l3lab/ntp-mathlib huggingface dataset:
/-- The parameterized integral `x ↦ ∫ y, g (y⁻¹ * x) ∂μ` depends continuously on `y` when `g` is a
compactly supported continuous function on a topological group `G`, and `μ` is finite on compact
sets. -/
@[to_additive]
lemma continuous_integral_apply_inv_mul
    {G : Type*} [TopologicalSpace G] [LocallyCompactSpace G] [Group G] [TopologicalGroup G]
    [MeasurableSpace G] [BorelSpace G]
    {μ : Measure G} [IsFiniteMeasureOnCompacts μ] {E : Type*} [NormedAddCommGroup E]
    [NormedSpace ℝ E] {g : G → E}
    (hg : Continuous g) (h'g : HasCompactSupport g) :
    Continuous (fun (x : G) ↦ ∫ y, g (y⁻¹ * x) ∂μ) := by
  let k := tsupport g
  have k_comp : IsCompact k := h'g
  apply continuous_iff_continuousAt.2 (fun x₀ ↦ ?_)
  obtain ⟨t, t_comp, ht⟩ : ∃ t, IsCompact t ∧ t ∈ 𝓝 x₀ := exists_compact_mem_nhds x₀
  let k' : Set G := t • k⁻¹
  have k'_comp : IsCompact k' := t_comp.smul_set k_comp.inv
  have A : ContinuousOn (fun (x : G) ↦ ∫ y, g (y⁻¹ * x) ∂μ) t := by
    apply continuousOn_integral_of_compact_support k'_comp
    · exact (hg.comp (continuous_snd.inv.mul continuous_fst)).continuousOn
    · intro p x hp hx
      contrapose! hx
      refine ⟨p, p⁻¹ * x, hp, ?_, by 

Proof state:
G : Type u_1
inst✝⁸ : TopologicalSpace G
inst✝⁷ : LocallyCompactSpace G
inst✝⁶ : Group G
inst✝⁵ : TopologicalGroup G
inst✝⁴ : MeasurableSpace G
inst✝³ : BorelSpace G
μ : Measure G
inst✝² : IsFiniteMeasureOnCompacts μ
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
g : G → E
hg : Continuous g
h'g : HasCompactSupport g
k : Set G := tsupport g
k_comp : IsCompact k
x₀ : G
t : Set G
t_comp : IsCompact t
ht : t ∈ 𝓝 x₀
k' : Set G := t • k⁻¹
k'_comp : IsCompact k'
p x : G
hp : p ∈ t
hx : g (x⁻¹ * p) ≠ 0
⊢ (fun x x_1 => x • x_1) p (p⁻¹ * x) = x

Next tactic:
simp
/-- The parameterized integral `x ↦ ∫ y, g (y⁻¹ * x) ∂μ` depends continuously on `y` when `g` is a
compactly supported continuous function on a topological group `G`, and `μ` is finite on compact
sets. -/
@[to_additive]
lemma continuous_integral_apply_inv_mul
    {G : Type*} [TopologicalSpace G] [LocallyCompactSpace G] [Group G] [TopologicalGroup G]
    [MeasurableSpace G] [BorelSpace G]
    {μ : Measure G} [IsFiniteMeasureOnCompacts μ] {E : Type*} [NormedAddCommGroup E]
    [NormedSpace ℝ E] {g : G → E}
    (hg : Continuous g) (h'g : HasCompactSupport g) :
    Continuous (fun (x : G) ↦ ∫ y, g (y⁻¹ * x) ∂μ) := by
  let k := tsupport g
  have k_comp : IsCompact k := h'g
  apply continuous_iff_continuousAt.2 (fun x₀ ↦ ?_)
  obtain ⟨t, t_comp, ht⟩ : ∃ t, IsCompact t ∧ t ∈ 𝓝 x₀ := exists_compact_mem_nhds x₀
  let k' : Set G := t • k⁻¹
  have k'_comp : IsCompact k' := t_comp.smul_set k_comp.inv
  have A : ContinuousOn (fun (x : G) ↦ ∫ y, g (y⁻¹ * x) ∂μ) t := by
    apply continuousOn_integral_of_compact_support k'_comp
    · exact (hg.comp (continuous_snd.inv.mul continuous_fst)).continuousOn
    · intro p x hp hx
      contrapose! hx
      refine ⟨p, p⁻¹ * x, hp, ?_, by simp⟩
      

