-- Mathlib/Topology/Metrizable/Urysohn.lean
theorem exists_isInducing_l_infty : ∃ f : X → ℕ →ᵇ ℝ, IsInducing f

Proof state:
case intro.intro.refine_1
X : Type u_1
inst✝² : TopologicalSpace X
inst✝¹ : RegularSpace X
inst✝ : SecondCountableTopology X
B : Set (Set X)
hBc : B.Countable
hB : IsTopologicalBasis B
s : Set (Set X × Set X) := {UV | UV ∈ B ×ˢ B ∧ closure UV.1 ⊆ UV.2}
this✝¹ : Encodable ↑s
this✝ : TopologicalSpace ↑s := ⊥
this : DiscreteTopology ↑s
hd : ∀ (UV : ↑s), Disjoint (closure (↑UV).1) (↑UV).2ᶜ
ε : ↑s → ℝ
ε01 : ∀ (UV : ↑s), ε UV ∈ Ioc 0 1
hε : Tendsto ε cofinite (𝓝 0)
f : ↑s → C(X, ℝ)
hf0 : ∀ (UV : ↑s), EqOn (⇑(f UV)) 0 (↑UV).1
hfε : ∀ (UV : ↑s), EqOn (⇑(f UV)) (fun x => ε UV) (↑UV).2ᶜ
hf0ε : ∀ (UV : ↑s) (x : X), (f UV) x ∈ Icc 0 (ε UV)
hf01 : ∀ (UV : ↑s) (x : X), (f UV) x ∈ Icc 0 1
F : X → ↑s →ᵇ ℝ := fun x => { toFun := fun UV => (f UV) x, continuous_toFun := ⋯, map_bounded' := ⋯ }
hF : ∀ (x : X) (UV : ↑s), (F x) UV = (f UV) x
x : X
⊢ 𝓝 x ≤ comap F (𝓝 (F x))

Tactic:
refine (nhds_basis_closedBall.comap _).ge_iff.2 fun δ δ0 => ?_

Result:
case intro.intro.refine_1
X : Type u_1
inst✝² : TopologicalSpace X
inst✝¹ : RegularSpace X
inst✝ : SecondCountableTopology X
B : Set (Set X)
hBc : B.Countable
hB : IsTopologicalBasis B
s : Set (Set X × Set X) := {UV | UV ∈ B ×ˢ B ∧ closure UV.1 ⊆ UV.2}
this✝¹ : Encodable ↑s
this✝ : TopologicalSpace ↑s := ⊥
this : DiscreteTopology ↑s
hd : ∀ (UV : ↑s), Disjoint (closure (↑UV).1) (↑UV).2ᶜ
ε : ↑s → ℝ
ε01 : ∀ (UV : ↑s), ε UV ∈ Ioc 0 1
hε : Tendsto ε cofinite (𝓝 0)
f : ↑s → C(X, ℝ)
hf0 : ∀ (UV : ↑s), EqOn (⇑(f UV)) 0 (↑UV).1
hfε : ∀ (UV : ↑s), EqOn (⇑(f UV)) (fun x => ε UV) (↑UV).2ᶜ
hf0ε : ∀ (UV : ↑s) (x : X), (f UV) x ∈ Icc 0 (ε UV)
hf01 : ∀ (UV : ↑s) (x : X), (f UV) x ∈ Icc 0 1
F : X → ↑s →ᵇ ℝ := fun x => { toFun := fun UV => (f UV) x, continuous_toFun := ⋯, map_bounded' := ⋯ }
hF : ∀ (x : X) (UV : ↑s), (F x) UV = (f UV) x
x : X
δ : ℝ
δ0 : 0 < δ
⊢ F ⁻¹' closedBall (F x) δ ∈ 𝓝 x

Proof state:
case intro.intro.intro
X : Type u_1
inst✝² : TopologicalSpace X
inst✝¹ : RegularSpace X
inst✝ : SecondCountableTopology X
B : Set (Set X)
hBc : B.Countable
hB : IsTopologicalBasis B
s : Set (Set X × Set X) := {UV | UV ∈ B ×ˢ B ∧ closure UV.1 ⊆ UV.2}
⊢ ∃ f, IsInducing f

Tactic:
haveI : Encodable s := ((hBc.prod hBc).mono inter_subset_left).toEncodable

Result:
case intro.intro.intro
X : Type u_1
inst✝² : TopologicalSpace X
inst✝¹ : RegularSpace X
inst✝ : SecondCountableTopology X
B : Set (Set X)
hBc : B.Countable
hB : IsTopologicalBasis B
s : Set (Set X × Set X) := {UV | UV ∈ B ×ˢ B ∧ closure UV.1 ⊆ UV.2}
this : Encodable ↑s
⊢ ∃ f, IsInducing f

