-- Mathlib/Topology/MetricSpace/Equicontinuity.lean
theorem equicontinuousAt_iff_pair {ι : Type*} [TopologicalSpace β] {F : ι → β → α}
    {x₀ : β} :
    EquicontinuousAt F x₀ ↔
      ∀ ε > 0, ∃ U ∈ 𝓝 x₀, ∀ x ∈ U, ∀ x' ∈ U, ∀ i, dist (F i x) (F i x') < ε

Proof state:
case mpr.intro.intro
α : Type u_1
β : Type u_2
inst✝¹ : PseudoMetricSpace α
ι : Type u_4
inst✝ : TopologicalSpace β
F : ι → β → α
x₀ : β
H : ∀ ε > 0, ∃ U ∈ 𝓝 x₀, ∀ x ∈ U, ∀ x' ∈ U, ∀ (i : ι), dist (F i x) (F i x') < ε
U : Set (α × α)
hU : U ∈ 𝓤 α
ε : ℝ
hε : ε > 0
hεU : ∀ ⦃a b : α⦄, dist a b < ε → (a, b) ∈ U
V : Set β
h : ∀ x ∈ V, ∀ x' ∈ V, ∀ (i : ι), dist (F i x) (F i x') < ε
⊢ ∀ x ∈ V, ∀ y ∈ V, ∀ (i : ι), (F i x, F i y) ∈ U

Tactic:
exact fun x hx x' hx' i => hεU (h _ hx _ hx' i)

Result:
no goals

Proof state:
case mp
α : Type u_1
β : Type u_2
inst✝¹ : PseudoMetricSpace α
ι : Type u_4
inst✝ : TopologicalSpace β
F : ι → β → α
x₀ : β
H : ∀ U ∈ 𝓤 α, ∃ V ∈ 𝓝 x₀, ∀ x ∈ V, ∀ y ∈ V, ∀ (i : ι), (F i x, F i y) ∈ U
⊢ ∀ ε > 0, ∃ U ∈ 𝓝 x₀, ∀ x ∈ U, ∀ x' ∈ U, ∀ (i : ι), dist (F i x) (F i x') < ε

Tactic:
intro ε hε

Result:
case mp
α : Type u_1
β : Type u_2
inst✝¹ : PseudoMetricSpace α
ι : Type u_4
inst✝ : TopologicalSpace β
F : ι → β → α
x₀ : β
H : ∀ U ∈ 𝓤 α, ∃ V ∈ 𝓝 x₀, ∀ x ∈ V, ∀ y ∈ V, ∀ (i : ι), (F i x, F i y) ∈ U
ε : ℝ
hε : ε > 0
⊢ ∃ U ∈ 𝓝 x₀, ∀ x ∈ U, ∀ x' ∈ U, ∀ (i : ι), dist (F i x) (F i x') < ε

Proof state:
case mpr
α : Type u_1
β : Type u_2
inst✝¹ : PseudoMetricSpace α
ι : Type u_4
inst✝ : TopologicalSpace β
F : ι → β → α
x₀ : β
H : ∀ ε > 0, ∃ U ∈ 𝓝 x₀, ∀ x ∈ U, ∀ x' ∈ U, ∀ (i : ι), dist (F i x) (F i x') < ε
⊢ ∀ U ∈ 𝓤 α, ∃ V ∈ 𝓝 x₀, ∀ x ∈ V, ∀ y ∈ V, ∀ (i : ι), (F i x, F i y) ∈ U

Tactic:
intro U hU

Result:
case mpr
α : Type u_1
β : Type u_2
inst✝¹ : PseudoMetricSpace α
ι : Type u_4
inst✝ : TopologicalSpace β
F : ι → β → α
x₀ : β
H : ∀ ε > 0, ∃ U ∈ 𝓝 x₀, ∀ x ∈ U, ∀ x' ∈ U, ∀ (i : ι), dist (F i x) (F i x') < ε
U : Set (α × α)
hU : U ∈ 𝓤 α
⊢ ∃ V ∈ 𝓝 x₀, ∀ x ∈ V, ∀ y ∈ V, ∀ (i : ι), (F i x, F i y) ∈ U

Proof state:
α : Type u_1
β : Type u_2
inst✝¹ : PseudoMetricSpace α
ι : Type u_4
inst✝ : TopologicalSpace β
F : ι → β → α
x₀ : β
⊢ EquicontinuousAt F x₀ ↔ ∀ ε > 0, ∃ U ∈ 𝓝 x₀, ∀ x ∈ U, ∀ x' ∈ U, ∀ (i : ι), dist (F i x) (F i x') < ε

Tactic:
rw [equicontinuousAt_iff_pair]

Result:
α : Type u_1
β : Type u_2
inst✝¹ : PseudoMetricSpace α
ι : Type u_4
inst✝ : TopologicalSpace β
F : ι → β → α
x₀ : β
⊢ (∀ U ∈ 𝓤 α, ∃ V ∈ 𝓝 x₀, ∀ x ∈ V, ∀ y ∈ V, ∀ (i : ι), (F i x, F i y) ∈ U) ↔
    ∀ ε > 0, ∃ U ∈ 𝓝 x₀, ∀ x ∈ U, ∀ x' ∈ U, ∀ (i : ι), dist (F i x) (F i x') < ε

Proof state:
case mpr
α : Type u_1
β : Type u_2
inst✝¹ : PseudoMetricSpace α
ι : Type u_4
inst✝ : TopologicalSpace β
F : ι → β → α
x₀ : β
H : ∀ ε > 0, ∃ U ∈ 𝓝 x₀, ∀ x ∈ U, ∀ x' ∈ U, ∀ (i : ι), dist (F i x) (F i x') < ε
U : Set (α × α)
hU : U ∈ 𝓤 α
⊢ ∃ V ∈ 𝓝 x₀, ∀ x ∈ V, ∀ y ∈ V, ∀ (i : ι), (F i x, F i y) ∈ U

