-- Mathlib/MeasureTheory/Function/ConvergenceInMeasure.lean
theorem tendstoInMeasure_of_tendsto_ae_of_stronglyMeasurable [IsFiniteMeasure μ]
    (hf : ∀ n, StronglyMeasurable (f n)) (hg : StronglyMeasurable g)
    (hfg : ∀ᵐ x ∂μ, Tendsto (fun n => f n x) atTop (𝓝 (g x))) : TendstoInMeasure μ f atTop g

Proof state:
case neg.intro.intro.intro.intro
α : Type u_1
E : Type u_4
m : MeasurableSpace α
μ : Measure α
inst✝¹ : MetricSpace E
f : ℕ → α → E
g : α → E
inst✝ : IsFiniteMeasure μ
hf : ∀ (n : ℕ), StronglyMeasurable (f n)
hg : StronglyMeasurable g
hfg : ∀ᵐ (x : α) ∂μ, Tendsto (fun n => f n x) atTop (𝓝 (g x))
ε : ℝ
hε : 0 < ε
δ : ℝ≥0
hδ : 0 < ↑δ
t : Set α
left✝ : MeasurableSet t
ht : μ t ≤ ↑δ
hunif : ∀ ε > 0, ∀ᶠ (n : ℕ) in atTop, ∀ x ∈ tᶜ, dist (g x) (f n x) < ε
⊢ ∃ N, ∀ n ≥ N, μ {x | ε ≤ dist (f n x) (g x)} ≤ ↑δ

Tactic:
obtain ⟨N, hN⟩ := eventually_atTop.1 (hunif ε hε)

Result:
case neg.intro.intro.intro.intro.intro
α : Type u_1
E : Type u_4
m : MeasurableSpace α
μ : Measure α
inst✝¹ : MetricSpace E
f : ℕ → α → E
g : α → E
inst✝ : IsFiniteMeasure μ
hf : ∀ (n : ℕ), StronglyMeasurable (f n)
hg : StronglyMeasurable g
hfg : ∀ᵐ (x : α) ∂μ, Tendsto (fun n => f n x) atTop (𝓝 (g x))
ε : ℝ
hε : 0 < ε
δ : ℝ≥0
hδ : 0 < ↑δ
t : Set α
left✝ : MeasurableSet t
ht : μ t ≤ ↑δ
hunif : ∀ ε > 0, ∀ᶠ (n : ℕ) in atTop, ∀ x ∈ tᶜ, dist (g x) (f n x) < ε
N : ℕ
hN : ∀ b ≥ N, ∀ x ∈ tᶜ, dist (g x) (f b x) < ε
⊢ ∃ N, ∀ n ≥ N, μ {x | ε ≤ dist (f n x) (g x)} ≤ ↑δ

Proof state:
case neg.intro.intro.intro.intro.intro
α : Type u_1
E : Type u_4
m : MeasurableSpace α
μ : Measure α
inst✝¹ : MetricSpace E
f : ℕ → α → E
g : α → E
inst✝ : IsFiniteMeasure μ
hf : ∀ (n : ℕ), StronglyMeasurable (f n)
hg : StronglyMeasurable g
hfg : ∀ᵐ (x : α) ∂μ, Tendsto (fun n => f n x) atTop (𝓝 (g x))
ε : ℝ
hε : 0 < ε
δ : ℝ≥0
hδ : 0 < ↑δ
t : Set α
left✝ : MeasurableSet t
ht : μ t ≤ ↑δ
hunif : ∀ ε > 0, ∀ᶠ (n : ℕ) in atTop, ∀ x ∈ tᶜ, dist (g x) (f n x) < ε
N : ℕ
hN : ∀ b ≥ N, ∀ x ∈ tᶜ, dist (g x) (f b x) < ε
⊢ ∃ N, ∀ n ≥ N, μ {x | ε ≤ dist (f n x) (g x)} ≤ ↑δ

Tactic:
refine ⟨N, fun n hn => ?_⟩

Result:
case neg.intro.intro.intro.intro.intro
α : Type u_1
E : Type u_4
m : MeasurableSpace α
μ : Measure α
inst✝¹ : MetricSpace E
f : ℕ → α → E
g : α → E
inst✝ : IsFiniteMeasure μ
hf : ∀ (n : ℕ), StronglyMeasurable (f n)
hg : StronglyMeasurable g
hfg : ∀ᵐ (x : α) ∂μ, Tendsto (fun n => f n x) atTop (𝓝 (g x))
ε : ℝ
hε : 0 < ε
δ : ℝ≥0
hδ : 0 < ↑δ
t : Set α
left✝ : MeasurableSet t
ht : μ t ≤ ↑δ
hunif : ∀ ε > 0, ∀ᶠ (n : ℕ) in atTop, ∀ x ∈ tᶜ, dist (g x) (f n x) < ε
N : ℕ
hN : ∀ b ≥ N, ∀ x ∈ tᶜ, dist (g x) (f b x) < ε
n : ℕ
hn : n ≥ N
⊢ μ {x | ε ≤ dist (f n x) (g x)} ≤ ↑δ

