Theorem: MeasureTheory.lintegral_rpow_eq_lintegral_meas_le_mul
File: Mathlib/Analysis/SpecialFunctions/Pow/Integral.lean

Proof steps:
state_before: α : Type u_1
inst✝ : MeasurableSpace α
f : α → ℝ
μ : Measure α
f_nn : 0 ≤ᶠ[ae μ] f
f_mble : AEMeasurable f μ
p : ℝ
p_pos : 0 < p
⊢ ∫⁻ (ω : α), ENNReal.ofReal (f ω ^ p) ∂μ =
    ENNReal.ofReal p * ∫⁻ (t : ℝ) in Ioi 0, μ {a | t ≤ f a} * ENNReal.ofReal (t ^ (p - 1))
state_after: α : Type u_1
inst✝ : MeasurableSpace α
f : α → ℝ
μ : Measure α
f_nn : 0 ≤ᶠ[ae μ] f
f_mble : AEMeasurable f μ
p : ℝ
p_pos : 0 < p
one_lt_p : -1 < p - 1
⊢ ∫⁻ (ω : α), ENNReal.ofReal (f ω ^ p) ∂μ =
    ENNReal.ofReal p * ∫⁻ (t : ℝ) in Ioi 0, μ {a | t ≤ f a} * ENNReal.ofReal (t ^ (p - 1))
tactic: have one_lt_p : -1 < p - 1 := by linarith
state_before: α : Type u_1
inst✝ : MeasurableSpace α
f : α → ℝ
μ : Measure α
f_nn : 0 ≤ᶠ[ae μ] f
f_mble : AEMeasurable f μ
p : ℝ
p_pos : 0 < p
one_lt_p : -1 < p - 1
⊢ ∫⁻ (ω : α), ENNReal.ofReal (f ω ^ p) ∂μ =
    ENNReal.ofReal p * ∫⁻ (t : ℝ) in Ioi 0, μ {a | t ≤ f a} * ENNReal.ofReal (t ^ (p - 1))
state_after: α : Type u_1
inst✝ : MeasurableSpace α
f : α → ℝ
μ : Measure α
f_nn : 0 ≤ᶠ[ae μ] f
f_mble : AEMeasurable f μ
p : ℝ
p_pos : 0 < p
one_lt_p : -1 < p - 1
obs : ∀ (x : ℝ), ∫ (t : ℝ) in 0 ..x, t ^ (p - 1) = x ^ p / p
⊢ ∫⁻ (ω : α), ENNReal.ofReal (f ω ^ p) ∂μ =
    ENNReal.ofReal p * ∫⁻ (t : ℝ) in Ioi 0, μ {a | t ≤ f a} * ENNReal.ofReal (t ^ (p - 1))
tactic: have obs : ∀ x : ℝ, ∫ t : ℝ in (0)..x, t ^ (p - 1) = x ^ p / p := by
    intro x
    rw [integral_rpow (Or.inl one_lt_p)]
    simp [Real.zero_rpow p_pos.ne.symm]
state_before: α : Type u_1
inst✝ : MeasurableSpace α
f : α → ℝ
μ : Measure α
f_nn : 0 ≤ᶠ[ae μ] f
f_mble : AEMeasurable f μ
p : ℝ
p_pos : 0 < p
one_lt_p : -1 < p - 1
obs : ∀ (x : ℝ), ∫ (t : ℝ) in 0 ..x, t ^ (p - 1) = x ^ p / p
⊢ ∫⁻ (ω : α), ENNReal.ofReal (f ω ^ p) ∂μ =
    ENNReal.ofReal p * ∫⁻ (t : ℝ) in Ioi 0, μ {a | t ≤ f a} * ENNReal.ofReal (t ^ (p - 1))
state_after: α : Type u_1
inst✝ : MeasurableSpace α
f : α → ℝ
μ : Measure α
f_nn : 0 ≤ᶠ[ae μ] f
f_mble : AEMeasurable f μ
p : ℝ
p_pos : 0 < p
one_lt_p : -1 < p - 1
g : ℝ → ℝ := fun t => t ^ (p - 1)
obs : ∀ (x : ℝ), intervalIntegral g 0 x volume = x ^ p / p
⊢ ∫⁻ (ω : α), ENNReal.ofReal (f ω ^ p) ∂μ =
    ENNReal.ofReal p * ∫⁻ (t : ℝ) in Ioi 0, μ {a | t ≤ f a} * ENNReal.ofReal (t ^ (p - 1))
tactic: set g := fun t : ℝ => t ^ (p - 1)
state_before: α : Type u_1
inst✝ : MeasurableSpace α
f : α → ℝ
μ : Measure α
f_nn : 0 ≤ᶠ[ae μ] f
f_mble : AEMeasurable f μ
p : ℝ
p_pos : 0 < p