Tactic:
rcases mem_uniformity_dist.mp hU with ⟨ε, hε, hεU⟩

Result:
case mpr.intro.intro
α : Type u_1
β : Type u_2
inst✝¹ : PseudoMetricSpace α
ι : Type u_4
inst✝ : TopologicalSpace β
F : ι → β → α
x₀ : β
H : ∀ ε > 0, ∃ U ∈ 𝓝 x₀, ∀ x ∈ U, ∀ x' ∈ U, ∀ (i : ι), dist (F i x) (F i x') < ε
U : Set (α × α)
hU : U ∈ 𝓤 α
ε : ℝ
hε : ε > 0
hεU : ∀ ⦃a b : α⦄, dist a b < ε → (a, b) ∈ U
⊢ ∃ V ∈ 𝓝 x₀, ∀ x ∈ V, ∀ y ∈ V, ∀ (i : ι), (F i x, F i y) ∈ U

Proof state:
α : Type u_1
β : Type u_2
inst✝¹ : PseudoMetricSpace α
ι : Type u_4
inst✝ : TopologicalSpace β
F : ι → β → α
x₀ : β
⊢ (∀ U ∈ 𝓤 α, ∃ V ∈ 𝓝 x₀, ∀ x ∈ V, ∀ y ∈ V, ∀ (i : ι), (F i x, F i y) ∈ U) ↔
    ∀ ε > 0, ∃ U ∈ 𝓝 x₀, ∀ x ∈ U, ∀ x' ∈ U, ∀ (i : ι), dist (F i x) (F i x') < ε

Tactic:
constructor <;> intro H

Result:
case mp
α : Type u_1
β : Type u_2
inst✝¹ : PseudoMetricSpace α
ι : Type u_4
inst✝ : TopologicalSpace β
F : ι → β → α
x₀ : β
H : ∀ U ∈ 𝓤 α, ∃ V ∈ 𝓝 x₀, ∀ x ∈ V, ∀ y ∈ V, ∀ (i : ι), (F i x, F i y) ∈ U
⊢ ∀ ε > 0, ∃ U ∈ 𝓝 x₀, ∀ x ∈ U, ∀ x' ∈ U, ∀ (i : ι), dist (F i x) (F i x') < ε

case mpr
α : Type u_1
β : Type u_2
inst✝¹ : PseudoMetricSpace α
ι : Type u_4
inst✝ : TopologicalSpace β
F : ι → β → α
x₀ : β
H : ∀ ε > 0, ∃ U ∈ 𝓝 x₀, ∀ x ∈ U, ∀ x' ∈ U, ∀ (i : ι), dist (F i x) (F i x') < ε
⊢ ∀ U ∈ 𝓤 α, ∃ V ∈ 𝓝 x₀, ∀ x ∈ V, ∀ y ∈ V, ∀ (i : ι), (F i x, F i y) ∈ U

Proof state:
case mp
α : Type u_1
β : Type u_2
inst✝¹ : PseudoMetricSpace α
ι : Type u_4
inst✝ : TopologicalSpace β
F : ι → β → α
x₀ : β
H : ∀ U ∈ 𝓤 α, ∃ V ∈ 𝓝 x₀, ∀ x ∈ V, ∀ y ∈ V, ∀ (i : ι), (F i x, F i y) ∈ U
ε : ℝ
hε : ε > 0
⊢ ∃ U ∈ 𝓝 x₀, ∀ x ∈ U, ∀ x' ∈ U, ∀ (i : ι), dist (F i x) (F i x') < ε

Tactic:
exact H _ (dist_mem_uniformity hε)

Result:
no goals

Proof state:
case mpr.intro.intro
α : Type u_1
β : Type u_2
inst✝¹ : PseudoMetricSpace α
ι : Type u_4
inst✝ : TopologicalSpace β
F : ι → β → α
x₀ : β
H : ∀ ε > 0, ∃ U ∈ 𝓝 x₀, ∀ x ∈ U, ∀ x' ∈ U, ∀ (i : ι), dist (F i x) (F i x') < ε
U : Set (α × α)
hU : U ∈ 𝓤 α
ε : ℝ
hε : ε > 0
hεU : ∀ ⦃a b : α⦄, dist a b < ε → (a, b) ∈ U
⊢ ∃ V ∈ 𝓝 x₀, ∀ x ∈ V, ∀ y ∈ V, ∀ (i : ι), (F i x, F i y) ∈ U

Tactic:
refine Exists.imp (fun V => And.imp_right fun h => ?_) (H _ hε)

Result:
case mpr.intro.intro
α : Type u_1
β : Type u_2
inst✝¹ : PseudoMetricSpace α
ι : Type u_4
inst✝ : TopologicalSpace β
F : ι → β → α
x₀ : β
H : ∀ ε > 0, ∃ U ∈ 𝓝 x₀, ∀ x ∈ U, ∀ x' ∈ U, ∀ (i : ι), dist (F i x) (F i x') < ε
U : Set (α × α)
hU : U ∈ 𝓤 α
ε : ℝ
hε : ε > 0
hεU : ∀ ⦃a b : α⦄, dist a b < ε → (a, b) ∈ U
V : Set β
h : ∀ x ∈ V, ∀ x' ∈ V, ∀ (i : ι), dist (F i x) (F i x') < ε
⊢ ∀ x ∈ V, ∀ y ∈ V, ∀ (i : ι), (F i x, F i y) ∈ U