-- Mathlib/Probability/StrongLaw.lean
/-- Preliminary lemma for the strong law of large numbers for vector-valued random variables:
the composition of the random variables with a simple function satisfies the strong law of large
numbers. -/
lemma strong_law_ae_simpleFunc_comp (X : ℕ → Ω → E) (h' : Measurable (X 0))
    (hindep : Pairwise ((IndepFun · · μ) on X))
    (hident : ∀ i, IdentDistrib (X i) (X 0) μ μ) (φ : SimpleFunc E E) :
    ∀ᵐ ω ∂μ,
      Tendsto (fun n : ℕ ↦ (n : ℝ) ⁻¹ • (∑ i ∈ range n, φ (X i ω))) atTop (𝓝 μ[φ ∘ (X 0)])

Proof state:
Ω : Type u_1
mΩ : MeasurableSpace Ω
μ : Measure Ω
inst✝⁴ : IsProbabilityMeasure μ
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace ℝ E
inst✝¹ : CompleteSpace E
inst✝ : MeasurableSpace E
X : ℕ → Ω → E
h' : Measurable (X 0)
hindep : Pairwise ((fun x1 x2 => IndepFun x1 x2 μ) on X)
hident : ∀ (i : ℕ), IdentDistrib (X i) (X 0) μ μ
φ : SimpleFunc E E
⊢ ∀ᵐ (ω : Ω) ∂μ, Tendsto (fun n => (↑n)⁻¹ • ∑ i ∈ Finset.range n, φ (X i ω)) atTop (𝓝 (∫ (x : Ω), (⇑φ ∘ X 0) x ∂μ))

Tactic:
refine SimpleFunc.induction (P := fun ψ ↦ ∀ᵐ ω ∂μ,
  Tendsto (fun n : ℕ ↦ (n : ℝ) ⁻¹ • (∑ i ∈ range n, ψ (X i ω))) atTop (𝓝 μ[ψ ∘ (X 0)])) ?_ ?_ φ

Result:
case refine_1
Ω : Type u_1
mΩ : MeasurableSpace Ω
μ : Measure Ω
inst✝⁴ : IsProbabilityMeasure μ
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace ℝ E
inst✝¹ : CompleteSpace E
inst✝ : MeasurableSpace E
X : ℕ → Ω → E
h' : Measurable (X 0)
hindep : Pairwise ((fun x1 x2 => IndepFun x1 x2 μ) on X)
hident : ∀ (i : ℕ), IdentDistrib (X i) (X 0) μ μ
φ : SimpleFunc E E
⊢ ∀ (c : E) {s : Set E} (hs : MeasurableSet s),
    (fun ψ =>
        ∀ᵐ (ω : Ω) ∂μ,
          Tendsto (fun n => (↑n)⁻¹ • ∑ i ∈ Finset.range n, ψ (X i ω)) atTop (𝓝 (∫ (x : Ω), (⇑ψ ∘ X 0) x ∂μ)))
      (SimpleFunc.piecewise s hs (SimpleFunc.const E c) (SimpleFunc.const E 0))

case refine_2
Ω : Type u_1
mΩ : MeasurableSpace Ω
μ : Measure Ω
inst✝⁴ : IsProbabilityMeasure μ
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace ℝ E
inst✝¹ : CompleteSpace E
inst✝ : MeasurableSpace E
X : ℕ → Ω → E
h' : Measurable (X 0)
hindep : Pairwise ((fun x1 x2 => IndepFun x1 x2 μ) on X)
hident : ∀ (i : ℕ), IdentDistrib (X i) (X 0) μ μ
φ : SimpleFunc E E
⊢ ∀ ⦃f g : SimpleFunc E E⦄,
    Disjoint (Function.support ⇑f) (Function.support ⇑g) →
      (fun ψ =>
            ∀ᵐ (ω : Ω) ∂μ,
              Tendsto (fun n => (↑n)⁻¹ • ∑ i ∈ Finset.range n, ψ (X i ω)) atTop (𝓝 (∫ (x : Ω), (⇑ψ ∘ X 0) x ∂μ)))
          f →
        (fun ψ =>
              ∀ᵐ (ω : Ω) ∂μ,
                Tendsto (fun n => (↑n)⁻¹ • ∑ i ∈ Finset.range n, ψ (X i ω)) atTop (𝓝 (∫ (x : Ω), (⇑ψ ∘ X 0) x ∂μ)))
            g →
          (fun ψ =>
              ∀ᵐ (ω : Ω) ∂μ,
                Tendsto (fun n => (↑n)⁻¹ • ∑ i ∈ Finset.range n, ψ (X i ω)) atTop (𝓝 (∫ (x : Ω), (⇑ψ ∘ X 0) x ∂μ)))
            (f + g)