Proof state:
case hfs
G : Type u_1
inst✝⁸ : TopologicalSpace G
inst✝⁷ : LocallyCompactSpace G
inst✝⁶ : Group G
inst✝⁵ : TopologicalGroup G
inst✝⁴ : MeasurableSpace G
inst✝³ : BorelSpace G
μ : Measure G
inst✝² : IsFiniteMeasureOnCompacts μ
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
g : G → E
hg : Continuous g
h'g : HasCompactSupport g
k : Set G := tsupport g
k_comp : IsCompact k
x₀ : G
t : Set G
t_comp : IsCompact t
ht : t ∈ 𝓝 x₀
k' : Set G := t • k⁻¹
k'_comp : IsCompact k'
p x : G
hp : p ∈ t
hx : g (x⁻¹ * p) ≠ 0
⊢ p⁻¹ * x ∈ k⁻¹

Next tactic:
simpa only [Set.mem_inv, mul_inv_rev, inv_inv] using subset_tsupport _ hx
/-- The parameterized integral `x ↦ ∫ y, g (y⁻¹ * x) ∂μ` depends continuously on `y` when `g` is a
compactly supported continuous function on a topological group `G`, and `μ` is finite on compact
sets. -/
@[to_additive]
lemma continuous_integral_apply_inv_mul
    {G : Type*} [TopologicalSpace G] [LocallyCompactSpace G] [Group G] [TopologicalGroup G]
    [MeasurableSpace G] [BorelSpace G]
    {μ : Measure G} [IsFiniteMeasureOnCompacts μ] {E : Type*} [NormedAddCommGroup E]
    [NormedSpace ℝ E] {g : G → E}
    (hg : Continuous g) (h'g : HasCompactSupport g) :
    Continuous (fun (x : G) ↦ ∫ y, g (y⁻¹ * x) ∂μ) := by
  let k := tsupport g
  have k_comp : IsCompact k := h'g
  apply continuous_iff_continuousAt.2 (fun x₀ ↦ ?_)
  obtain ⟨t, t_comp, ht⟩ : ∃ t, IsCompact t ∧ t ∈ 𝓝 x₀ := exists_compact_mem_nhds x₀
  let k' : Set G := t • k⁻¹
  have k'_comp : IsCompact k' := t_comp.smul_set k_comp.inv
  have A : ContinuousOn (fun (x : G) ↦ ∫ y, g (y⁻¹ * x) ∂μ) t := by
    apply continuousOn_integral_of_compact_support k'_comp
    · exact (hg.comp (continuous_snd.inv.mul continuous_fst)).continuousOn
    · intro p x hp hx
      contrapose! hx
      refine ⟨p, p⁻¹ * x, hp, ?_, by simp⟩
      simpa only [Set.mem_inv, mul_inv_rev, inv_inv] using subset_tsupport _ hx
  

Proof state:
case intro.intro
G : Type u_1
inst✝⁸ : TopologicalSpace G
inst✝⁷ : LocallyCompactSpace G
inst✝⁶ : Group G
inst✝⁵ : TopologicalGroup G
inst✝⁴ : MeasurableSpace G
inst✝³ : BorelSpace G
μ : Measure G
inst✝² : IsFiniteMeasureOnCompacts μ
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
g : G → E
hg : Continuous g
h'g : HasCompactSupport g
k : Set G := tsupport g
k_comp : IsCompact k
x₀ : G
t : Set G
t_comp : IsCompact t
ht : t ∈ 𝓝 x₀
k' : Set G := t • k⁻¹
k'_comp : IsCompact k'
A : ContinuousOn (fun x => ∫ (y : G), g (y⁻¹ * x) ∂μ) t
⊢ ContinuousAt (fun x => ∫ (y : G), g (y⁻¹ * x) ∂μ) x₀

Next tactic:
exact A.continuousAt ht
/-- In a group with a left invariant measure `μ` and a right invariant measure `ν`, one can express
integrals with respect to `μ` as integrals with respect to `ν` up to a constant scaling factor
(given in the statement as `∫ x, g x ∂μ` where `g` is a fixed reference function) and an
explicit density `y ↦ 1/∫ z, g (z⁻¹ * y) ∂ν`. -/
@[to_additive]
lemma integral_isMulLeftInvariant_isMulRightInvariant_combo
    {μ ν : Measure G} [IsFiniteMeasureOnCompacts μ] [IsFiniteMeasureOnCompacts ν]
    [IsMulLeftInvariant μ] [IsMulRightInvariant ν] [IsOpenPosMeasure ν]
    {f g : G → ℝ} (hf : Continuous f) (h'f : HasCompactSupport f)
    (hg : Continuous g) (h'g : HasCompactSupport g) (g_nonneg : 0 ≤ g) {x₀ : G} (g_pos : g x₀ ≠ 0) :
    ∫ x, f x ∂μ = (∫ y, f y * (∫ z, g (z⁻¹ * y) ∂ν)⁻¹ ∂ν) * ∫ x, g x ∂μ := by
  