one_lt_p : -1 < p - 1
g : ℝ → ℝ := fun t => t ^ (p - 1)
obs : ∀ (x : ℝ), intervalIntegral g 0 x volume = x ^ p / p
⊢ ∫⁻ (ω : α), ENNReal.ofReal (f ω ^ p) ∂μ =
    ENNReal.ofReal p * ∫⁻ (t : ℝ) in Ioi 0, μ {a | t ≤ f a} * ENNReal.ofReal (t ^ (p - 1))
state_after: α : Type u_1
inst✝ : MeasurableSpace α
f : α → ℝ
μ : Measure α
f_nn : 0 ≤ᶠ[ae μ] f
f_mble : AEMeasurable f μ
p : ℝ
p_pos : 0 < p
one_lt_p : -1 < p - 1
g : ℝ → ℝ := fun t => t ^ (p - 1)
obs : ∀ (x : ℝ), intervalIntegral g 0 x volume = x ^ p / p
g_nn : ∀ᵐ (t : ℝ) ∂volume.restrict (Ioi 0), 0 ≤ g t
⊢ ∫⁻ (ω : α), ENNReal.ofReal (f ω ^ p) ∂μ =
    ENNReal.ofReal p * ∫⁻ (t : ℝ) in Ioi 0, μ {a | t ≤ f a} * ENNReal.ofReal (t ^ (p - 1))
tactic: have g_nn : ∀ᵐ t ∂volume.restrict (Ioi (0 : ℝ)), 0 ≤ g t := by
    filter_upwards [self_mem_ae_restrict (measurableSet_Ioi : MeasurableSet (Ioi (0 : ℝ)))]
    intro t t_pos
    exact Real.rpow_nonneg (mem_Ioi.mp t_pos).le (p - 1)
state_before: α : Type u_1
inst✝ : MeasurableSpace α
f : α → ℝ
μ : Measure α
f_nn : 0 ≤ᶠ[ae μ] f
f_mble : AEMeasurable f μ
p : ℝ
p_pos : 0 < p
one_lt_p : -1 < p - 1
g : ℝ → ℝ := fun t => t ^ (p - 1)
obs : ∀ (x : ℝ), intervalIntegral g 0 x volume = x ^ p / p
g_nn : ∀ᵐ (t : ℝ) ∂volume.restrict (Ioi 0), 0 ≤ g t
⊢ ∫⁻ (ω : α), ENNReal.ofReal (f ω ^ p) ∂μ =
    ENNReal.ofReal p * ∫⁻ (t : ℝ) in Ioi 0, μ {a | t ≤ f a} * ENNReal.ofReal (t ^ (p - 1))
state_after: α : Type u_1
inst✝ : MeasurableSpace α
f : α → ℝ
μ : Measure α
f_nn : 0 ≤ᶠ[ae μ] f
f_mble : AEMeasurable f μ
p : ℝ
p_pos : 0 < p
one_lt_p : -1 < p - 1
g : ℝ → ℝ := fun t => t ^ (p - 1)
obs : ∀ (x : ℝ), intervalIntegral g 0 x volume = x ^ p / p
g_nn : ∀ᵐ (t : ℝ) ∂volume.restrict (Ioi 0), 0 ≤ g t
g_intble : ∀ t > 0, IntervalIntegrable g volume 0 t
⊢ ∫⁻ (ω : α), ENNReal.ofReal (f ω ^ p) ∂μ =
    ENNReal.ofReal p * ∫⁻ (t : ℝ) in Ioi 0, μ {a | t ≤ f a} * ENNReal.ofReal (t ^ (p - 1))
tactic: have g_intble : ∀ t > 0, IntervalIntegrable g volume 0 t := fun _ _ =>
    intervalIntegral.intervalIntegrable_rpow' one_lt_p
state_before: α : Type u_1
inst✝ : MeasurableSpace α
f : α → ℝ
μ : Measure α
f_nn : 0 ≤ᶠ[ae μ] f
f_mble : AEMeasurable f μ
p : ℝ
p_pos : 0 < p
one_lt_p : -1 < p - 1
g : ℝ → ℝ := fun t => t ^ (p - 1)
obs : ∀ (x : ℝ), intervalIntegral g 0 x volume = x ^ p / p
g_nn : ∀ᵐ (t : ℝ) ∂volume.restrict (Ioi 0), 0 ≤ g t
g_intble : ∀ t > 0, IntervalIntegrable g volume 0 t
⊢ ∫⁻ (ω : α), ENNReal.ofReal (f ω ^ p) ∂μ =
    ENNReal.ofReal p * ∫⁻ (t : ℝ) in Ioi 0, μ {a | t ≤ f a} * ENNReal.ofReal (t ^ (p - 1))
state_after: α : Type u_1
