formal/lean/mathlib/topology/metric_space/closeds.lean

/-
Copyright (c) 2019 Sébastien Gouëzel. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Sébastien Gouëzel
-/
import analysis.specific_limits.basic
import topology.metric_space.hausdorff_distance
import topology.sets.compacts

/-!
# Closed subsets

This file defines the metric and emetric space structure on the types of closed subsets and nonempty
compact subsets of a metric or emetric space.

The Hausdorff distance induces an emetric space structure on the type of closed subsets
of an emetric space, called `closeds`. Its completeness, resp. compactness, resp.
second-countability, follow from the corresponding properties of the original space.

In a metric space, the type of nonempty compact subsets (called `nonempty_compacts`) also
inherits a metric space structure from the Hausdorff distance, as the Hausdorff edistance is
always finite in this context.
-/

noncomputable theory
open_locale classical topological_space ennreal

universe u
open classical set function topological_space filter

namespace emetric
section
variables {α : Type u} [emetric_space α] {s : set α}

/-- In emetric spaces, the Hausdorff edistance defines an emetric space structure
on the type of closed subsets -/
instance closeds.emetric_space : emetric_space (closeds α) :=
{ edist               := λs t, Hausdorff_edist (s : set α) t,
  edist_self          := λs, Hausdorff_edist_self,
  edist_comm          := λs t, Hausdorff_edist_comm,
  edist_triangle      := λs t u, Hausdorff_edist_triangle,
  eq_of_edist_eq_zero :=
    λ s t h, closeds.ext $ (Hausdorff_edist_zero_iff_eq_of_closed s.closed t.closed).1 h }

/-- The edistance to a closed set depends continuously on the point and the set -/
lemma continuous_inf_edist_Hausdorff_edist :
  continuous (λ p : α × (closeds α), inf_edist p.1 p.2) :=
begin
  refine continuous_of_le_add_edist 2 (by simp) _,
  rintros ⟨x, s⟩ ⟨y, t⟩,
  calc inf_edist x s ≤ inf_edist x t + Hausdorff_edist (t : set α) s :
    inf_edist_le_inf_edist_add_Hausdorff_edist
  ... ≤ inf_edist y t + edist x y + Hausdorff_edist (t : set α) s :
    add_le_add_right inf_edist_le_inf_edist_add_edist _
  ... = inf_edist y t + (edist x y + Hausdorff_edist (s : set α) t)
      : by rw [add_assoc, Hausdorff_edist_comm]
  ... ≤ inf_edist y t + (edist (x, s) (y, t) + edist (x, s) (y, t)) :
    add_le_add_left (add_le_add (le_max_left _ _) (le_max_right _ _)) _
  ... = inf_edist y t + 2 * edist (x, s) (y, t) :
    by rw [← mul_two, mul_comm]
end

/-- Subsets of a given closed subset form a closed set -/
lemma is_closed_subsets_of_is_closed (hs : is_closed s) :
  is_closed {t : closeds α | (t : set α) ⊆ s} :=
begin
  refine is_closed_of_closure_subset (λt ht x hx, _),
  -- t : closeds α,  ht : t ∈ closure {t : closeds α | t ⊆ s},
  -- x : α,  hx : x ∈ t
  -- goal : x ∈ s
  have : x ∈ closure s,
  { refine mem_closure_iff.2 (λε εpos, _),
    rcases mem_closure_iff.1 ht ε εpos with ⟨u, hu, Dtu⟩,
    -- u : closeds α,  hu : u ∈ {t : closeds α | t ⊆ s},  hu' : edist t u < ε
    rcases exists_edist_lt_of_Hausdorff_edist_lt hx Dtu with ⟨y, hy, Dxy⟩,
    -- y : α,  hy : y ∈ u, Dxy : edist x y < ε
    exact ⟨y, hu hy, Dxy⟩ },
  rwa hs.closure_eq at this,
end

/-- By definition, the edistance on `closeds α` is given by the Hausdorff edistance -/
lemma closeds.edist_eq {s t : closeds α} : edist s t = Hausdorff_edist (s : set α) t := rfl