Proof state:
G : Type u_1
inst✝¹⁰ : TopologicalSpace G
inst✝⁹ : LocallyCompactSpace G
inst✝⁸ : Group G
inst✝⁷ : TopologicalGroup G
inst✝⁶ : MeasurableSpace G
inst✝⁵ : BorelSpace G
μ ν : Measure G
inst✝⁴ : IsFiniteMeasureOnCompacts μ
inst✝³ : IsFiniteMeasureOnCompacts ν
inst✝² : IsMulLeftInvariant μ
inst✝¹ : IsMulRightInvariant ν
inst✝ : IsOpenPosMeasure ν
f g : G → ℝ
hf : Continuous f
h'f : HasCompactSupport f
hg : Continuous g
h'g : HasCompactSupport g
g_nonneg : 0 ≤ g
x₀ : G
g_pos : g x₀ ≠ 0
⊢ ∫ (x : G), f x ∂μ = (∫ (y : G), f y * (∫ (z : G), g (z⁻¹ * y) ∂ν)⁻¹ ∂ν) * ∫ (x : G), g x ∂μ

Next tactic:
let D : G → ℝ := fun (x : G) ↦ ∫ y, g (y⁻¹ * x) ∂ν
/-- In a group with a left invariant measure `μ` and a right invariant measure `ν`, one can express
integrals with respect to `μ` as integrals with respect to `ν` up to a constant scaling factor
(given in the statement as `∫ x, g x ∂μ` where `g` is a fixed reference function) and an
explicit density `y ↦ 1/∫ z, g (z⁻¹ * y) ∂ν`. -/
@[to_additive]
lemma integral_isMulLeftInvariant_isMulRightInvariant_combo
    {μ ν : Measure G} [IsFiniteMeasureOnCompacts μ] [IsFiniteMeasureOnCompacts ν]
    [IsMulLeftInvariant μ] [IsMulRightInvariant ν] [IsOpenPosMeasure ν]
    {f g : G → ℝ} (hf : Continuous f) (h'f : HasCompactSupport f)
    (hg : Continuous g) (h'g : HasCompactSupport g) (g_nonneg : 0 ≤ g) {x₀ : G} (g_pos : g x₀ ≠ 0) :
    ∫ x, f x ∂μ = (∫ y, f y * (∫ z, g (z⁻¹ * y) ∂ν)⁻¹ ∂ν) * ∫ x, g x ∂μ := by
  let D : G → ℝ := fun (x : G) ↦ ∫ y, g (y⁻¹ * x) ∂ν
  

Proof state:
G : Type u_1
inst✝¹⁰ : TopologicalSpace G
inst✝⁹ : LocallyCompactSpace G
inst✝⁸ : Group G
inst✝⁷ : TopologicalGroup G
inst✝⁶ : MeasurableSpace G
inst✝⁵ : BorelSpace G
μ ν : Measure G
inst✝⁴ : IsFiniteMeasureOnCompacts μ
inst✝³ : IsFiniteMeasureOnCompacts ν
inst✝² : IsMulLeftInvariant μ
inst✝¹ : IsMulRightInvariant ν
inst✝ : IsOpenPosMeasure ν
f g : G → ℝ
hf : Continuous f
h'f : HasCompactSupport f
hg : Continuous g
h'g : HasCompactSupport g
g_nonneg : 0 ≤ g
x₀ : G
g_pos : g x₀ ≠ 0
D : G → ℝ := fun x => ∫ (y : G), g (y⁻¹ * x) ∂ν
⊢ ∫ (x : G), f x ∂μ = (∫ (y : G), f y * (∫ (z : G), g (z⁻¹ * y) ∂ν)⁻¹ ∂ν) * ∫ (x : G), g x ∂μ

