-- Mathlib/MeasureTheory/Measure/Haar/Disintegration.lean
theorem LinearMap.exists_map_addHaar_eq_smul_addHaar' (h : Function.Surjective L) :
    ∃ (c : ℝ≥0∞), 0 < c ∧ c < ∞ ∧ μ.map L = (c * addHaar (univ : Set (LinearMap.ker L))) • ν

Proof state:
𝕜 : Type u_1
E : Type u_2
F : Type u_3
inst✝¹² : NontriviallyNormedField 𝕜
inst✝¹¹ : CompleteSpace 𝕜
inst✝¹⁰ : NormedAddCommGroup E
inst✝⁹ : MeasurableSpace E
inst✝⁸ : BorelSpace E
inst✝⁷ : NormedSpace 𝕜 E
inst✝⁶ : NormedAddCommGroup F
inst✝⁵ : MeasurableSpace F
inst✝⁴ : BorelSpace F
inst✝³ : NormedSpace 𝕜 F
L : E →ₗ[𝕜] F
μ : Measure E
ν : Measure F
inst✝² : μ.IsAddHaarMeasure
inst✝¹ : ν.IsAddHaarMeasure
inst✝ : LocallyCompactSpace E
h : Function.Surjective ⇑L
this✝ : FiniteDimensional 𝕜 E
this : ProperSpace F
S : Submodule 𝕜 E := ker L
T : Submodule 𝕜 E
hT : IsCompl S T
M : (↥S × ↥T) ≃ₗ[𝕜] E := S.prodEquivOfIsCompl T hT
M_cont : Continuous ⇑M.symm
P : ↥S × ↥T →ₗ[𝕜] ↥T := snd 𝕜 ↥S ↥T
P_cont : Continuous ⇑P
I : Function.Bijective ⇑(L.domRestrict T)
L' : ↥T ≃ₗ[𝕜] F := LinearEquiv.ofBijective (L.domRestrict T) I
L'_cont : Continuous ⇑L'
⊢ L = ↑L' ∘ₗ P ∘ₗ ↑M.symm

Tactic:
ext x

Result:
case h
𝕜 : Type u_1
E : Type u_2
F : Type u_3
inst✝¹² : NontriviallyNormedField 𝕜
inst✝¹¹ : CompleteSpace 𝕜
inst✝¹⁰ : NormedAddCommGroup E
inst✝⁹ : MeasurableSpace E
inst✝⁸ : BorelSpace E
inst✝⁷ : NormedSpace 𝕜 E
inst✝⁶ : NormedAddCommGroup F
inst✝⁵ : MeasurableSpace F
inst✝⁴ : BorelSpace F
inst✝³ : NormedSpace 𝕜 F
L : E →ₗ[𝕜] F
μ : Measure E
ν : Measure F
inst✝² : μ.IsAddHaarMeasure
inst✝¹ : ν.IsAddHaarMeasure
inst✝ : LocallyCompactSpace E
h : Function.Surjective ⇑L
this✝ : FiniteDimensional 𝕜 E
this : ProperSpace F
S : Submodule 𝕜 E := ker L
T : Submodule 𝕜 E
hT : IsCompl S T
M : (↥S × ↥T) ≃ₗ[𝕜] E := S.prodEquivOfIsCompl T hT
M_cont : Continuous ⇑M.symm
P : ↥S × ↥T →ₗ[𝕜] ↥T := snd 𝕜 ↥S ↥T
P_cont : Continuous ⇑P
I : Function.Bijective ⇑(L.domRestrict T)
L' : ↥T ≃ₗ[𝕜] F := LinearEquiv.ofBijective (L.domRestrict T) I
L'_cont : Continuous ⇑L'
x : E
⊢ L x = (↑L' ∘ₗ P ∘ₗ ↑M.symm) x

