-- Mathlib/MeasureTheory/Function/Jacobian.lean
theorem mul_le_addHaar_image_of_lt_det (A : E →L[ℝ] E) {m : ℝ≥0}
    (hm : (m : ℝ≥0∞) < ENNReal.ofReal |A.det|) :
    ∀ᶠ δ in 𝓝[>] (0 : ℝ≥0),
      ∀ (s : Set E) (f : E → E), ApproximatesLinearOn f A s δ → (m : ℝ≥0∞) * μ s ≤ μ (f '' s)

Proof state:
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : FiniteDimensional ℝ E
inst✝² : MeasurableSpace E
inst✝¹ : BorelSpace E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
A : E →L[ℝ] E
m : ℝ≥0
hm : ↑m < ENNReal.ofReal |A.det|
mpos : 0 < m
hA : A.det ≠ 0
B : E ≃L[ℝ] E := A.toContinuousLinearEquivOfDetNeZero hA
I : ENNReal.ofReal |(↑B.symm).det| < ↑m⁻¹
δ₀ : ℝ≥0
δ₀pos : 0 < δ₀
hδ₀ : ∀ (t : Set E) (g : E → E), ApproximatesLinearOn g (↑B.symm) t δ₀ → μ (g '' t) ≤ ↑m⁻¹ * μ t
⊢ ∀ᶠ (δ : ℝ≥0) in 𝓝 0, Subsingleton E ∨ δ < ‖↑B.symm‖₊⁻¹

Tactic:
by_cases h : Subsingleton E

Result:
case pos
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : FiniteDimensional ℝ E
inst✝² : MeasurableSpace E
inst✝¹ : BorelSpace E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
A : E →L[ℝ] E
m : ℝ≥0
hm : ↑m < ENNReal.ofReal |A.det|
mpos : 0 < m
hA : A.det ≠ 0
B : E ≃L[ℝ] E := A.toContinuousLinearEquivOfDetNeZero hA
I : ENNReal.ofReal |(↑B.symm).det| < ↑m⁻¹
δ₀ : ℝ≥0
δ₀pos : 0 < δ₀
hδ₀ : ∀ (t : Set E) (g : E → E), ApproximatesLinearOn g (↑B.symm) t δ₀ → μ (g '' t) ≤ ↑m⁻¹ * μ t
h : Subsingleton E
⊢ ∀ᶠ (δ : ℝ≥0) in 𝓝 0, Subsingleton E ∨ δ < ‖↑B.symm‖₊⁻¹

case neg
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : FiniteDimensional ℝ E
inst✝² : MeasurableSpace E
inst✝¹ : BorelSpace E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
A : E →L[ℝ] E
m : ℝ≥0
hm : ↑m < ENNReal.ofReal |A.det|
mpos : 0 < m
hA : A.det ≠ 0
B : E ≃L[ℝ] E := A.toContinuousLinearEquivOfDetNeZero hA
I : ENNReal.ofReal |(↑B.symm).det| < ↑m⁻¹
δ₀ : ℝ≥0
δ₀pos : 0 < δ₀
hδ₀ : ∀ (t : Set E) (g : E → E), ApproximatesLinearOn g (↑B.symm) t δ₀ → μ (g '' t) ≤ ↑m⁻¹ * μ t
h : ¬Subsingleton E
⊢ ∀ᶠ (δ : ℝ≥0) in 𝓝 0, Subsingleton E ∨ δ < ‖↑B.symm‖₊⁻¹

Proof state:
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : FiniteDimensional ℝ E
inst✝² : MeasurableSpace E
inst✝¹ : BorelSpace E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
A : E →L[ℝ] E
m : ℝ≥0
hm : ↑m < ENNReal.ofReal |A.det|
mpos : 0 < m
hA : A.det ≠ 0
B : E ≃L[ℝ] E := A.toContinuousLinearEquivOfDetNeZero hA
I : ENNReal.ofReal |(↑B.symm).det| < ↑m⁻¹
δ₀ : ℝ≥0
δ₀pos : 0 < δ₀
hδ₀ : ∀ (t : Set E) (g : E → E), ApproximatesLinearOn g (↑B.symm) t δ₀ → μ (g '' t) ≤ ↑m⁻¹ * μ t
L1 : ∀ᶠ (δ : ℝ≥0) in 𝓝 0, Subsingleton E ∨ δ < ‖↑B.symm‖₊⁻¹
this : Tendsto (fun δ => ‖↑B.symm‖₊ * (‖↑B.symm‖₊⁻¹ - δ)⁻¹ * δ) (𝓝 0) (𝓝 (‖↑B.symm‖₊ * (‖↑B.symm‖₊⁻¹ - 0)⁻¹ * 0))
⊢ ∀ᶠ (δ : ℝ≥0) in 𝓝 0, ‖↑B.symm‖₊ * (‖↑B.symm‖₊⁻¹ - δ)⁻¹ * δ < δ₀