Next tactic:
have D_cont : Continuous D := continuous_integral_apply_inv_mul hg h'g
/-- In a group with a left invariant measure `μ` and a right invariant measure `ν`, one can express
integrals with respect to `μ` as integrals with respect to `ν` up to a constant scaling factor
(given in the statement as `∫ x, g x ∂μ` where `g` is a fixed reference function) and an
explicit density `y ↦ 1/∫ z, g (z⁻¹ * y) ∂ν`. -/
@[to_additive]
lemma integral_isMulLeftInvariant_isMulRightInvariant_combo
    {μ ν : Measure G} [IsFiniteMeasureOnCompacts μ] [IsFiniteMeasureOnCompacts ν]
    [IsMulLeftInvariant μ] [IsMulRightInvariant ν] [IsOpenPosMeasure ν]
    {f g : G → ℝ} (hf : Continuous f) (h'f : HasCompactSupport f)
    (hg : Continuous g) (h'g : HasCompactSupport g) (g_nonneg : 0 ≤ g) {x₀ : G} (g_pos : g x₀ ≠ 0) :
    ∫ x, f x ∂μ = (∫ y, f y * (∫ z, g (z⁻¹ * y) ∂ν)⁻¹ ∂ν) * ∫ x, g x ∂μ := by
  let D : G → ℝ := fun (x : G) ↦ ∫ y, g (y⁻¹ * x) ∂ν
  have D_cont : Continuous D := continuous_integral_apply_inv_mul hg h'g
  

Proof state:
G : Type u_1
inst✝¹⁰ : TopologicalSpace G
inst✝⁹ : LocallyCompactSpace G
inst✝⁸ : Group G
inst✝⁷ : TopologicalGroup G
inst✝⁶ : MeasurableSpace G
inst✝⁵ : BorelSpace G
μ ν : Measure G
inst✝⁴ : IsFiniteMeasureOnCompacts μ
inst✝³ : IsFiniteMeasureOnCompacts ν
inst✝² : IsMulLeftInvariant μ
inst✝¹ : IsMulRightInvariant ν
inst✝ : IsOpenPosMeasure ν
f g : G → ℝ
hf : Continuous f
h'f : HasCompactSupport f
hg : Continuous g
h'g : HasCompactSupport g
g_nonneg : 0 ≤ g
x₀ : G
g_pos : g x₀ ≠ 0
D : G → ℝ := fun x => ∫ (y : G), g (y⁻¹ * x) ∂ν
D_cont : Continuous D
⊢ ∫ (x : G), f x ∂μ = (∫ (y : G), f y * (∫ (z : G), g (z⁻¹ * y) ∂ν)⁻¹ ∂ν) * ∫ (x : G), g x ∂μ

Next tactic:
have D_pos : ∀ x, 0 < D x := by
    intro x
    have C : Continuous (fun y ↦ g (y⁻¹ * x)) := hg.comp (continuous_inv.mul continuous_const)
    apply (integral_pos_iff_support_of_nonneg _ _).2
    · apply C.isOpen_support.measure_pos ν
      exact ⟨x * x₀⁻¹, by simpa using g_pos⟩
    · exact fun y ↦ g_nonneg (y⁻¹ * x)
    · apply C.integrable_of_hasCompactSupport
      exact h'g.comp_homeomorph ((Homeomorph.inv G).trans (Homeomorph.mulRight x))
/-- In a group with a left invariant measure `μ` and a right invariant measure `ν`, one can express
integrals with respect to `μ` as integrals with respect to `ν` up to a constant scaling factor
(given in the statement as `∫ x, g x ∂μ` where `g` is a fixed reference function) and an
explicit density `y ↦ 1/∫ z, g (z⁻¹ * y) ∂ν`. -/
@[to_additive]
lemma integral_isMulLeftInvariant_isMulRightInvariant_combo
    {μ ν : Measure G} [IsFiniteMeasureOnCompacts μ] [IsFiniteMeasureOnCompacts ν]
    [IsMulLeftInvariant μ] [IsMulRightInvariant ν] [IsOpenPosMeasure ν]
    {f g : G → ℝ} (hf : Continuous f) (h'f : HasCompactSupport f)
    (hg : Continuous g) (h'g : HasCompactSupport g) (g_nonneg : 0 ≤ g) {x₀ : G} (g_pos : g x₀ ≠ 0) :
    ∫ x, f x ∂μ = (∫ y, f y * (∫ z, g (z⁻¹ * y) ∂ν)⁻¹ ∂ν) * ∫ x, g x ∂μ := by
  let D : G → ℝ := fun (x : G) ↦ ∫ y, g (y⁻¹ * x) ∂ν
  have D_cont : Continuous D := continuous_integral_apply_inv_mul hg h'g
  have D_pos : ∀ x, 0 < D x := by
    

