-- Mathlib/MeasureTheory/Measure/Lebesgue/VolumeOfBalls.lean
theorem MeasureTheory.measure_unitBall_eq_integral_div_gamma {E : Type*} {p : ℝ}
    [NormedAddCommGroup E] [NormedSpace ℝ E] [FiniteDimensional ℝ E] [MeasurableSpace E]
    [BorelSpace E] (μ : Measure E) [IsAddHaarMeasure μ] (hp : 0 < p) :
    μ (Metric.ball 0 1) =
      .ofReal ((∫ (x : E), Real.exp (- ‖x‖ ^ p) ∂μ) / Real.Gamma (finrank ℝ E / p + 1))

Proof state:
case inl
E : Type u_1
p : ℝ
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
hp : 0 < p
hE : Subsingleton E
⊢ μ (Metric.ball 0 1) = ENNReal.ofReal ((∫ (x : E), Real.exp (-‖x‖ ^ p) ∂μ) / Real.Gamma (↑(finrank ℝ E) / p + 1))

Tactic:
rw [(Metric.nonempty_ball.mpr zero_lt_one).eq_zero, ← setIntegral_univ,
  Set.univ_nonempty.eq_zero, integral_singleton, finrank_zero_of_subsingleton, Nat.cast_zero,
  zero_div, zero_add, Real.Gamma_one, div_one, norm_zero, Real.zero_rpow hp.ne', neg_zero,
  Real.exp_zero, smul_eq_mul, mul_one, ofReal_toReal (measure_ne_top μ {0})]

Result:
no goals

Proof state:
E : Type u_1
p : ℝ
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
hp : 0 < p
hE : Nontrivial E
this : 0 < ↑(finrank ℝ E)
⊢ 0 < ↑(finrank ℝ E) / p

Tactic:
positivity

Result:
no goals

Proof state:
case inr
E : Type u_1
p : ℝ
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
hp : 0 < p
hE : Nontrivial E
this✝ : 0 < ↑(finrank ℝ E)
this :
  (∫ (y : ℝ) in Set.Ioi 0, y ^ (finrank ℝ E - 1) • Real.exp (-y ^ p)) / Real.Gamma (↑(finrank ℝ E) / p + 1) *
      ↑(finrank ℝ E) =
    1
⊢ μ (Metric.ball 0 1) ≠ ⊤

Tactic:
exact ne_of_lt measure_ball_lt_top

Result:
no goals

Proof state:
E : Type u_1
p : ℝ
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
hp : 0 < p
⊢ μ (Metric.ball 0 1) = ENNReal.ofReal ((∫ (x : E), Real.exp (-‖x‖ ^ p) ∂μ) / Real.Gamma (↑(finrank ℝ E) / p + 1))

Tactic:
obtain hE | hE := subsingleton_or_nontrivial E

Result:
case inl
E : Type u_1
p : ℝ
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
hp : 0 < p
hE : Subsingleton E
⊢ μ (Metric.ball 0 1) = ENNReal.ofReal ((∫ (x : E), Real.exp (-‖x‖ ^ p) ∂μ) / Real.Gamma (↑(finrank ℝ E) / p + 1))

case inr
E : Type u_1
p : ℝ
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
hp : 0 < p
hE : Nontrivial E
⊢ μ (Metric.ball 0 1) = ENNReal.ofReal ((∫ (x : E), Real.exp (-‖x‖ ^ p) ∂μ) / Real.Gamma (↑(finrank ℝ E) / p + 1))

Proof state:
E : Type u_1
p : ℝ
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
hp : 0 < p
hE : Nontrivial E
this : 0 < ↑(finrank ℝ E)
⊢ 1 / p * Real.Gamma (↑(finrank ℝ E) / p) / (↑(finrank ℝ E) / p * Real.Gamma (↑(finrank ℝ E) / p)) * ↑(finrank ℝ E) = 1