Proof state:
𝕜 : Type u_1
E : Type u_2
F : Type u_3
inst✝¹² : NontriviallyNormedField 𝕜
inst✝¹¹ : CompleteSpace 𝕜
inst✝¹⁰ : NormedAddCommGroup E
inst✝⁹ : MeasurableSpace E
inst✝⁸ : BorelSpace E
inst✝⁷ : NormedSpace 𝕜 E
inst✝⁶ : NormedAddCommGroup F
inst✝⁵ : MeasurableSpace F
inst✝⁴ : BorelSpace F
inst✝³ : NormedSpace 𝕜 F
L : E →ₗ[𝕜] F
μ : Measure E
ν : Measure F
inst✝² : μ.IsAddHaarMeasure
inst✝¹ : ν.IsAddHaarMeasure
inst✝ : LocallyCompactSpace E
h : Function.Surjective ⇑L
this✝¹ : FiniteDimensional 𝕜 E
this✝ : ProperSpace F
S : Submodule 𝕜 E := ker L
T : Submodule 𝕜 E
hT : IsCompl S T
M : (↥S × ↥T) ≃ₗ[𝕜] E := S.prodEquivOfIsCompl T hT
M_cont : Continuous ⇑M.symm
P : ↥S × ↥T →ₗ[𝕜] ↥T := snd 𝕜 ↥S ↥T
P_cont : Continuous ⇑P
I✝ : Function.Bijective ⇑(L.domRestrict T)
L' : ↥T ≃ₗ[𝕜] F := LinearEquiv.ofBijective (L.domRestrict T) I✝
L'_cont : Continuous ⇑L'
A : L = ↑L' ∘ₗ P ∘ₗ ↑M.symm
I : map (⇑L) μ = map (⇑L') (map (⇑P) (map (⇑M.symm) μ))
μS : Measure ↥S := addHaar
μT : Measure ↥T := addHaar
c₀ : ℝ≥0∞
c₀_pos : c₀ ≠ 0
c₀_fin : c₀ ≠ ⊤
h₀ : map (⇑M.symm) μ = c₀ • μS.prod μT
J : map (⇑P) (μS.prod μT) = μS univ • μT
this : (map (⇑L') μT).IsAddHaarMeasure
⊢ ∃ c₁, c₁ ≠ 0 ∧ c₁ ≠ ⊤ ∧ map (⇑L') μT = c₁ • ν

Tactic:
refine ⟨addHaarScalarFactor (μT.map L') ν, ?_, ENNReal.coe_ne_top,
  isAddLeftInvariant_eq_smul _ _⟩

Result:
𝕜 : Type u_1
E : Type u_2
F : Type u_3
inst✝¹² : NontriviallyNormedField 𝕜
inst✝¹¹ : CompleteSpace 𝕜
inst✝¹⁰ : NormedAddCommGroup E
inst✝⁹ : MeasurableSpace E
inst✝⁸ : BorelSpace E
inst✝⁷ : NormedSpace 𝕜 E
inst✝⁶ : NormedAddCommGroup F
inst✝⁵ : MeasurableSpace F
inst✝⁴ : BorelSpace F
inst✝³ : NormedSpace 𝕜 F
L : E →ₗ[𝕜] F
μ : Measure E
ν : Measure F
inst✝² : μ.IsAddHaarMeasure
inst✝¹ : ν.IsAddHaarMeasure
inst✝ : LocallyCompactSpace E
h : Function.Surjective ⇑L
this✝¹ : FiniteDimensional 𝕜 E
this✝ : ProperSpace F
S : Submodule 𝕜 E := ker L
T : Submodule 𝕜 E
hT : IsCompl S T
M : (↥S × ↥T) ≃ₗ[𝕜] E := S.prodEquivOfIsCompl T hT
M_cont : Continuous ⇑M.symm
P : ↥S × ↥T →ₗ[𝕜] ↥T := snd 𝕜 ↥S ↥T
P_cont : Continuous ⇑P
I✝ : Function.Bijective ⇑(L.domRestrict T)
L' : ↥T ≃ₗ[𝕜] F := LinearEquiv.ofBijective (L.domRestrict T) I✝
L'_cont : Continuous ⇑L'
A : L = ↑L' ∘ₗ P ∘ₗ ↑M.symm
I : map (⇑L) μ = map (⇑L') (map (⇑P) (map (⇑M.symm) μ))
μS : Measure ↥S := addHaar
μT : Measure ↥T := addHaar
c₀ : ℝ≥0∞
c₀_pos : c₀ ≠ 0
c₀_fin : c₀ ≠ ⊤
h₀ : map (⇑M.symm) μ = c₀ • μS.prod μT
J : map (⇑P) (μS.prod μT) = μS univ • μT
this : (map (⇑L') μT).IsAddHaarMeasure
⊢ ↑((map (⇑L') μT).addHaarScalarFactor ν) ≠ 0