Proof state:
G : Type u_1
inst✝¹⁰ : TopologicalSpace G
inst✝⁹ : LocallyCompactSpace G
inst✝⁸ : Group G
inst✝⁷ : TopologicalGroup G
inst✝⁶ : MeasurableSpace G
inst✝⁵ : BorelSpace G
μ ν : Measure G
inst✝⁴ : IsFiniteMeasureOnCompacts μ
inst✝³ : IsFiniteMeasureOnCompacts ν
inst✝² : IsMulLeftInvariant μ
inst✝¹ : IsMulRightInvariant ν
inst✝ : IsOpenPosMeasure ν
f g : G → ℝ
hf : Continuous f
h'f : HasCompactSupport f
hg : Continuous g
h'g : HasCompactSupport g
g_nonneg : 0 ≤ g
x₀ : G
g_pos : g x₀ ≠ 0
D : G → ℝ := fun x => ∫ (y : G), g (y⁻¹ * x) ∂ν
D_cont : Continuous D
⊢ ∀ (x : G), 0 < D x

Next tactic:
intro x
/-- In a group with a left invariant measure `μ` and a right invariant measure `ν`, one can express
integrals with respect to `μ` as integrals with respect to `ν` up to a constant scaling factor
(given in the statement as `∫ x, g x ∂μ` where `g` is a fixed reference function) and an
explicit density `y ↦ 1/∫ z, g (z⁻¹ * y) ∂ν`. -/
@[to_additive]
lemma integral_isMulLeftInvariant_isMulRightInvariant_combo
    {μ ν : Measure G} [IsFiniteMeasureOnCompacts μ] [IsFiniteMeasureOnCompacts ν]
    [IsMulLeftInvariant μ] [IsMulRightInvariant ν] [IsOpenPosMeasure ν]
    {f g : G → ℝ} (hf : Continuous f) (h'f : HasCompactSupport f)
    (hg : Continuous g) (h'g : HasCompactSupport g) (g_nonneg : 0 ≤ g) {x₀ : G} (g_pos : g x₀ ≠ 0) :
    ∫ x, f x ∂μ = (∫ y, f y * (∫ z, g (z⁻¹ * y) ∂ν)⁻¹ ∂ν) * ∫ x, g x ∂μ := by
  let D : G → ℝ := fun (x : G) ↦ ∫ y, g (y⁻¹ * x) ∂ν
  have D_cont : Continuous D := continuous_integral_apply_inv_mul hg h'g
  have D_pos : ∀ x, 0 < D x := by
    intro x
    

Proof state:
G : Type u_1
inst✝¹⁰ : TopologicalSpace G
inst✝⁹ : LocallyCompactSpace G
inst✝⁸ : Group G
inst✝⁷ : TopologicalGroup G
inst✝⁶ : MeasurableSpace G
inst✝⁵ : BorelSpace G
μ ν : Measure G
inst✝⁴ : IsFiniteMeasureOnCompacts μ
inst✝³ : IsFiniteMeasureOnCompacts ν
inst✝² : IsMulLeftInvariant μ
inst✝¹ : IsMulRightInvariant ν
inst✝ : IsOpenPosMeasure ν
f g : G → ℝ
hf : Continuous f
h'f : HasCompactSupport f
hg : Continuous g
h'g : HasCompactSupport g
g_nonneg : 0 ≤ g
x₀ : G
g_pos : g x₀ ≠ 0
D : G → ℝ := fun x => ∫ (y : G), g (y⁻¹ * x) ∂ν
D_cont : Continuous D
x : G
⊢ 0 < D x

Next tactic:
have C : Continuous (fun y ↦ g (y⁻¹ * x)) := hg.comp (continuous_inv.mul continuous_const)
/-- In a group with a left invariant measure `μ` and a right invariant measure `ν`, one can express
integrals with respect to `μ` as integrals with respect to `ν` up to a constant scaling factor
(given in the statement as `∫ x, g x ∂μ` where `g` is a fixed reference function) and an
explicit density `y ↦ 1/∫ z, g (z⁻¹ * y) ∂ν`. -/
@[to_additive]
lemma integral_isMulLeftInvariant_isMulRightInvariant_combo
    {μ ν : Measure G} [IsFiniteMeasureOnCompacts μ] [IsFiniteMeasureOnCompacts ν]
    [IsMulLeftInvariant μ] [IsMulRightInvariant ν] [IsOpenPosMeasure ν]
    {f g : G → ℝ} (hf : Continuous f) (h'f : HasCompactSupport f)
    (hg : Continuous g) (h'g : HasCompactSupport g) (g_nonneg : 0 ≤ g) {x₀ : G} (g_pos : g x₀ ≠ 0) :
    ∫ x, f x ∂μ = (∫ y, f y * (∫ z, g (z⁻¹ * y) ∂ν)⁻¹ ∂ν) * ∫ x, g x ∂μ := by
  let D : G → ℝ := fun (x : G) ↦ ∫ y, g (y⁻¹ * x) ∂ν
  have D_cont : Continuous D := continuous_integral_apply_inv_mul hg h'g
  have D_pos : ∀ x, 0 < D x := by
    intro x
    have C : Continuous (fun y ↦ g (y⁻¹ * x)) := hg.comp (continuous_inv.mul continuous_const)
    

