-- Mathlib/MeasureTheory/Measure/Lebesgue/EqHaar.lean
theorem addHaar_image_linearMap (f : E →ₗ[ℝ] E) (s : Set E) :
    μ (f '' s) = ENNReal.ofReal |LinearMap.det f| * μ s

Proof state:
case inl
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : MeasurableSpace E
inst✝² : BorelSpace E
inst✝¹ : FiniteDimensional ℝ E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
f : E →ₗ[ℝ] E
s : Set E
hf : LinearMap.det f ≠ 0
⊢ μ (⇑f '' s) = ENNReal.ofReal |LinearMap.det f| * μ s

Tactic:
let g := (f.equivOfDetNeZero hf).toContinuousLinearEquiv

Result:
case inl
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : MeasurableSpace E
inst✝² : BorelSpace E
inst✝¹ : FiniteDimensional ℝ E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
f : E →ₗ[ℝ] E
s : Set E
hf : LinearMap.det f ≠ 0
g : E ≃L[ℝ] E := (f.equivOfDetNeZero hf).toContinuousLinearEquiv
⊢ μ (⇑f '' s) = ENNReal.ofReal |LinearMap.det f| * μ s

Proof state:
case inl
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : MeasurableSpace E
inst✝² : BorelSpace E
inst✝¹ : FiniteDimensional ℝ E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
f : E →ₗ[ℝ] E
s : Set E
hf : LinearMap.det f ≠ 0
g : E ≃L[ℝ] E := (f.equivOfDetNeZero hf).toContinuousLinearEquiv
⊢ μ (⇑g '' s) = ENNReal.ofReal |LinearMap.det f| * μ s

Tactic:
rw [ContinuousLinearEquiv.image_eq_preimage g s, addHaar_preimage_continuousLinearEquiv]

Result:
case inl
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : MeasurableSpace E
inst✝² : BorelSpace E
inst✝¹ : FiniteDimensional ℝ E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
f : E →ₗ[ℝ] E
s : Set E
hf : LinearMap.det f ≠ 0
g : E ≃L[ℝ] E := (f.equivOfDetNeZero hf).toContinuousLinearEquiv
⊢ ENNReal.ofReal |LinearMap.det ↑↑g.symm.symm| * μ s = ENNReal.ofReal |LinearMap.det f| * μ s

Proof state:
case inr
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : MeasurableSpace E
inst✝² : BorelSpace E
inst✝¹ : FiniteDimensional ℝ E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
f : E →ₗ[ℝ] E
s : Set E
hf : LinearMap.det f = 0
this : μ ↑(LinearMap.range f) = 0
⊢ μ (⇑f '' s) = 0

Tactic:
exact le_antisymm (le_trans (measure_mono (image_subset_range _ _)) this.le) (zero_le _)

Result:
no goals

Proof state:
case inr
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : MeasurableSpace E
inst✝² : BorelSpace E
inst✝¹ : FiniteDimensional ℝ E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
f : E →ₗ[ℝ] E
s : Set E
hf : LinearMap.det f = 0
⊢ μ (⇑f '' s) = ENNReal.ofReal |LinearMap.det f| * μ s

Tactic:
simp only [hf, zero_mul, ENNReal.ofReal_zero, abs_zero]

Result:
case inr
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : MeasurableSpace E
inst✝² : BorelSpace E
inst✝¹ : FiniteDimensional ℝ E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
f : E →ₗ[ℝ] E
s : Set E
hf : LinearMap.det f = 0
⊢ μ (⇑f '' s) = 0

Proof state:
case inl
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : MeasurableSpace E
inst✝² : BorelSpace E
inst✝¹ : FiniteDimensional ℝ E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
f : E →ₗ[ℝ] E
s : Set E
hf : LinearMap.det f ≠ 0
g : E ≃L[ℝ] E := (f.equivOfDetNeZero hf).toContinuousLinearEquiv
⊢ ENNReal.ofReal |LinearMap.det ↑↑g.symm.symm| * μ s = ENNReal.ofReal |LinearMap.det f| * μ s

Tactic:
congr

Result:
no goals

Proof state:
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : MeasurableSpace E
inst✝² : BorelSpace E
inst✝¹ : FiniteDimensional ℝ E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
f : E →ₗ[ℝ] E
s : Set E
⊢ μ (⇑f '' s) = ENNReal.ofReal |LinearMap.det f| * μ s

Tactic:
rcases ne_or_eq (LinearMap.det f) 0 with (hf | hf)

Result:
case inl
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : MeasurableSpace E
inst✝² : BorelSpace E
inst✝¹ : FiniteDimensional ℝ E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
f : E →ₗ[ℝ] E
s : Set E
hf : LinearMap.det f ≠ 0
⊢ μ (⇑f '' s) = ENNReal.ofReal |LinearMap.det f| * μ s

case inr
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : MeasurableSpace E
inst✝² : BorelSpace E
inst✝¹ : FiniteDimensional ℝ E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
f : E →ₗ[ℝ] E
s : Set E
hf : LinearMap.det f = 0
⊢ μ (⇑f '' s) = ENNReal.ofReal |LinearMap.det f| * μ s

Proof state:
case inr
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : MeasurableSpace E
inst✝² : BorelSpace E
inst✝¹ : FiniteDimensional ℝ E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
f : E →ₗ[ℝ] E
s : Set E
hf : LinearMap.det f = 0
⊢ μ (⇑f '' s) = 0

Tactic:
have : μ (LinearMap.range f) = 0 :=
  addHaar_submodule μ _ (LinearMap.range_lt_top_of_det_eq_zero hf).ne

Result:
case inr
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : MeasurableSpace E
inst✝² : BorelSpace E
inst✝¹ : FiniteDimensional ℝ E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
f : E →ₗ[ℝ] E
s : Set E
hf : LinearMap.det f = 0
this : μ ↑(LinearMap.range f) = 0
⊢ μ (⇑f '' s) = 0

Proof state:
case inl
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : MeasurableSpace E
inst✝² : BorelSpace E
inst✝¹ : FiniteDimensional ℝ E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
f : E →ₗ[ℝ] E
s : Set E
hf : LinearMap.det f ≠ 0
g : E ≃L[ℝ] E := (f.equivOfDetNeZero hf).toContinuousLinearEquiv
⊢ μ (⇑f '' s) = ENNReal.ofReal |LinearMap.det f| * μ s

Tactic:
change μ (g '' s) = _

Result:
case inl
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : MeasurableSpace E
inst✝² : BorelSpace E
inst✝¹ : FiniteDimensional ℝ E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
f : E →ₗ[ℝ] E
s : Set E
hf : LinearMap.det f ≠ 0
g : E ≃L[ℝ] E := (f.equivOfDetNeZero hf).toContinuousLinearEquiv
⊢ μ (⇑g '' s) = ENNReal.ofReal |LinearMap.det f| * μ s