-- Mathlib/MeasureTheory/Integral/Bochner.lean
theorem integral_trim (hm : m ≤ m0) {f : β → G} (hf : StronglyMeasurable[m] f) :
    ∫ x, f x ∂μ = ∫ x, f x ∂μ.trim hm

Proof state:
case pos
G : Type u_5
inst✝¹ : NormedAddCommGroup G
inst✝ : NormedSpace ℝ G
β : Type u_6
m m0 : MeasurableSpace β
μ : Measure β
hm : m ≤ m0
f : β → G
hf : StronglyMeasurable f
hG : CompleteSpace G
this✝¹ : MeasurableSpace G := borel G
this✝ : BorelSpace G
hf_int : Integrable f μ
this : SeparableSpace ↑(range f ∪ {0})
f_seq : ℕ → β →ₛ G := SimpleFunc.approxOn f ⋯ (range f ∪ {0}) 0 ⋯
hf_seq_meas : ∀ (n : ℕ), StronglyMeasurable ⇑(f_seq n)
hf_seq_int : ∀ (n : ℕ), Integrable (⇑(f_seq n)) μ
hf_seq_int_m : ∀ (n : ℕ), Integrable (⇑(f_seq n)) (μ.trim hm)
hf_seq_eq : ∀ (n : ℕ), ∫ (x : β), (f_seq n) x ∂μ = ∫ (x : β), (f_seq n) x ∂μ.trim hm
⊢ ∫ (x : β), f x ∂μ = ∫ (x : β), f x ∂μ.trim hm

Tactic:
have h_lim_1 : atTop.Tendsto (fun n => ∫ x, f_seq n x ∂μ) (𝓝 (∫ x, f x ∂μ)) := by
  refine tendsto_integral_of_L1 f hf_int (Eventually.of_forall hf_seq_int) ?_
  exact SimpleFunc.tendsto_approxOn_range_L1_enorm (hf.mono hm).measurable hf_int

Result:
case pos
G : Type u_5
inst✝¹ : NormedAddCommGroup G
inst✝ : NormedSpace ℝ G
β : Type u_6
m m0 : MeasurableSpace β
μ : Measure β
hm : m ≤ m0
f : β → G
hf : StronglyMeasurable f
hG : CompleteSpace G
this✝¹ : MeasurableSpace G := borel G
this✝ : BorelSpace G
hf_int : Integrable f μ
this : SeparableSpace ↑(range f ∪ {0})
f_seq : ℕ → β →ₛ G := SimpleFunc.approxOn f ⋯ (range f ∪ {0}) 0 ⋯
hf_seq_meas : ∀ (n : ℕ), StronglyMeasurable ⇑(f_seq n)
hf_seq_int : ∀ (n : ℕ), Integrable (⇑(f_seq n)) μ
hf_seq_int_m : ∀ (n : ℕ), Integrable (⇑(f_seq n)) (μ.trim hm)
hf_seq_eq : ∀ (n : ℕ), ∫ (x : β), (f_seq n) x ∂μ = ∫ (x : β), (f_seq n) x ∂μ.trim hm
h_lim_1 : Tendsto (fun n => ∫ (x : β), (f_seq n) x ∂μ) atTop (𝓝 (∫ (x : β), f x ∂μ))
⊢ ∫ (x : β), f x ∂μ = ∫ (x : β), f x ∂μ.trim hm

Proof state:
G : Type u_5
inst✝¹ : NormedAddCommGroup G
inst✝ : NormedSpace ℝ G
β : Type u_6
m m0 : MeasurableSpace β
μ : Measure β
hm : m ≤ m0
f : β → G
hf : StronglyMeasurable f
⊢ ∫ (x : β), f x ∂μ = ∫ (x : β), f x ∂μ.trim hm

Tactic:
by_cases hG : CompleteSpace G

Result:
case pos
G : Type u_5
inst✝¹ : NormedAddCommGroup G
inst✝ : NormedSpace ℝ G
β : Type u_6
m m0 : MeasurableSpace β
μ : Measure β
hm : m ≤ m0
f : β → G
hf : StronglyMeasurable f
hG : CompleteSpace G
⊢ ∫ (x : β), f x ∂μ = ∫ (x : β), f x ∂μ.trim hm

case neg
G : Type u_5
inst✝¹ : NormedAddCommGroup G
inst✝ : NormedSpace ℝ G
β : Type u_6
m m0 : MeasurableSpace β
μ : Measure β
hm : m ≤ m0
f : β → G
hf : StronglyMeasurable f
hG : ¬CompleteSpace G
⊢ ∫ (x : β), f x ∂μ = ∫ (x : β), f x ∂μ.trim hm