Proof state:
G : Type u_1
inst✝¹⁰ : TopologicalSpace G
inst✝⁹ : LocallyCompactSpace G
inst✝⁸ : Group G
inst✝⁷ : TopologicalGroup G
inst✝⁶ : MeasurableSpace G
inst✝⁵ : BorelSpace G
μ ν : Measure G
inst✝⁴ : IsFiniteMeasureOnCompacts μ
inst✝³ : IsFiniteMeasureOnCompacts ν
inst✝² : IsMulLeftInvariant μ
inst✝¹ : IsMulRightInvariant ν
inst✝ : IsOpenPosMeasure ν
f g : G → ℝ
hf : Continuous f
h'f : HasCompactSupport f
hg : Continuous g
h'g : HasCompactSupport g
g_nonneg : 0 ≤ g
x₀ : G
g_pos : g x₀ ≠ 0
D : G → ℝ := fun x => ∫ (y : G), g (y⁻¹ * x) ∂ν
D_cont : Continuous D
x : G
C : Continuous fun y => g (y⁻¹ * x)
⊢ 0 < D x

Next tactic:
apply (integral_pos_iff_support_of_nonneg _ _).2
/-- In a group with a left invariant measure `μ` and a right invariant measure `ν`, one can express
integrals with respect to `μ` as integrals with respect to `ν` up to a constant scaling factor
(given in the statement as `∫ x, g x ∂μ` where `g` is a fixed reference function) and an
explicit density `y ↦ 1/∫ z, g (z⁻¹ * y) ∂ν`. -/
@[to_additive]
lemma integral_isMulLeftInvariant_isMulRightInvariant_combo
    {μ ν : Measure G} [IsFiniteMeasureOnCompacts μ] [IsFiniteMeasureOnCompacts ν]
    [IsMulLeftInvariant μ] [IsMulRightInvariant ν] [IsOpenPosMeasure ν]
    {f g : G → ℝ} (hf : Continuous f) (h'f : HasCompactSupport f)
    (hg : Continuous g) (h'g : HasCompactSupport g) (g_nonneg : 0 ≤ g) {x₀ : G} (g_pos : g x₀ ≠ 0) :
    ∫ x, f x ∂μ = (∫ y, f y * (∫ z, g (z⁻¹ * y) ∂ν)⁻¹ ∂ν) * ∫ x, g x ∂μ := by
  let D : G → ℝ := fun (x : G) ↦ ∫ y, g (y⁻¹ * x) ∂ν
  have D_cont : Continuous D := continuous_integral_apply_inv_mul hg h'g
  have D_pos : ∀ x, 0 < D x := by
    intro x
    have C : Continuous (fun y ↦ g (y⁻¹ * x)) := hg.comp (continuous_inv.mul continuous_const)
    apply (integral_pos_iff_support_of_nonneg _ _).2
    · 

Proof state:
G : Type u_1
inst✝¹⁰ : TopologicalSpace G
inst✝⁹ : LocallyCompactSpace G
inst✝⁸ : Group G
inst✝⁷ : TopologicalGroup G
inst✝⁶ : MeasurableSpace G
inst✝⁵ : BorelSpace G
μ ν : Measure G
inst✝⁴ : IsFiniteMeasureOnCompacts μ
inst✝³ : IsFiniteMeasureOnCompacts ν
inst✝² : IsMulLeftInvariant μ
inst✝¹ : IsMulRightInvariant ν
inst✝ : IsOpenPosMeasure ν
f g : G → ℝ
hf : Continuous f
h'f : HasCompactSupport f
hg : Continuous g
h'g : HasCompactSupport g
g_nonneg : 0 ≤ g
x₀ : G
g_pos : g x₀ ≠ 0
D : G → ℝ := fun x => ∫ (y : G), g (y⁻¹ * x) ∂ν
D_cont : Continuous D
x : G
C : Continuous fun y => g (y⁻¹ * x)
⊢ 0 < ↑↑ν (support fun x_1 => g (x_1⁻¹ * x))