Proof state:
case hint
Ω : Type u_1
mΩ : MeasurableSpace Ω
μ : Measure Ω
inst✝⁴ : IsProbabilityMeasure μ
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace ℝ E
inst✝¹ : CompleteSpace E
inst✝ : MeasurableSpace E
X : ℕ → Ω → E
h' : Measurable (X 0)
hindep : Pairwise ((fun x1 x2 => IndepFun x1 x2 μ) on X)
hident : ∀ (i : ℕ), IdentDistrib (X i) (X 0) μ μ
φ : SimpleFunc E E
c : E
s : Set E
hs : MeasurableSet s
F : E → ℝ := s.indicator 1
F_meas : Measurable F
Y : ℕ → Ω → ℝ := fun n => F ∘ X n
⊢ Integrable (Y 0) μ

Tactic:
exact SimpleFunc.integrable_of_isFiniteMeasure
  ((SimpleFunc.piecewise s hs (SimpleFunc.const _ (1 : ℝ))
    (SimpleFunc.const _ (0 : ℝ))).comp (X 0) h')

Result:
no goals

Proof state:
case h.e'_3
Ω : Type u_1
mΩ : MeasurableSpace Ω
μ : Measure Ω
inst✝⁴ : IsProbabilityMeasure μ
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace ℝ E
inst✝¹ : CompleteSpace E
inst✝ : MeasurableSpace E
X : ℕ → Ω → E
h' : Measurable (X 0)
hindep : Pairwise ((fun x1 x2 => IndepFun x1 x2 μ) on X)
hident : ∀ (i : ℕ), IdentDistrib (X i) (X 0) μ μ
φ✝ φ ψ : SimpleFunc E E
hφ : ∀ᵐ (ω : Ω) ∂μ, Tendsto (fun n => (↑n)⁻¹ • ∑ i ∈ Finset.range n, φ (X i ω)) atTop (𝓝 (∫ (x : Ω), (⇑φ ∘ X 0) x ∂μ))
hψ : ∀ᵐ (ω : Ω) ∂μ, Tendsto (fun n => (↑n)⁻¹ • ∑ i ∈ Finset.range n, ψ (X i ω)) atTop (𝓝 (∫ (x : Ω), (⇑ψ ∘ X 0) x ∂μ))
ω : Ω
hωφ : Tendsto (fun n => (↑n)⁻¹ • ∑ i ∈ Finset.range n, φ (X i ω)) atTop (𝓝 (∫ (x : Ω), (⇑φ ∘ X 0) x ∂μ))
hωψ : Tendsto (fun n => (↑n)⁻¹ • ∑ i ∈ Finset.range n, ψ (X i ω)) atTop (𝓝 (∫ (x : Ω), (⇑ψ ∘ X 0) x ∂μ))
⊢ (fun n => (↑n)⁻¹ • ∑ i ∈ Finset.range n, (φ + ψ) (X i ω)) = fun x =>
    (↑x)⁻¹ • ∑ i ∈ Finset.range x, φ (X i ω) + (↑x)⁻¹ • ∑ i ∈ Finset.range x, ψ (X i ω)

Tactic:
simp [sum_add_distrib]

Result:
no goals

Proof state:
case h
Ω : Type u_1
mΩ : MeasurableSpace Ω
μ : Measure Ω
inst✝⁴ : IsProbabilityMeasure μ
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace ℝ E
inst✝¹ : CompleteSpace E
inst✝ : MeasurableSpace E
X : ℕ → Ω → E
h' : Measurable (X 0)
hindep : Pairwise ((fun x1 x2 => IndepFun x1 x2 μ) on X)
hident : ∀ (i : ℕ), IdentDistrib (X i) (X 0) μ μ
φ : SimpleFunc E E
c : E
s : Set E
hs : MeasurableSet s
F : E → ℝ := s.indicator 1
F_meas : Measurable F
Y : ℕ → Ω → ℝ := fun n => F ∘ X n
this : ∀ᵐ (ω : Ω) ∂μ, Tendsto (fun n => (↑n)⁻¹ • ∑ i ∈ Finset.range n, Y i ω) atTop (𝓝 (∫ (x : Ω), Y 0 x ∂μ))
ω : Ω
hω : Tendsto (fun n => (↑n)⁻¹ • ∑ i ∈ Finset.range n, Y i ω) atTop (𝓝 (∫ (x : Ω), Y 0 x ∂μ))
I : s.indicator (Function.const E c) = fun x => s.indicator 1 x • c
⊢ Tendsto (fun n => (↑n)⁻¹ • ∑ x ∈ Finset.range n, s.indicator 1 (X x ω) • c) atTop
    (𝓝 ((∫ (x : Ω), s.indicator 1 (X 0 x) ∂μ) • c))