Proof state:
case pos
G : Type u_5
inst✝¹ : NormedAddCommGroup G
inst✝ : NormedSpace ℝ G
β : Type u_6
m m0 : MeasurableSpace β
μ : Measure β
hm : m ≤ m0
f : β → G
hf : StronglyMeasurable f
hG : CompleteSpace G
this✝¹ : MeasurableSpace G := borel G
this✝ : BorelSpace G
hf_int : Integrable f μ
this : SeparableSpace ↑(range f ∪ {0})
f_seq : ℕ → β →ₛ G := SimpleFunc.approxOn f ⋯ (range f ∪ {0}) 0 ⋯
hf_seq_meas : ∀ (n : ℕ), StronglyMeasurable ⇑(f_seq n)
⊢ ∫ (x : β), f x ∂μ = ∫ (x : β), f x ∂μ.trim hm

Tactic:
have hf_seq_int : ∀ n, Integrable (f_seq n) μ :=
  SimpleFunc.integrable_approxOn_range (hf.mono hm).measurable hf_int

Result:
case pos
G : Type u_5
inst✝¹ : NormedAddCommGroup G
inst✝ : NormedSpace ℝ G
β : Type u_6
m m0 : MeasurableSpace β
μ : Measure β
hm : m ≤ m0
f : β → G
hf : StronglyMeasurable f
hG : CompleteSpace G
this✝¹ : MeasurableSpace G := borel G
this✝ : BorelSpace G
hf_int : Integrable f μ
this : SeparableSpace ↑(range f ∪ {0})
f_seq : ℕ → β →ₛ G := SimpleFunc.approxOn f ⋯ (range f ∪ {0}) 0 ⋯
hf_seq_meas : ∀ (n : ℕ), StronglyMeasurable ⇑(f_seq n)
hf_seq_int : ∀ (n : ℕ), Integrable (⇑(f_seq n)) μ
⊢ ∫ (x : β), f x ∂μ = ∫ (x : β), f x ∂μ.trim hm

Proof state:
case pos
G : Type u_5
inst✝¹ : NormedAddCommGroup G
inst✝ : NormedSpace ℝ G
β : Type u_6
m m0 : MeasurableSpace β
μ : Measure β
hm : m ≤ m0
f : β → G
hf : StronglyMeasurable f
hG : CompleteSpace G
this✝¹ : MeasurableSpace G := borel G
this✝ : BorelSpace G
hf_int : Integrable f μ
this : SeparableSpace ↑(range f ∪ {0})
f_seq : ℕ → β →ₛ G := SimpleFunc.approxOn f ⋯ (range f ∪ {0}) 0 ⋯
⊢ ∫ (x : β), f x ∂μ = ∫ (x : β), f x ∂μ.trim hm

Tactic:
have hf_seq_meas : ∀ n, StronglyMeasurable[m] (f_seq n) := fun n =>
  @SimpleFunc.stronglyMeasurable β G m _ (f_seq n)

Result:
case pos
G : Type u_5
inst✝¹ : NormedAddCommGroup G
inst✝ : NormedSpace ℝ G
β : Type u_6
m m0 : MeasurableSpace β
μ : Measure β
hm : m ≤ m0
f : β → G
hf : StronglyMeasurable f
hG : CompleteSpace G
this✝¹ : MeasurableSpace G := borel G
this✝ : BorelSpace G
hf_int : Integrable f μ
this : SeparableSpace ↑(range f ∪ {0})
f_seq : ℕ → β →ₛ G := SimpleFunc.approxOn f ⋯ (range f ∪ {0}) 0 ⋯
hf_seq_meas : ∀ (n : ℕ), StronglyMeasurable ⇑(f_seq n)
⊢ ∫ (x : β), f x ∂μ = ∫ (x : β), f x ∂μ.trim hm

Proof state:
case pos
G : Type u_5
inst✝¹ : NormedAddCommGroup G
inst✝ : NormedSpace ℝ G
β : Type u_6
m m0 : MeasurableSpace β
μ : Measure β
hm : m ≤ m0
f : β → G
hf : StronglyMeasurable f
hG : CompleteSpace G
this✝¹ : MeasurableSpace G := borel G
this✝ : BorelSpace G
⊢ ∫ (x : β), f x ∂μ = ∫ (x : β), f x ∂μ.trim hm