Next tactic:
apply C.isOpen_support.measure_pos ν
/-- In a group with a left invariant measure `μ` and a right invariant measure `ν`, one can express
integrals with respect to `μ` as integrals with respect to `ν` up to a constant scaling factor
(given in the statement as `∫ x, g x ∂μ` where `g` is a fixed reference function) and an
explicit density `y ↦ 1/∫ z, g (z⁻¹ * y) ∂ν`. -/
@[to_additive]
lemma integral_isMulLeftInvariant_isMulRightInvariant_combo
    {μ ν : Measure G} [IsFiniteMeasureOnCompacts μ] [IsFiniteMeasureOnCompacts ν]
    [IsMulLeftInvariant μ] [IsMulRightInvariant ν] [IsOpenPosMeasure ν]
    {f g : G → ℝ} (hf : Continuous f) (h'f : HasCompactSupport f)
    (hg : Continuous g) (h'g : HasCompactSupport g) (g_nonneg : 0 ≤ g) {x₀ : G} (g_pos : g x₀ ≠ 0) :
    ∫ x, f x ∂μ = (∫ y, f y * (∫ z, g (z⁻¹ * y) ∂ν)⁻¹ ∂ν) * ∫ x, g x ∂μ := by
  let D : G → ℝ := fun (x : G) ↦ ∫ y, g (y⁻¹ * x) ∂ν
  have D_cont : Continuous D := continuous_integral_apply_inv_mul hg h'g
  have D_pos : ∀ x, 0 < D x := by
    intro x
    have C : Continuous (fun y ↦ g (y⁻¹ * x)) := hg.comp (continuous_inv.mul continuous_const)
    apply (integral_pos_iff_support_of_nonneg _ _).2
    · apply C.isOpen_support.measure_pos ν
      

Proof state:
G : Type u_1
inst✝¹⁰ : TopologicalSpace G
inst✝⁹ : LocallyCompactSpace G
inst✝⁸ : Group G
inst✝⁷ : TopologicalGroup G
inst✝⁶ : MeasurableSpace G
inst✝⁵ : BorelSpace G
μ ν : Measure G
inst✝⁴ : IsFiniteMeasureOnCompacts μ
inst✝³ : IsFiniteMeasureOnCompacts ν
inst✝² : IsMulLeftInvariant μ
inst✝¹ : IsMulRightInvariant ν
inst✝ : IsOpenPosMeasure ν
f g : G → ℝ
hf : Continuous f
h'f : HasCompactSupport f
hg : Continuous g
h'g : HasCompactSupport g
g_nonneg : 0 ≤ g
x₀ : G
g_pos : g x₀ ≠ 0
D : G → ℝ := fun x => ∫ (y : G), g (y⁻¹ * x) ∂ν
D_cont : Continuous D
x : G
C : Continuous fun y => g (y⁻¹ * x)
⊢ Set.Nonempty (support fun y => g (y⁻¹ * x))

Next tactic:
exact ⟨x * x₀⁻¹, by simpa using g_pos⟩
/-- In a group with a left invariant measure `μ` and a right invariant measure `ν`, one can express
integrals with respect to `μ` as integrals with respect to `ν` up to a constant scaling factor
(given in the statement as `∫ x, g x ∂μ` where `g` is a fixed reference function) and an
explicit density `y ↦ 1/∫ z, g (z⁻¹ * y) ∂ν`. -/
@[to_additive]
lemma integral_isMulLeftInvariant_isMulRightInvariant_combo
    {μ ν : Measure G} [IsFiniteMeasureOnCompacts μ] [IsFiniteMeasureOnCompacts ν]
    [IsMulLeftInvariant μ] [IsMulRightInvariant ν] [IsOpenPosMeasure ν]
    {f g : G → ℝ} (hf : Continuous f) (h'f : HasCompactSupport f)
    (hg : Continuous g) (h'g : HasCompactSupport g) (g_nonneg : 0 ≤ g) {x₀ : G} (g_pos : g x₀ ≠ 0) :
    ∫ x, f x ∂μ = (∫ y, f y * (∫ z, g (z⁻¹ * y) ∂ν)⁻¹ ∂ν) * ∫ x, g x ∂μ := by
  let D : G → ℝ := fun (x : G) ↦ ∫ y, g (y⁻¹ * x) ∂ν
  have D_cont : Continuous D := continuous_integral_apply_inv_mul hg h'g
  have D_pos : ∀ x, 0 < D x := by
    intro x
    have C : Continuous (fun y ↦ g (y⁻¹ * x)) := hg.comp (continuous_inv.mul continuous_const)
    apply (integral_pos_iff_support_of_nonneg _ _).2
    · apply C.isOpen_support.measure_pos ν
      exact ⟨x * x₀⁻¹, by 