Tactic:
convert Tendsto.smul_const hω c using 1

Result:
case h.e'_3
Ω : Type u_1
mΩ : MeasurableSpace Ω
μ : Measure Ω
inst✝⁴ : IsProbabilityMeasure μ
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace ℝ E
inst✝¹ : CompleteSpace E
inst✝ : MeasurableSpace E
X : ℕ → Ω → E
h' : Measurable (X 0)
hindep : Pairwise ((fun x1 x2 => IndepFun x1 x2 μ) on X)
hident : ∀ (i : ℕ), IdentDistrib (X i) (X 0) μ μ
φ : SimpleFunc E E
c : E
s : Set E
hs : MeasurableSet s
F : E → ℝ := s.indicator 1
F_meas : Measurable F
Y : ℕ → Ω → ℝ := fun n => F ∘ X n
this : ∀ᵐ (ω : Ω) ∂μ, Tendsto (fun n => (↑n)⁻¹ • ∑ i ∈ Finset.range n, Y i ω) atTop (𝓝 (∫ (x : Ω), Y 0 x ∂μ))
ω : Ω
hω : Tendsto (fun n => (↑n)⁻¹ • ∑ i ∈ Finset.range n, Y i ω) atTop (𝓝 (∫ (x : Ω), Y 0 x ∂μ))
I : s.indicator (Function.const E c) = fun x => s.indicator 1 x • c
⊢ (fun n => (↑n)⁻¹ • ∑ x ∈ Finset.range n, s.indicator 1 (X x ω) • c) = fun x =>
    ((↑x)⁻¹ • ∑ i ∈ Finset.range x, Y i ω) • c

Proof state:
Ω : Type u_1
mΩ : MeasurableSpace Ω
μ : Measure Ω
inst✝⁴ : IsProbabilityMeasure μ
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace ℝ E
inst✝¹ : CompleteSpace E
inst✝ : MeasurableSpace E
X : ℕ → Ω → E
h' : Measurable (X 0)
hindep : Pairwise ((fun x1 x2 => IndepFun x1 x2 μ) on X)
hident : ∀ (i : ℕ), IdentDistrib (X i) (X 0) μ μ
φ : SimpleFunc E E
c : E
s : Set E
hs : MeasurableSet s
F : E → ℝ := s.indicator 1
F_meas : Measurable F
Y : ℕ → Ω → ℝ := fun n => F ∘ X n
⊢ ∀ᵐ (ω : Ω) ∂μ, Tendsto (fun n => (↑n)⁻¹ • ∑ i ∈ Finset.range n, Y i ω) atTop (𝓝 (∫ (x : Ω), Y 0 x ∂μ))

Tactic:
simp only [Function.const_one, smul_eq_mul, ← div_eq_inv_mul]

Result:
Ω : Type u_1
mΩ : MeasurableSpace Ω
μ : Measure Ω
inst✝⁴ : IsProbabilityMeasure μ
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace ℝ E
inst✝¹ : CompleteSpace E
inst✝ : MeasurableSpace E
X : ℕ → Ω → E
h' : Measurable (X 0)
hindep : Pairwise ((fun x1 x2 => IndepFun x1 x2 μ) on X)
hident : ∀ (i : ℕ), IdentDistrib (X i) (X 0) μ μ
φ : SimpleFunc E E
c : E
s : Set E
hs : MeasurableSet s
F : E → ℝ := s.indicator 1
F_meas : Measurable F
Y : ℕ → Ω → ℝ := fun n => F ∘ X n
⊢ ∀ᵐ (ω : Ω) ∂μ, Tendsto (fun n => (∑ i ∈ Finset.range n, Y i ω) / ↑n) atTop (𝓝 (∫ (x : Ω), Y 0 x ∂μ))