Tactic:
by_cases hf_int : Integrable f μ

Result:
case pos
G : Type u_5
inst✝¹ : NormedAddCommGroup G
inst✝ : NormedSpace ℝ G
β : Type u_6
m m0 : MeasurableSpace β
μ : Measure β
hm : m ≤ m0
f : β → G
hf : StronglyMeasurable f
hG : CompleteSpace G
this✝¹ : MeasurableSpace G := borel G
this✝ : BorelSpace G
hf_int : Integrable f μ
⊢ ∫ (x : β), f x ∂μ = ∫ (x : β), f x ∂μ.trim hm

case neg
G : Type u_5
inst✝¹ : NormedAddCommGroup G
inst✝ : NormedSpace ℝ G
β : Type u_6
m m0 : MeasurableSpace β
μ : Measure β
hm : m ≤ m0
f : β → G
hf : StronglyMeasurable f
hG : CompleteSpace G
this✝¹ : MeasurableSpace G := borel G
this✝ : BorelSpace G
hf_int : ¬Integrable f μ
⊢ ∫ (x : β), f x ∂μ = ∫ (x : β), f x ∂μ.trim hm

Proof state:
G : Type u_5
inst✝¹ : NormedAddCommGroup G
inst✝ : NormedSpace ℝ G
β : Type u_6
m m0 : MeasurableSpace β
μ : Measure β
hm : m ≤ m0
f : β → G
hf : StronglyMeasurable f
hG : CompleteSpace G
this✝¹ : MeasurableSpace G := borel G
this✝ : BorelSpace G
hf_int : Integrable f μ
this : SeparableSpace ↑(range f ∪ {0})
f_seq : ℕ → β →ₛ G := SimpleFunc.approxOn f ⋯ (range f ∪ {0}) 0 ⋯
hf_seq_meas : ∀ (n : ℕ), StronglyMeasurable ⇑(f_seq n)
hf_seq_int : ∀ (n : ℕ), Integrable (⇑(f_seq n)) μ
hf_seq_int_m : ∀ (n : ℕ), Integrable (⇑(f_seq n)) (μ.trim hm)
hf_seq_eq : ∀ (n : ℕ), ∫ (x : β), (f_seq n) x ∂μ = ∫ (x : β), (f_seq n) x ∂μ.trim hm
⊢ Tendsto (fun i => ∫⁻ (x : β), ‖(f_seq i) x - f x‖ₑ ∂μ) atTop (𝓝 0)

Tactic:
exact SimpleFunc.tendsto_approxOn_range_L1_enorm (hf.mono hm).measurable hf_int

Result:
no goals

Proof state:
case pos
G : Type u_5
inst✝¹ : NormedAddCommGroup G
inst✝ : NormedSpace ℝ G
β : Type u_6
m m0 : MeasurableSpace β
μ : Measure β
hm : m ≤ m0
f : β → G
hf : StronglyMeasurable f
hG : CompleteSpace G
this✝¹ : MeasurableSpace G := borel G
this✝ : BorelSpace G
hf_int : Integrable f μ
this : SeparableSpace ↑(range f ∪ {0})
⊢ ∫ (x : β), f x ∂μ = ∫ (x : β), f x ∂μ.trim hm

Tactic:
let f_seq := @SimpleFunc.approxOn G β _ _ _ m _ hf.measurable (range f ∪ {0}) 0 (by simp) _

Result:
case pos
G : Type u_5
inst✝¹ : NormedAddCommGroup G
inst✝ : NormedSpace ℝ G
β : Type u_6
m m0 : MeasurableSpace β
μ : Measure β
hm : m ≤ m0
f : β → G
hf : StronglyMeasurable f
hG : CompleteSpace G
this✝¹ : MeasurableSpace G := borel G
this✝ : BorelSpace G
hf_int : Integrable f μ
this : SeparableSpace ↑(range f ∪ {0})
f_seq : ℕ → β →ₛ G := SimpleFunc.approxOn f ⋯ (range f ∪ {0}) 0 ⋯
⊢ ∫ (x : β), f x ∂μ = ∫ (x : β), f x ∂μ.trim hm