Proof state:
G : Type u_1
inst✝¹⁰ : TopologicalSpace G
inst✝⁹ : LocallyCompactSpace G
inst✝⁸ : Group G
inst✝⁷ : TopologicalGroup G
inst✝⁶ : MeasurableSpace G
inst✝⁵ : BorelSpace G
μ ν : Measure G
inst✝⁴ : IsFiniteMeasureOnCompacts μ
inst✝³ : IsFiniteMeasureOnCompacts ν
inst✝² : IsMulLeftInvariant μ
inst✝¹ : IsMulRightInvariant ν
inst✝ : IsOpenPosMeasure ν
f g : G → ℝ
hf : Continuous f
h'f : HasCompactSupport f
hg : Continuous g
h'g : HasCompactSupport g
g_nonneg : 0 ≤ g
x₀ : G
g_pos : g x₀ ≠ 0
D : G → ℝ := fun x => ∫ (y : G), g (y⁻¹ * x) ∂ν
D_cont : Continuous D
x : G
C : Continuous fun y => g (y⁻¹ * x)
⊢ x * x₀⁻¹ ∈ support fun y => g (y⁻¹ * x)

Next tactic:
simpa using g_pos
/-- In a group with a left invariant measure `μ` and a right invariant measure `ν`, one can express
integrals with respect to `μ` as integrals with respect to `ν` up to a constant scaling factor
(given in the statement as `∫ x, g x ∂μ` where `g` is a fixed reference function) and an
explicit density `y ↦ 1/∫ z, g (z⁻¹ * y) ∂ν`. -/
@[to_additive]
lemma integral_isMulLeftInvariant_isMulRightInvariant_combo
    {μ ν : Measure G} [IsFiniteMeasureOnCompacts μ] [IsFiniteMeasureOnCompacts ν]
    [IsMulLeftInvariant μ] [IsMulRightInvariant ν] [IsOpenPosMeasure ν]
    {f g : G → ℝ} (hf : Continuous f) (h'f : HasCompactSupport f)
    (hg : Continuous g) (h'g : HasCompactSupport g) (g_nonneg : 0 ≤ g) {x₀ : G} (g_pos : g x₀ ≠ 0) :
    ∫ x, f x ∂μ = (∫ y, f y * (∫ z, g (z⁻¹ * y) ∂ν)⁻¹ ∂ν) * ∫ x, g x ∂μ := by
  let D : G → ℝ := fun (x : G) ↦ ∫ y, g (y⁻¹ * x) ∂ν
  have D_cont : Continuous D := continuous_integral_apply_inv_mul hg h'g
  have D_pos : ∀ x, 0 < D x := by
    intro x
    have C : Continuous (fun y ↦ g (y⁻¹ * x)) := hg.comp (continuous_inv.mul continuous_const)
    apply (integral_pos_iff_support_of_nonneg _ _).2
    · apply C.isOpen_support.measure_pos ν
      exact ⟨x * x₀⁻¹, by simpa using g_pos⟩
    · 

Proof state:
G : Type u_1
inst✝¹⁰ : TopologicalSpace G
inst✝⁹ : LocallyCompactSpace G
inst✝⁸ : Group G
inst✝⁷ : TopologicalGroup G
inst✝⁶ : MeasurableSpace G
inst✝⁵ : BorelSpace G
μ ν : Measure G
inst✝⁴ : IsFiniteMeasureOnCompacts μ
inst✝³ : IsFiniteMeasureOnCompacts ν
inst✝² : IsMulLeftInvariant μ
inst✝¹ : IsMulRightInvariant ν
inst✝ : IsOpenPosMeasure ν
f g : G → ℝ
hf : Continuous f
h'f : HasCompactSupport f
hg : Continuous g
h'g : HasCompactSupport g
g_nonneg : 0 ≤ g
x₀ : G
g_pos : g x₀ ≠ 0
D : G → ℝ := fun x => ∫ (y : G), g (y⁻¹ * x) ∂ν
D_cont : Continuous D
x : G
C : Continuous fun y => g (y⁻¹ * x)
⊢ 0 ≤ fun x_1 => g (x_1⁻¹ * x)

Next tactic:
exact fun y ↦ g_nonneg (y⁻¹ * x)