Lean4
/-- For a regular topological space with second countable topology,
there exists an inducing map to `l^∞ = ℕ →ᵇ ℝ`. -/
theorem exists_isInducing_l_infty : ∃ f : X → ℕ →ᵇ ℝ, IsInducing f := by
-- Choose a countable basis, and consider the set `s` of pairs of set `(U, V)` such that `U ∈ B`,
-- `V ∈ B`, and `closure U ⊆ V`.
rcases exists_countable_basis X with ⟨B, hBc, -, hB⟩
let s : Set (Set X × Set X) :=
{UV ∈ B ×ˢ B | closure UV.1 ⊆ UV.2}
-- `s` is a countable set.
haveI : Encodable s := ((hBc.prod hBc).mono inter_subset_left).toEncodable
letI : TopologicalSpace s := ⊥
haveI : DiscreteTopology s := ⟨rfl⟩
rsuffices ⟨f, hf⟩ : ∃ f : X → s →ᵇ ℝ, IsInducing f
·
exact
⟨fun x => (f x).extend (Encodable.encode' s) 0,
(BoundedContinuousFunction.isometry_extend (Encodable.encode' s) (0 : ℕ →ᵇ ℝ)).isEmbedding.isInducing.comp hf⟩
have hd : ∀ UV : s, Disjoint (closure UV.1.1) UV.1.2ᶜ := fun UV =>
disjoint_compl_right.mono_right
(compl_subset_compl.2 UV.2.2)
-- Choose a sequence of `εₙ > 0`, `n : s`, that is bounded above by `1` and tends to zero
-- along the `cofinite` filter.
obtain ⟨ε, ε01, hε⟩ : ∃ ε : s → ℝ, (∀ UV, ε UV ∈ Ioc (0 : ℝ) 1) ∧ Tendsto ε cofinite (𝓝 0) :=
by
rcases posSumOfEncodable zero_lt_one s with ⟨ε, ε0, c, hεc, hc1⟩
refine ⟨ε, fun UV => ⟨ε0 UV, ?_⟩, hεc.summable.tendsto_cofinite_zero⟩
exact (le_hasSum hεc UV fun _ _ => (ε0 _).le).trans hc1
have : ∀ UV : s, ∃ f : C(X, ℝ), EqOn f 0 UV.1.1 ∧ EqOn f (fun _ => ε UV) UV.1.2ᶜ ∧ ∀ x, f x ∈ Icc 0 (ε UV) :=
by
intro UV
rcases exists_continuous_zero_one_of_isClosed isClosed_closure (hB.isOpen UV.2.1.2).isClosed_compl (hd UV) with
⟨f, hf₀, hf₁, hf01⟩
exact
⟨ε UV • f, fun x hx => by simp [hf₀ (subset_closure hx)], fun x hx => by simp [hf₁ hx], fun x =>
⟨mul_nonneg (ε01 _).1.le (hf01 _).1, mul_le_of_le_one_right (ε01 _).1.le (hf01 _).2⟩⟩
choose f hf0 hfε hf0ε using this
have hf01 : ∀ UV x, f UV x ∈ Icc (0 : ℝ) 1 := fun UV x =>
Icc_subset_Icc_right (ε01 _).2
(hf0ε _ _)
-- The embedding is given by `F x UV = f UV x`.
set F : X → s →ᵇ ℝ := fun x =>
⟨⟨fun UV => f UV x, continuous_of_discreteTopology⟩, 1, fun UV₁ UV₂ =>
Real.dist_le_of_mem_Icc_01 (hf01 _ _) (hf01 _ _)⟩
have hF : ∀ x UV, F x UV = f UV x := fun _ _ => rfl
refine ⟨F, isInducing_iff_nhds.2 fun x => le_antisymm ?_ ?_⟩
· /- First we prove that `F` is continuous. Given `δ > 0`, consider the set `T` of `(U, V) ∈ s`
such that `ε (U, V) ≥ δ`. Since `ε` tends to zero, `T` is finite. Since each `f` is continuous,
we can choose a neighborhood such that `dist (F y (U, V)) (F x (U, V)) ≤ δ` for any
`(U, V) ∈ T`. For `(U, V) ∉ T`, the same inequality is true because both `F y (U, V)` and
`F x (U, V)` belong to the interval `[0, ε (U, V)]`. -/
refine (nhds_basis_closedBall.comap _).ge_iff.2 fun δ δ0 => ?_
have h_fin : {UV : s | δ ≤ ε UV}.Finite := by simpa only [← not_lt] using hε (gt_mem_nhds δ0)
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)
refine this.mono fun y hy => (BoundedContinuousFunction.dist_le δ0.le).2 fun UV => ?_
rcases le_total δ (ε UV) with hle | hle
exacts [hy _ hle, (Real.dist_le_of_mem_Icc (hf0ε _ _) (hf0ε _ _)).trans (by rwa [sub_zero])]
· /- Finally, we prove that each neighborhood `V` of `x : X`
includes a preimage of a neighborhood of `F x` under `F`.
Without loss of generality, `V` belongs to `B`.
Choose `U ∈ B` such that `x ∈ V` and `closure V ⊆ U`.
Then the preimage of the `(ε (U, V))`-neighborhood of `F x` is included by `V`. -/
refine ((nhds_basis_ball.comap _).le_basis_iff hB.nhds_hasBasis).2 ?_
rintro V ⟨hVB, hxV⟩
rcases hB.exists_closure_subset (hB.mem_nhds hVB hxV) with ⟨U, hUB, hxU, hUV⟩
set UV : ↥s := ⟨(U, V), ⟨hUB, hVB⟩, hUV⟩
refine ⟨ε UV, (ε01 UV).1, fun y (hy : dist (F y) (F x) < ε UV) => ?_⟩
replace hy : dist (F y UV) (F x UV) < ε UV := (BoundedContinuousFunction.dist_coe_le_dist _).trans_lt hy
contrapose! hy
rw [hF, hF, hfε UV hy, hf0 UV hxU, Pi.zero_apply, dist_zero_right]
exact le_abs_self _
