inst✝² : ν.IsHaarMeasure
inst✝¹ : IsFiniteMeasureOnCompacts μ'
inst✝ : μ'.IsMulLeftInvariant
hG : LocallyCompactSpace G
g : G → ℝ
g_cont : Continuous g
g_comp : HasCompactSupport ⇑{ toFun := g, continuous_toFun := g_cont }
g_nonneg : 0 ≤ { toFun := g, continuous_toFun := g_cont }
g_one : { toFun := g, continuous_toFun := g_cont } 1 ≠ 0
⊢ μ'.haarScalarFactor ν = μ'.haarScalarFactor μ * μ.haarScalarFactor ν

Tactic:
have Z := integral_isMulLeftInvariant_eq_smul_of_hasCompactSupport μ' μ g_cont g_comp

Result:
case pos.intro.mk.intro.intro
G : Type u_1
inst✝⁸ : TopologicalSpace G
inst✝⁷ : Group G
inst✝⁶ : IsTopologicalGroup G
inst✝⁵ : MeasurableSpace G
inst✝⁴ : BorelSpace G
μ' μ ν : Measure G
inst✝³ : μ.IsHaarMeasure
inst✝² : ν.IsHaarMeasure
inst✝¹ : IsFiniteMeasureOnCompacts μ'
inst✝ : μ'.IsMulLeftInvariant
hG : LocallyCompactSpace G
g : G → ℝ
g_cont : Continuous g
g_comp : HasCompactSupport ⇑{ toFun := g, continuous_toFun := g_cont }
g_nonneg : 0 ≤ { toFun := g, continuous_toFun := g_cont }
g_one : { toFun := g, continuous_toFun := g_cont } 1 ≠ 0
Z : ∫ (x : G), g x ∂μ' = ∫ (x : G), g x ∂μ'.haarScalarFactor μ • μ
⊢ μ'.haarScalarFactor ν = μ'.haarScalarFactor μ * μ.haarScalarFactor ν

Proof state:
case pos
G : Type u_1
inst✝⁸ : TopologicalSpace G
inst✝⁷ : Group G
inst✝⁶ : IsTopologicalGroup G
inst✝⁵ : MeasurableSpace G
inst✝⁴ : BorelSpace G
μ' μ ν : Measure G
inst✝³ : μ.IsHaarMeasure
inst✝² : ν.IsHaarMeasure
inst✝¹ : IsFiniteMeasureOnCompacts μ'
inst✝ : μ'.IsMulLeftInvariant
hG : LocallyCompactSpace G
⊢ μ'.haarScalarFactor ν = μ'.haarScalarFactor μ * μ.haarScalarFactor ν

case neg
G : Type u_1
inst✝⁸ : TopologicalSpace G
inst✝⁷ : Group G
inst✝⁶ : IsTopologicalGroup G
inst✝⁵ : MeasurableSpace G
inst✝⁴ : BorelSpace G
μ' μ ν : Measure G
inst✝³ : μ.IsHaarMeasure
inst✝² : ν.IsHaarMeasure
inst✝¹ : IsFiniteMeasureOnCompacts μ'
inst✝ : μ'.IsMulLeftInvariant
hG : ¬LocallyCompactSpace G
⊢ μ'.haarScalarFactor ν = μ'.haarScalarFactor μ * μ.haarScalarFactor ν

Tactic:
swap

Result:
case neg
G : Type u_1
inst✝⁸ : TopologicalSpace G
inst✝⁷ : Group G
inst✝⁶ : IsTopologicalGroup G
inst✝⁵ : MeasurableSpace G
inst✝⁴ : BorelSpace G
μ' μ ν : Measure G
inst✝³ : μ.IsHaarMeasure
inst✝² : ν.IsHaarMeasure
inst✝¹ : IsFiniteMeasureOnCompacts μ'
inst✝ : μ'.IsMulLeftInvariant
hG : ¬LocallyCompactSpace G
⊢ μ'.haarScalarFactor ν = μ'.haarScalarFactor μ * μ.haarScalarFactor ν

case pos
G : Type u_1
inst✝⁸ : TopologicalSpace G
inst✝⁷ : Group G
inst✝⁶ : IsTopologicalGroup G
inst✝⁵ : MeasurableSpace G
inst✝⁴ : BorelSpace G
μ' μ ν : Measure G
inst✝³ : μ.IsHaarMeasure
inst✝² : ν.IsHaarMeasure
inst✝¹ : IsFiniteMeasureOnCompacts μ'
inst✝ : μ'.IsMulLeftInvariant
hG : LocallyCompactSpace G
⊢ μ'.haarScalarFactor ν = μ'.haarScalarFactor μ * μ.haarScalarFactor ν