Proof state:
case intro.intro
X : Type u_1
inst✝² : TopologicalSpace X
inst✝¹ : RegularSpace X
inst✝ : SecondCountableTopology X
B : Set (Set X)
hBc : B.Countable
hB : IsTopologicalBasis B
s : Set (Set X × Set X) := {UV | UV ∈ B ×ˢ B ∧ closure UV.1 ⊆ UV.2}
this✝¹ : Encodable ↑s
this✝ : TopologicalSpace ↑s := ⊥
this : DiscreteTopology ↑s
hd : ∀ (UV : ↑s), Disjoint (closure (↑UV).1) (↑UV).2ᶜ
ε : ↑s → ℝ
ε01 : ∀ (UV : ↑s), ε UV ∈ Ioc 0 1
hε : Tendsto ε cofinite (𝓝 0)
f : ↑s → C(X, ℝ)
hf0 : ∀ (UV : ↑s), EqOn (⇑(f UV)) 0 (↑UV).1
hfε : ∀ (UV : ↑s), EqOn (⇑(f UV)) (fun x => ε UV) (↑UV).2ᶜ
hf0ε : ∀ (UV : ↑s) (x : X), (f UV) x ∈ Icc 0 (ε UV)
⊢ ∃ f, IsInducing f

Tactic:
have hf01 : ∀ UV x, f UV x ∈ Icc (0 : ℝ) 1 :=
  fun UV x => Icc_subset_Icc_right (ε01 _).2 (hf0ε _ _)

Result:
case intro.intro
X : Type u_1
inst✝² : TopologicalSpace X
inst✝¹ : RegularSpace X
inst✝ : SecondCountableTopology X
B : Set (Set X)
hBc : B.Countable
hB : IsTopologicalBasis B
s : Set (Set X × Set X) := {UV | UV ∈ B ×ˢ B ∧ closure UV.1 ⊆ UV.2}
this✝¹ : Encodable ↑s
this✝ : TopologicalSpace ↑s := ⊥
this : DiscreteTopology ↑s
hd : ∀ (UV : ↑s), Disjoint (closure (↑UV).1) (↑UV).2ᶜ
ε : ↑s → ℝ
ε01 : ∀ (UV : ↑s), ε UV ∈ Ioc 0 1
hε : Tendsto ε cofinite (𝓝 0)
f : ↑s → C(X, ℝ)
hf0 : ∀ (UV : ↑s), EqOn (⇑(f UV)) 0 (↑UV).1
hfε : ∀ (UV : ↑s), EqOn (⇑(f UV)) (fun x => ε UV) (↑UV).2ᶜ
hf0ε : ∀ (UV : ↑s) (x : X), (f UV) x ∈ Icc 0 (ε UV)
hf01 : ∀ (UV : ↑s) (x : X), (f UV) x ∈ Icc 0 1
⊢ ∃ f, IsInducing f

Proof state:
case intro.intro.refine_1
X : Type u_1
inst✝² : TopologicalSpace X
inst✝¹ : RegularSpace X
inst✝ : SecondCountableTopology X
B : Set (Set X)
hBc : B.Countable
hB : IsTopologicalBasis B
s : Set (Set X × Set X) := {UV | UV ∈ B ×ˢ B ∧ closure UV.1 ⊆ UV.2}
this✝¹ : Encodable ↑s
this✝ : TopologicalSpace ↑s := ⊥
this : DiscreteTopology ↑s
hd : ∀ (UV : ↑s), Disjoint (closure (↑UV).1) (↑UV).2ᶜ
ε : ↑s → ℝ
ε01 : ∀ (UV : ↑s), ε UV ∈ Ioc 0 1
hε : Tendsto ε cofinite (𝓝 0)
f : ↑s → C(X, ℝ)
hf0 : ∀ (UV : ↑s), EqOn (⇑(f UV)) 0 (↑UV).1
hfε : ∀ (UV : ↑s), EqOn (⇑(f UV)) (fun x => ε UV) (↑UV).2ᶜ
hf0ε : ∀ (UV : ↑s) (x : X), (f UV) x ∈ Icc 0 (ε UV)
hf01 : ∀ (UV : ↑s) (x : X), (f UV) x ∈ Icc 0 1
F : X → ↑s →ᵇ ℝ := fun x => { toFun := fun UV => (f UV) x, continuous_toFun := ⋯, map_bounded' := ⋯ }
hF : ∀ (x : X) (UV : ↑s), (F x) UV = (f UV) x
x : X
δ : ℝ
δ0 : 0 < δ
h_fin : {UV | δ ≤ ε UV}.Finite
⊢ F ⁻¹' closedBall (F x) δ ∈ 𝓝 x