Proof state:
case neg.intro.intro.intro.intro.intro
α : Type u_1
E : Type u_4
m : MeasurableSpace α
μ : Measure α
inst✝¹ : MetricSpace E
f : ℕ → α → E
g : α → E
inst✝ : IsFiniteMeasure μ
hf : ∀ (n : ℕ), StronglyMeasurable (f n)
hg : StronglyMeasurable g
hfg : ∀ᵐ (x : α) ∂μ, Tendsto (fun n => f n x) atTop (𝓝 (g x))
ε : ℝ
hε : 0 < ε
δ : ℝ≥0
hδ : 0 < ↑δ
t : Set α
left✝ : MeasurableSet t
ht : μ t ≤ ↑δ
hunif : ∀ ε > 0, ∀ᶠ (n : ℕ) in atTop, ∀ x ∈ tᶜ, dist (g x) (f n x) < ε
N : ℕ
hN : ∀ b ≥ N, ∀ x ∈ tᶜ, dist (g x) (f b x) < ε
n : ℕ
hn : n ≥ N
x : α
hx : x ∈ tᶜ
⊢ x ∈ {x | ε ≤ dist (f n x) (g x)}ᶜ

Tactic:
rw [Set.mem_compl_iff, Set.nmem_setOf_iff, dist_comm, not_le]

Result:
case neg.intro.intro.intro.intro.intro
α : Type u_1
E : Type u_4
m : MeasurableSpace α
μ : Measure α
inst✝¹ : MetricSpace E
f : ℕ → α → E
g : α → E
inst✝ : IsFiniteMeasure μ
hf : ∀ (n : ℕ), StronglyMeasurable (f n)
hg : StronglyMeasurable g
hfg : ∀ᵐ (x : α) ∂μ, Tendsto (fun n => f n x) atTop (𝓝 (g x))
ε : ℝ
hε : 0 < ε
δ : ℝ≥0
hδ : 0 < ↑δ
t : Set α
left✝ : MeasurableSet t
ht : μ t ≤ ↑δ
hunif : ∀ ε > 0, ∀ᶠ (n : ℕ) in atTop, ∀ x ∈ tᶜ, dist (g x) (f n x) < ε
N : ℕ
hN : ∀ b ≥ N, ∀ x ∈ tᶜ, dist (g x) (f b x) < ε
n : ℕ
hn : n ≥ N
x : α
hx : x ∈ tᶜ
⊢ dist (g x) (f n x) < ε

Proof state:
case neg.intro.intro.intro.intro.intro
α : Type u_1
E : Type u_4
m : MeasurableSpace α
μ : Measure α
inst✝¹ : MetricSpace E
f : ℕ → α → E
g : α → E
inst✝ : IsFiniteMeasure μ
hf : ∀ (n : ℕ), StronglyMeasurable (f n)
hg : StronglyMeasurable g
hfg : ∀ᵐ (x : α) ∂μ, Tendsto (fun n => f n x) atTop (𝓝 (g x))
ε : ℝ
hε : 0 < ε
δ : ℝ≥0
hδ : 0 < ↑δ
t : Set α
left✝ : MeasurableSet t
ht : μ t ≤ ↑δ
hunif : ∀ ε > 0, ∀ᶠ (n : ℕ) in atTop, ∀ x ∈ tᶜ, dist (g x) (f n x) < ε
N : ℕ
hN : ∀ b ≥ N, ∀ x ∈ tᶜ, dist (g x) (f b x) < ε
n : ℕ
hn : n ≥ N
⊢ μ {x | ε ≤ dist (f n x) (g x)} ≤ ↑δ

Tactic:
suffices { x : α | ε ≤ dist (f n x) (g x) } ⊆ t from (measure_mono this).trans ht

Result:
case neg.intro.intro.intro.intro.intro
α : Type u_1
E : Type u_4
m : MeasurableSpace α
μ : Measure α
inst✝¹ : MetricSpace E
f : ℕ → α → E
g : α → E
inst✝ : IsFiniteMeasure μ
hf : ∀ (n : ℕ), StronglyMeasurable (f n)
hg : StronglyMeasurable g
hfg : ∀ᵐ (x : α) ∂μ, Tendsto (fun n => f n x) atTop (𝓝 (g x))
ε : ℝ
hε : 0 < ε
δ : ℝ≥0
hδ : 0 < ↑δ
t : Set α
left✝ : MeasurableSet t
ht : μ t ≤ ↑δ
hunif : ∀ ε > 0, ∀ᶠ (n : ℕ) in atTop, ∀ x ∈ tᶜ, dist (g x) (f n x) < ε
N : ℕ
hN : ∀ b ≥ N, ∀ x ∈ tᶜ, dist (g x) (f b x) < ε
n : ℕ
hn : n ≥ N
⊢ {x | ε ≤ dist (f n x) (g x)} ⊆ t