Proof state:
case intro
𝕜 : Type u_1
E : Type u_2
F : Type u_3
inst✝¹² : NontriviallyNormedField 𝕜
inst✝¹¹ : CompleteSpace 𝕜
inst✝¹⁰ : NormedAddCommGroup E
inst✝⁹ : MeasurableSpace E
inst✝⁸ : BorelSpace E
inst✝⁷ : NormedSpace 𝕜 E
inst✝⁶ : NormedAddCommGroup F
inst✝⁵ : MeasurableSpace F
inst✝⁴ : BorelSpace F
inst✝³ : NormedSpace 𝕜 F
L : E →ₗ[𝕜] F
μ : Measure E
ν : Measure F
inst✝² : μ.IsAddHaarMeasure
inst✝¹ : ν.IsAddHaarMeasure
inst✝ : LocallyCompactSpace E
h : Function.Surjective ⇑L
this✝ : FiniteDimensional 𝕜 E
this : ProperSpace F
S : Submodule 𝕜 E := ker L
T : Submodule 𝕜 E
hT : IsCompl S T
M : (↥S × ↥T) ≃ₗ[𝕜] E := S.prodEquivOfIsCompl T hT
M_cont : Continuous ⇑M.symm
⊢ ∃ c, 0 < c ∧ c < ⊤ ∧ map (⇑L) μ = (c * addHaar univ) • ν

Tactic:
let P : S × T →ₗ[𝕜] T := LinearMap.snd 𝕜 S T

Result:
case intro
𝕜 : Type u_1
E : Type u_2
F : Type u_3
inst✝¹² : NontriviallyNormedField 𝕜
inst✝¹¹ : CompleteSpace 𝕜
inst✝¹⁰ : NormedAddCommGroup E
inst✝⁹ : MeasurableSpace E
inst✝⁸ : BorelSpace E
inst✝⁷ : NormedSpace 𝕜 E
inst✝⁶ : NormedAddCommGroup F
inst✝⁵ : MeasurableSpace F
inst✝⁴ : BorelSpace F
inst✝³ : NormedSpace 𝕜 F
L : E →ₗ[𝕜] F
μ : Measure E
ν : Measure F
inst✝² : μ.IsAddHaarMeasure
inst✝¹ : ν.IsAddHaarMeasure
inst✝ : LocallyCompactSpace E
h : Function.Surjective ⇑L
this✝ : FiniteDimensional 𝕜 E
this : ProperSpace F
S : Submodule 𝕜 E := ker L
T : Submodule 𝕜 E
hT : IsCompl S T
M : (↥S × ↥T) ≃ₗ[𝕜] E := S.prodEquivOfIsCompl T hT
M_cont : Continuous ⇑M.symm
P : ↥S × ↥T →ₗ[𝕜] ↥T := snd 𝕜 ↥S ↥T
⊢ ∃ c, 0 < c ∧ c < ⊤ ∧ map (⇑L) μ = (c * addHaar univ) • ν