Tactic:
have : ∀ᶠ y in 𝓝 x, ∀ UV, δ ≤ ε UV → dist (F y UV) (F x UV) ≤ δ := by
  refine (eventually_all_finite h_fin).2 fun UV _ => ?_
  exact (f UV).continuous.tendsto x (closedBall_mem_nhds _ δ0)

Result:
case intro.intro.refine_1
X : Type u_1
inst✝² : TopologicalSpace X
inst✝¹ : RegularSpace X
inst✝ : SecondCountableTopology X
B : Set (Set X)
hBc : B.Countable
hB : IsTopologicalBasis B
s : Set (Set X × Set X) := {UV | UV ∈ B ×ˢ B ∧ closure UV.1 ⊆ UV.2}
this✝² : Encodable ↑s
this✝¹ : TopologicalSpace ↑s := ⊥
this✝ : DiscreteTopology ↑s
hd : ∀ (UV : ↑s), Disjoint (closure (↑UV).1) (↑UV).2ᶜ
ε : ↑s → ℝ
ε01 : ∀ (UV : ↑s), ε UV ∈ Ioc 0 1
hε : Tendsto ε cofinite (𝓝 0)
f : ↑s → C(X, ℝ)
hf0 : ∀ (UV : ↑s), EqOn (⇑(f UV)) 0 (↑UV).1
hfε : ∀ (UV : ↑s), EqOn (⇑(f UV)) (fun x => ε UV) (↑UV).2ᶜ
hf0ε : ∀ (UV : ↑s) (x : X), (f UV) x ∈ Icc 0 (ε UV)
hf01 : ∀ (UV : ↑s) (x : X), (f UV) x ∈ Icc 0 1
F : X → ↑s →ᵇ ℝ := fun x => { toFun := fun UV => (f UV) x, continuous_toFun := ⋯, map_bounded' := ⋯ }
hF : ∀ (x : X) (UV : ↑s), (F x) UV = (f UV) x
x : X
δ : ℝ
δ0 : 0 < δ
h_fin : {UV | δ ≤ ε UV}.Finite
this : ∀ᶠ (y : X) in 𝓝 x, ∀ (UV : ↑s), δ ≤ ε UV → dist ((F y) UV) ((F x) UV) ≤ δ
⊢ F ⁻¹' closedBall (F x) δ ∈ 𝓝 x

Proof state:
case mk.intro.intro.intro
X : Type u_1
inst✝² : TopologicalSpace X
inst✝¹ : RegularSpace X
inst✝ : SecondCountableTopology X
B : Set (Set X)
hBc : B.Countable
hB : IsTopologicalBasis B
s : Set (Set X × Set X) := {UV | UV ∈ B ×ˢ B ∧ closure UV.1 ⊆ UV.2}
this✝¹ : Encodable ↑s
this✝ : TopologicalSpace ↑s := ⊥
this : DiscreteTopology ↑s
hd : ∀ (UV : ↑s), Disjoint (closure (↑UV).1) (↑UV).2ᶜ
ε : ↑s → ℝ
ε0 : ∀ (i : ↑s), 0 < ε i
c : ℝ
hεc : HasSum ε c
hc1 : c ≤ 1
⊢ ∃ ε, (∀ (UV : ↑s), ε UV ∈ Ioc 0 1) ∧ Tendsto ε cofinite (𝓝 0)

Tactic:
refine ⟨ε, fun UV => ⟨ε0 UV, ?_⟩, hεc.summable.tendsto_cofinite_zero⟩

Result:
case mk.intro.intro.intro
X : Type u_1
inst✝² : TopologicalSpace X
inst✝¹ : RegularSpace X
inst✝ : SecondCountableTopology X
B : Set (Set X)
hBc : B.Countable
hB : IsTopologicalBasis B
s : Set (Set X × Set X) := {UV | UV ∈ B ×ˢ B ∧ closure UV.1 ⊆ UV.2}
this✝¹ : Encodable ↑s
this✝ : TopologicalSpace ↑s := ⊥
this : DiscreteTopology ↑s
hd : ∀ (UV : ↑s), Disjoint (closure (↑UV).1) (↑UV).2ᶜ
ε : ↑s → ℝ
ε0 : ∀ (i : ↑s), 0 < ε i
c : ℝ
hεc : HasSum ε c
hc1 : c ≤ 1
UV : ↑s
⊢ ε UV ≤ 1