Tactic:
field_simp

Result:
E : Type u_1
p : ℝ
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
hp : 0 < p
hE : Nontrivial E
this : 0 < ↑(finrank ℝ E)
⊢ Real.Gamma (↑(finrank ℝ E) / p) * ↑(finrank ℝ E) = ↑(finrank ℝ E) * Real.Gamma (↑(finrank ℝ E) / p)

Proof state:
E : Type u_1
p : ℝ
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
hp : 0 < p
hE : Nontrivial E
this : 0 < ↑(finrank ℝ E)
⊢ -1 < ↑(finrank ℝ E) - 1

Tactic:
linarith

Result:
no goals

Proof state:
case inr
E : Type u_1
p : ℝ
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
hp : 0 < p
hE : Nontrivial E
⊢ μ (Metric.ball 0 1) = ENNReal.ofReal ((∫ (x : E), Real.exp (-‖x‖ ^ p) ∂μ) / Real.Gamma (↑(finrank ℝ E) / p + 1))

Tactic:
have : (0 : ℝ) < finrank ℝ E := Nat.cast_pos.mpr finrank_pos

Result:
case inr
E : Type u_1
p : ℝ
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
hp : 0 < p
hE : Nontrivial E
this : 0 < ↑(finrank ℝ E)
⊢ μ (Metric.ball 0 1) = ENNReal.ofReal ((∫ (x : E), Real.exp (-‖x‖ ^ p) ∂μ) / Real.Gamma (↑(finrank ℝ E) / p + 1))

Proof state:
E : Type u_1
p : ℝ
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
hp : 0 < p
hE : Nontrivial E
this : 0 < ↑(finrank ℝ E)
⊢ (∫ (y : ℝ) in Set.Ioi 0, y ^ (↑(finrank ℝ E) - 1) * Real.exp (-y ^ p)) / Real.Gamma (↑(finrank ℝ E) / p + 1) *
      ↑(finrank ℝ E) =
    1

Tactic:
rw [integral_rpow_mul_exp_neg_rpow hp (by linarith), sub_add_cancel,
  Real.Gamma_add_one (ne_of_gt (by positivity))]

Result:
E : Type u_1
p : ℝ
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
hp : 0 < p
hE : Nontrivial E
this : 0 < ↑(finrank ℝ E)
⊢ 1 / p * Real.Gamma (↑(finrank ℝ E) / p) / (↑(finrank ℝ E) / p * Real.Gamma (↑(finrank ℝ E) / p)) * ↑(finrank ℝ E) = 1

Proof state:
case inr
E : Type u_1
p : ℝ
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
hp : 0 < p
hE : Nontrivial E
this : 0 < ↑(finrank ℝ E)
⊢ μ (Metric.ball 0 1) = ENNReal.ofReal ((∫ (x : E), Real.exp (-‖x‖ ^ p) ∂μ) / Real.Gamma (↑(finrank ℝ E) / p + 1))

Tactic:
have : ((∫ y in Set.Ioi (0 : ℝ), y ^ (finrank ℝ E - 1) • Real.exp (-y ^ p)) /
    Real.Gamma ((finrank ℝ E) / p + 1)) * (finrank ℝ E) = 1 := by
  simp_rw [← Real.rpow_natCast _ (finrank ℝ E - 1), smul_eq_mul, Nat.cast_sub finrank_pos,
    Nat.cast_one]
  rw [integral_rpow_mul_exp_neg_rpow hp (by linarith), sub_add_cancel,
    Real.Gamma_add_one (ne_of_gt (by positivity))]
  field_simp; ring

Result:
case inr
E : Type u_1
p : ℝ
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
hp : 0 < p
hE : Nontrivial E
this✝ : 0 < ↑(finrank ℝ E)
this :
  (∫ (y : ℝ) in Set.Ioi 0, y ^ (finrank ℝ E - 1) • Real.exp (-y ^ p)) / Real.Gamma (↑(finrank ℝ E) / p + 1) *
      ↑(finrank ℝ E) =
    1
⊢ μ (Metric.ball 0 1) = ENNReal.ofReal ((∫ (x : E), Real.exp (-‖x‖ ^ p) ∂μ) / Real.Gamma (↑(finrank ℝ E) / p + 1))