Tactic:
simp only [mul_zero] at this

Result:
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : FiniteDimensional ℝ E
inst✝² : MeasurableSpace E
inst✝¹ : BorelSpace E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
A : E →L[ℝ] E
m : ℝ≥0
hm : ↑m < ENNReal.ofReal |A.det|
mpos : 0 < m
hA : A.det ≠ 0
B : E ≃L[ℝ] E := A.toContinuousLinearEquivOfDetNeZero hA
I : ENNReal.ofReal |(↑B.symm).det| < ↑m⁻¹
δ₀ : ℝ≥0
δ₀pos : 0 < δ₀
hδ₀ : ∀ (t : Set E) (g : E → E), ApproximatesLinearOn g (↑B.symm) t δ₀ → μ (g '' t) ≤ ↑m⁻¹ * μ t
L1 : ∀ᶠ (δ : ℝ≥0) in 𝓝 0, Subsingleton E ∨ δ < ‖↑B.symm‖₊⁻¹
this : Tendsto (fun δ => ‖↑B.symm‖₊ * (‖↑B.symm‖₊⁻¹ - δ)⁻¹ * δ) (𝓝 0) (𝓝 0)
⊢ ∀ᶠ (δ : ℝ≥0) in 𝓝 0, ‖↑B.symm‖₊ * (‖↑B.symm‖₊⁻¹ - δ)⁻¹ * δ < δ₀

Proof state:
case h
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : FiniteDimensional ℝ E
inst✝² : MeasurableSpace E
inst✝¹ : BorelSpace E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
A : E →L[ℝ] E
m : ℝ≥0
hm : ↑m < ENNReal.ofReal |A.det|
mpos : 0 < m
hA : A.det ≠ 0
B : E ≃L[ℝ] E := A.toContinuousLinearEquivOfDetNeZero hA
I : ENNReal.ofReal |(↑B.symm).det| < ↑m⁻¹
δ₀ : ℝ≥0
δ₀pos : 0 < δ₀
hδ₀ : ∀ (t : Set E) (g : E → E), ApproximatesLinearOn g (↑B.symm) t δ₀ → μ (g '' t) ≤ ↑m⁻¹ * μ t
L1 : ∀ᶠ (δ : ℝ≥0) in 𝓝 0, Subsingleton E ∨ δ < ‖↑B.symm‖₊⁻¹
L2 : ∀ᶠ (δ : ℝ≥0) in 𝓝 0, ‖↑B.symm‖₊ * (‖↑B.symm‖₊⁻¹ - δ)⁻¹ * δ < δ₀
δ : ℝ≥0
h1δ : Subsingleton E ∨ δ < ‖↑B.symm‖₊⁻¹
h2δ : ‖↑B.symm‖₊ * (‖↑B.symm‖₊⁻¹ - δ)⁻¹ * δ < δ₀
s : Set E
f : E → E
hf : ApproximatesLinearOn f A s δ
hf' : ApproximatesLinearOn f (↑B) s δ
F : PartialEquiv E E := hf'.toPartialEquiv h1δ
⊢ μ (↑F.symm '' F.target) ≤ ↑m⁻¹ * μ F.target

Tactic:
exact hδ₀ _ _ ((hf'.to_inv h1δ).mono_num h2δ.le)

Result:
no goals

Proof state:
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : FiniteDimensional ℝ E
inst✝² : MeasurableSpace E
inst✝¹ : BorelSpace E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
A : E →L[ℝ] E
m : ℝ≥0
hm : ↑m < ENNReal.ofReal |A.det|
mpos : 0 < m
hA : A.det ≠ 0
B : E ≃L[ℝ] E := A.toContinuousLinearEquivOfDetNeZero hA
I : ENNReal.ofReal |(↑B.symm).det| < ↑m⁻¹
δ₀ : ℝ≥0
δ₀pos : 0 < δ₀
hδ₀ : ∀ (t : Set E) (g : E → E), ApproximatesLinearOn g (↑B.symm) t δ₀ → μ (g '' t) ≤ ↑m⁻¹ * μ t
L1 : ∀ᶠ (δ : ℝ≥0) in 𝓝 0, Subsingleton E ∨ δ < ‖↑B.symm‖₊⁻¹
L2 : ∀ᶠ (δ : ℝ≥0) in 𝓝 0, ‖↑B.symm‖₊ * (‖↑B.symm‖₊⁻¹ - δ)⁻¹ * δ < δ₀
δ : ℝ≥0
h1δ : Subsingleton E ∨ δ < ‖↑B.symm‖₊⁻¹
h2δ : ‖↑B.symm‖₊ * (‖↑B.symm‖₊⁻¹ - δ)⁻¹ * δ < δ₀
s : Set E
f : E → E
hf : ApproximatesLinearOn f A s δ
⊢ ApproximatesLinearOn f (↑B) s δ