inst✝ : MeasurableSpace α
f : α → ℝ
μ : Measure α
f_nn : 0 ≤ᶠ[ae μ] f
f_mble : AEMeasurable f μ
p : ℝ
p_pos : 0 < p
one_lt_p : -1 < p - 1
g : ℝ → ℝ := fun t => t ^ (p - 1)
obs : ∀ (x : ℝ), intervalIntegral g 0 x volume = x ^ p / p
g_nn : ∀ᵐ (t : ℝ) ∂volume.restrict (Ioi 0), 0 ≤ g t
g_intble : ∀ t > 0, IntervalIntegrable g volume 0 t
key :
  ∫⁻ (ω : α), ENNReal.ofReal (∫ (t : ℝ) in 0 ..f ω, g t) ∂μ =
    ∫⁻ (t : ℝ) in Ioi 0, μ {a | t ≤ f a} * ENNReal.ofReal (g t)
⊢ ∫⁻ (ω : α), ENNReal.ofReal (f ω ^ p) ∂μ =
    ENNReal.ofReal p * ∫⁻ (t : ℝ) in Ioi 0, μ {a | t ≤ f a} * ENNReal.ofReal (t ^ (p - 1))
tactic: have key := lintegral_comp_eq_lintegral_meas_le_mul μ f_nn f_mble g_intble g_nn
state_before: α : Type u_1
inst✝ : MeasurableSpace α
f : α → ℝ
μ : Measure α
f_nn : 0 ≤ᶠ[ae μ] f
f_mble : AEMeasurable f μ
p : ℝ
p_pos : 0 < p
one_lt_p : -1 < p - 1
g : ℝ → ℝ := fun t => t ^ (p - 1)
obs : ∀ (x : ℝ), intervalIntegral g 0 x volume = x ^ p / p
g_nn : ∀ᵐ (t : ℝ) ∂volume.restrict (Ioi 0), 0 ≤ g t
g_intble : ∀ t > 0, IntervalIntegrable g volume 0 t
key :
  ∫⁻ (ω : α), ENNReal.ofReal (∫ (t : ℝ) in 0 ..f ω, g t) ∂μ =
    ∫⁻ (t : ℝ) in Ioi 0, μ {a | t ≤ f a} * ENNReal.ofReal (g t)
⊢ ∫⁻ (ω : α), ENNReal.ofReal (f ω ^ p) ∂μ =
    ENNReal.ofReal p * ∫⁻ (t : ℝ) in Ioi 0, μ {a | t ≤ f a} * ENNReal.ofReal (t ^ (p - 1))
state_after: α : Type u_1
inst✝ : MeasurableSpace α
f : α → ℝ
μ : Measure α
f_nn : 0 ≤ᶠ[ae μ] f
f_mble : AEMeasurable f μ
p : ℝ
p_pos : 0 < p
one_lt_p : -1 < p - 1
g : ℝ → ℝ := fun t => t ^ (p - 1)
obs : ∀ (x : ℝ), intervalIntegral g 0 x volume = x ^ p / p
g_nn : ∀ᵐ (t : ℝ) ∂volume.restrict (Ioi 0), 0 ≤ g t
g_intble : ∀ t > 0, IntervalIntegrable g volume 0 t
key :
  ∫⁻ (ω : α), ENNReal.ofReal (∫ (t : ℝ) in 0 ..f ω, g t) ∂μ =
    ∫⁻ (t : ℝ) in Ioi 0, μ {a | t ≤ f a} * ENNReal.ofReal (g t)
⊢ ∫⁻ (ω : α), ENNReal.ofReal (f ω ^ p) ∂μ = ∫⁻ (a : α), ENNReal.ofReal p * ENNReal.ofReal (f a ^ p / p) ∂μ

α : Type u_1
inst✝ : MeasurableSpace α
f : α → ℝ
μ : Measure α
f_nn : 0 ≤ᶠ[ae μ] f
f_mble : AEMeasurable f μ
p : ℝ
p_pos : 0 < p
one_lt_p : -1 < p - 1
g : ℝ → ℝ := fun t => t ^ (p - 1)
obs : ∀ (x : ℝ), intervalIntegral g 0 x volume = x ^ p / p
g_nn : ∀ᵐ (t : ℝ) ∂volume.restrict (Ioi 0), 0 ≤ g t
g_intble : ∀ t > 0, IntervalIntegrable g volume 0 t
key :
  ∫⁻ (ω : α), ENNReal.ofReal (∫ (t : ℝ) in 0 ..f ω, g t) ∂μ =
    ∫⁻ (t : ℝ) in Ioi 0, μ {a | t ≤ f a} * ENNReal.ofReal (g t)
⊢ AEMeasurable (fun ω => ENNReal.ofReal (f ω ^ p / p)) μ
tactic: rw [← key, ← lintegral_const_mul'' (ENNReal.ofReal p)] <;> simp_rw [obs]