theorem MeasureTheory.measure_lt_one_eq_integral_div_gamma {p : ℝ} (hp : 0 < p) :
    μ {x : E | g x < 1} =
      .ofReal ((∫ (x : E), Real.exp (- (g x) ^ p) ∂μ) / Real.Gamma (finrank ℝ E / p + 1)) := by
  -- We copy `E` to a new type `F` on which we will put the norm defined by `g`
  letI F : Type _ := E
  letI : NormedAddCommGroup F :=
  { norm := g
    dist := fun x y => g (x - y)
    dist_self := by simp only [_root_.sub_self, h1, forall_const]
    dist_comm := fun _ _ => by dsimp [dist]; rw [← h2, neg_sub]
    dist_triangle := fun x y z => by convert h3 (x - y) (y - z) using 1; abel_nf
    edist := fun x y => .ofReal (g (x - y))
    edist_dist := fun _ _ => rfl
    eq_of_dist_eq_zero := by convert fun _ _ h => eq_of_sub_eq_zero (h4 h) }
  letI : NormedSpace ℝ F :=
  { norm_smul_le := fun _ _ =>  h5 _ _ }
  -- We put the new topology on F
  letI : TopologicalSpace F := UniformSpace.toTopologicalSpace
  letI : MeasurableSpace F := borel F
  have : BorelSpace F := { measurable_eq := rfl }
  -- The map between `E` and `F` as a continuous linear equivalence
  let φ := @LinearEquiv.toContinuousLinearEquiv ℝ _ E _ _ tE _ _ F _ _ _ _ _ _ _ _ _
    (LinearEquiv.refl ℝ E : E ≃ₗ[ℝ] F)
  -- The measure `ν` is the measure on `F` defined by `μ`
  -- Since we have two different topologies, it is necessary to specify the topology of E
  let ν : Measure F := @Measure.map E F _ mE φ μ
  have : IsAddHaarMeasure ν :=
    @ContinuousLinearEquiv.isAddHaarMeasure_map E F ℝ ℝ _ _ _ _ _ _ tE _ _ _ _ _ _ _ mE _ _ _ φ μ _
  convert (measure_unitBall_eq_integral_div_gamma ν hp) using 1
  · rw [@Measure.map_apply E F mE _ μ φ _ _ measurableSet_ball]
    · congr!
      simp_rw [Metric.ball, dist_zero_right]
      rfl
    · refine @Continuous.measurable E F tE mE _ _ _ _ φ ?_
      exact @ContinuousLinearEquiv.continuous ℝ ℝ _ _ _ _ _ _ E tE _ F _ _ _ _ φ
  · -- The map between `E` and `F` as a measurable equivalence
    let ψ := @Homeomorph.toMeasurableEquiv E F tE mE _ _ _ _
      (@ContinuousLinearEquiv.toHomeomorph ℝ ℝ _ _ _ _ _ _ E tE _ F _ _ _ _ φ)
    -- The map `ψ` is measure preserving by construction
    have : @MeasurePreserving E F mE _ ψ μ ν :=
      @Measurable.measurePreserving E F mE _ ψ (@MeasurableEquiv.measurable E F mE _ ψ) _
    

Proof state:
case h.e'_3
E : Type u_1
inst✝⁷ : AddCommGroup E
inst✝⁶ : Module ℝ E
inst✝⁵ : FiniteDimensional ℝ E
mE : MeasurableSpace E
tE : TopologicalSpace E
inst✝⁴ : TopologicalAddGroup E
inst✝³ : BorelSpace E
inst✝² : T2Space E
inst✝¹ : ContinuousSMul ℝ E
μ : Measure E
inst✝ : IsAddHaarMeasure μ
g : E → ℝ
h1 : g 0 = 0
h2 : ∀ (x : E), g (-x) = g x
h3 : ∀ (x y : E), g (x + y) ≤ g x + g y
h4 : ∀ {x : E}, g x = 0 → x = 0
h5 : ∀ (r : ℝ) (x : E), g (r • x) ≤ |r| * g x
p : ℝ
hp : 0 < p
F : Type u_1 := E
this✝⁵ : NormedAddCommGroup F := NormedAddCommGroup.mk
this✝⁴ : NormedSpace ℝ F := NormedSpace.mk (_ : ∀ (x : ℝ) (x_1 : F), g (x • x_1) ≤ |x| * g x_1)
this✝³ : TopologicalSpace F := UniformSpace.toTopologicalSpace
this✝² : MeasurableSpace F := borel F
this✝¹ : BorelSpace F
φ : E ≃L[ℝ] F := LinearEquiv.toContinuousLinearEquiv (LinearEquiv.refl ℝ E)
ν : Measure F := map (⇑φ) μ
this✝ : IsAddHaarMeasure ν
ψ : E ≃ᵐ F := Homeomorph.toMeasurableEquiv (ContinuousLinearEquiv.toHomeomorph φ)
this : MeasurePreserving ⇑ψ
⊢ ENNReal.ofReal ((∫ (x : E), Real.exp (-g x ^ p) ∂μ) / Real.Gamma (↑(finrank ℝ E) / p + 1)) =
    ENNReal.ofReal ((∫ (x : F), Real.exp (-‖x‖ ^ p) ∂ν) / Real.Gamma (↑(finrank ℝ F) / p + 1))

Next tactic:
erw [← this.integral_comp']