Tactic:
convert hf

Result:
no goals

Proof state:
case h
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : FiniteDimensional ℝ E
inst✝² : MeasurableSpace E
inst✝¹ : BorelSpace E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
A : E →L[ℝ] E
m : ℝ≥0
hm : ↑m < ENNReal.ofReal |A.det|
mpos : 0 < m
hA : A.det ≠ 0
B : E ≃L[ℝ] E := A.toContinuousLinearEquivOfDetNeZero hA
I : ENNReal.ofReal |(↑B.symm).det| < ↑m⁻¹
δ₀ : ℝ≥0
δ₀pos : 0 < δ₀
hδ₀ : ∀ (t : Set E) (g : E → E), ApproximatesLinearOn g (↑B.symm) t δ₀ → μ (g '' t) ≤ ↑m⁻¹ * μ t
L1 : ∀ᶠ (δ : ℝ≥0) in 𝓝 0, Subsingleton E ∨ δ < ‖↑B.symm‖₊⁻¹
L2 : ∀ᶠ (δ : ℝ≥0) in 𝓝 0, ‖↑B.symm‖₊ * (‖↑B.symm‖₊⁻¹ - δ)⁻¹ * δ < δ₀
δ : ℝ≥0
h1δ : Subsingleton E ∨ δ < ‖↑B.symm‖₊⁻¹
h2δ : ‖↑B.symm‖₊ * (‖↑B.symm‖₊⁻¹ - δ)⁻¹ * δ < δ₀
s : Set E
f : E → E
hf : ApproximatesLinearOn f A s δ
hf' : ApproximatesLinearOn f (↑B) s δ
⊢ ↑m * μ s ≤ μ (f '' s)

Tactic:
let F := hf'.toPartialEquiv h1δ

Result:
case h
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : FiniteDimensional ℝ E
inst✝² : MeasurableSpace E
inst✝¹ : BorelSpace E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
A : E →L[ℝ] E
m : ℝ≥0
hm : ↑m < ENNReal.ofReal |A.det|
mpos : 0 < m
hA : A.det ≠ 0
B : E ≃L[ℝ] E := A.toContinuousLinearEquivOfDetNeZero hA
I : ENNReal.ofReal |(↑B.symm).det| < ↑m⁻¹
δ₀ : ℝ≥0
δ₀pos : 0 < δ₀
hδ₀ : ∀ (t : Set E) (g : E → E), ApproximatesLinearOn g (↑B.symm) t δ₀ → μ (g '' t) ≤ ↑m⁻¹ * μ t
L1 : ∀ᶠ (δ : ℝ≥0) in 𝓝 0, Subsingleton E ∨ δ < ‖↑B.symm‖₊⁻¹
L2 : ∀ᶠ (δ : ℝ≥0) in 𝓝 0, ‖↑B.symm‖₊ * (‖↑B.symm‖₊⁻¹ - δ)⁻¹ * δ < δ₀
δ : ℝ≥0
h1δ : Subsingleton E ∨ δ < ‖↑B.symm‖₊⁻¹
h2δ : ‖↑B.symm‖₊ * (‖↑B.symm‖₊⁻¹ - δ)⁻¹ * δ < δ₀
s : Set E
f : E → E
hf : ApproximatesLinearOn f A s δ
hf' : ApproximatesLinearOn f (↑B) s δ
F : PartialEquiv E E := hf'.toPartialEquiv h1δ
⊢ ↑m * μ s ≤ μ (f '' s)

Proof state:
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : FiniteDimensional ℝ E
inst✝² : MeasurableSpace E
inst✝¹ : BorelSpace E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
A : E →L[ℝ] E
m : ℝ≥0
hm : ↑m < ENNReal.ofReal |A.det|
mpos : 0 < m
hA : A.det ≠ 0
B : E ≃L[ℝ] E := A.toContinuousLinearEquivOfDetNeZero hA
I : ENNReal.ofReal |(↑B.symm).det| < ↑m⁻¹
δ₀ : ℝ≥0
δ₀pos : 0 < δ₀
hδ₀ : ∀ (t : Set E) (g : E → E), ApproximatesLinearOn g (↑B.symm) t δ₀ → μ (g '' t) ≤ ↑m⁻¹ * μ t
L1 : ∀ᶠ (δ : ℝ≥0) in 𝓝 0, Subsingleton E ∨ δ < ‖↑B.symm‖₊⁻¹
this : Tendsto (fun δ => ‖↑B.symm‖₊ * (‖↑B.symm‖₊⁻¹ - δ)⁻¹ * δ) (𝓝 0) (𝓝 0)
⊢ ∀ᶠ (δ : ℝ≥0) in 𝓝 0, ‖↑B.symm‖₊ * (‖↑B.symm‖₊⁻¹ - δ)⁻¹ * δ < δ₀

Tactic:
exact (tendsto_order.1 this).2 δ₀ δ₀pos

Result:
no goals