Proof state:
case neg.intro.intro.intro.intro
α : Type u_1
E : Type u_4
m : MeasurableSpace α
μ : Measure α
inst✝¹ : MetricSpace E
f : ℕ → α → E
g : α → E
inst✝ : IsFiniteMeasure μ
hf : ∀ (n : ℕ), StronglyMeasurable (f n)
hg : StronglyMeasurable g
hfg : ∀ᵐ (x : α) ∂μ, Tendsto (fun n => f n x) atTop (𝓝 (g x))
ε : ℝ
hε : 0 < ε
δ : ℝ≥0
hδ : 0 < ↑δ
t : Set α
left✝ : MeasurableSet t
ht : μ t ≤ ↑δ
hunif : TendstoUniformlyOn f g atTop tᶜ
⊢ ∃ N, ∀ n ≥ N, μ {x | ε ≤ dist (f n x) (g x)} ≤ ↑δ

Tactic:
rw [Metric.tendstoUniformlyOn_iff] at hunif

Result:
case neg.intro.intro.intro.intro
α : Type u_1
E : Type u_4
m : MeasurableSpace α
μ : Measure α
inst✝¹ : MetricSpace E
f : ℕ → α → E
g : α → E
inst✝ : IsFiniteMeasure μ
hf : ∀ (n : ℕ), StronglyMeasurable (f n)
hg : StronglyMeasurable g
hfg : ∀ᵐ (x : α) ∂μ, Tendsto (fun n => f n x) atTop (𝓝 (g x))
ε : ℝ
hε : 0 < ε
δ : ℝ≥0
hδ : 0 < ↑δ
t : Set α
left✝ : MeasurableSet t
ht : μ t ≤ ↑δ
hunif : ∀ ε > 0, ∀ᶠ (n : ℕ) in atTop, ∀ x ∈ tᶜ, dist (g x) (f n x) < ε
⊢ ∃ N, ∀ n ≥ N, μ {x | ε ≤ dist (f n x) (g x)} ≤ ↑δ

Proof state:
case pos
α : Type u_1
E : Type u_4
m : MeasurableSpace α
μ : Measure α
inst✝¹ : MetricSpace E
f : ℕ → α → E
g : α → E
inst✝ : IsFiniteMeasure μ
hf : ∀ (n : ℕ), StronglyMeasurable (f n)
hg : StronglyMeasurable g
hfg : ∀ᵐ (x : α) ∂μ, Tendsto (fun n => f n x) atTop (𝓝 (g x))
ε : ℝ
hε : 0 < ε
δ : ℝ≥0∞
hδ : δ > 0
hδi : δ = ⊤
⊢ ∃ N, ∀ n ≥ N, μ {x | ε ≤ dist (f n x) (g x)} ≤ δ

Tactic:
simp only [hδi, imp_true_iff, le_top, exists_const]

Result:
no goals

Proof state:
case neg.intro.intro.intro.intro.intro
α : Type u_1
E : Type u_4
m : MeasurableSpace α
μ : Measure α
inst✝¹ : MetricSpace E
f : ℕ → α → E
g : α → E
inst✝ : IsFiniteMeasure μ
hf : ∀ (n : ℕ), StronglyMeasurable (f n)
hg : StronglyMeasurable g
hfg : ∀ᵐ (x : α) ∂μ, Tendsto (fun n => f n x) atTop (𝓝 (g x))
ε : ℝ
hε : 0 < ε
δ : ℝ≥0
hδ : 0 < ↑δ
t : Set α
left✝ : MeasurableSet t
ht : μ t ≤ ↑δ
hunif : ∀ ε > 0, ∀ᶠ (n : ℕ) in atTop, ∀ x ∈ tᶜ, dist (g x) (f n x) < ε
N : ℕ
hN : ∀ b ≥ N, ∀ x ∈ tᶜ, dist (g x) (f b x) < ε
n : ℕ
hn : n ≥ N
⊢ {x | ε ≤ dist (f n x) (g x)} ⊆ t

Tactic:
rw [← Set.compl_subset_compl]

Result:
case neg.intro.intro.intro.intro.intro
α : Type u_1
E : Type u_4
m : MeasurableSpace α
μ : Measure α
inst✝¹ : MetricSpace E
f : ℕ → α → E
g : α → E
inst✝ : IsFiniteMeasure μ
hf : ∀ (n : ℕ), StronglyMeasurable (f n)
hg : StronglyMeasurable g
hfg : ∀ᵐ (x : α) ∂μ, Tendsto (fun n => f n x) atTop (𝓝 (g x))
ε : ℝ
hε : 0 < ε
δ : ℝ≥0
hδ : 0 < ↑δ
t : Set α
left✝ : MeasurableSet t
ht : μ t ≤ ↑δ
hunif : ∀ ε > 0, ∀ᶠ (n : ℕ) in atTop, ∀ x ∈ tᶜ, dist (g x) (f n x) < ε
N : ℕ
hN : ∀ b ≥ N, ∀ x ∈ tᶜ, dist (g x) (f b x) < ε
n : ℕ
hn : n ≥ N
⊢ tᶜ ⊆ {x | ε ≤ dist (f n x) (g x)}ᶜ