/-- In a complete space, the type of closed subsets is complete for the
Hausdorff edistance. -/
instance closeds.complete_space [complete_space α] : complete_space (closeds α) :=
begin
  /- We will show that, if a sequence of sets `s n` satisfies
  `edist (s n) (s (n+1)) < 2^{-n}`, then it converges. This is enough to guarantee
  completeness, by a standard completeness criterion.
  We use the shorthand `B n = 2^{-n}` in ennreal. -/
  let B : ℕ → ℝ≥0∞ := λ n, (2⁻¹)^n,
  have B_pos : ∀ n, (0:ℝ≥0∞) < B n,
    by simp [B, ennreal.pow_pos],
  have B_ne_top : ∀ n, B n ≠ ⊤,
    by simp [B, ennreal.pow_ne_top],
  /- Consider a sequence of closed sets `s n` with `edist (s n) (s (n+1)) < B n`.
  We will show that it converges. The limit set is t0 = ⋂n, closure (⋃m≥n, s m).
  We will have to show that a point in `s n` is close to a point in `t0`, and a point
  in `t0` is close to a point in `s n`. The completeness then follows from a
  standard criterion. -/
  refine complete_of_convergent_controlled_sequences B B_pos (λs hs, _),
  let t0 := ⋂ n, closure (⋃ m ≥ n, s m : set α),
  let t : closeds α := ⟨t0, is_closed_Inter (λ_, is_closed_closure)⟩,
  use t,
  -- The inequality is written this way to agree with `edist_le_of_edist_le_geometric_of_tendsto₀`
  have I1 : ∀ n, ∀ x ∈ s n, ∃ y ∈ t0, edist x y ≤ 2 * B n,
  { /- This is the main difficulty of the proof. Starting from `x ∈ s n`, we want
       to find a point in `t0` which is close to `x`. Define inductively a sequence of
       points `z m` with `z n = x` and `z m ∈ s m` and `edist (z m) (z (m+1)) ≤ B m`. This is
       possible since the Hausdorff distance between `s m` and `s (m+1)` is at most `B m`.
       This sequence is a Cauchy sequence, therefore converging as the space is complete, to
       a limit which satisfies the required properties. -/
    assume n x hx,
    obtain ⟨z, hz₀, hz⟩ : ∃ z : Π l, s (n + l), (z 0 : α) = x ∧
      ∀ k, edist (z k:α) (z (k+1):α) ≤ B n / 2^k,
    { -- We prove existence of the sequence by induction.
      have : ∀ l (z : s (n + l)), ∃ z' : s (n + l + 1), edist (z : α) z' ≤ B n / 2^l,
      { assume l z,
        obtain ⟨z', z'_mem, hz'⟩ : ∃ z' ∈ s (n + l + 1), edist (z : α) z' < B n / 2^l,
        { refine exists_edist_lt_of_Hausdorff_edist_lt _ _,
          { exact s (n + l) },
          { exact z.2 },
          simp only [B, ennreal.inv_pow, div_eq_mul_inv],
          rw [← pow_add],
          apply hs; simp },
        exact ⟨⟨z', z'_mem⟩, le_of_lt hz'⟩ },
      use [λ k, nat.rec_on k ⟨x, hx⟩ (λl z, some (this l z)), rfl],
      exact λ k, some_spec (this k _) },
    -- it follows from the previous bound that `z` is a Cauchy sequence
    have : cauchy_seq (λ k, ((z k):α)),
      from cauchy_seq_of_edist_le_geometric_two (B n) (B_ne_top n) hz,
    -- therefore, it converges
    rcases cauchy_seq_tendsto_of_complete this with ⟨y, y_lim⟩,
    use y,
    -- the limit point `y` will be the desired point, in `t0` and close to our initial point `x`.
    -- First, we check it belongs to `t0`.
    have : y ∈ t0 := mem_Inter.2 (λk, mem_closure_of_tendsto y_lim
    begin
      simp only [exists_prop, set.mem_Union, filter.eventually_at_top, set.mem_preimage,
        set.preimage_Union],
      exact ⟨k, λ m hm, ⟨n+m, zero_add k ▸ add_le_add (zero_le n) hm, (z m).2⟩⟩
    end),
    use this,
    -- Then, we check that `y` is close to `x = z n`. This follows from the fact that `y`
    -- is the limit of `z k`, and the distance between `z n` and `z k` has already been estimated.
    rw [← hz₀],
    exact edist_le_of_edist_le_geometric_two_of_tendsto₀ (B n) hz y_lim },
  have I2 : ∀ n, ∀ x ∈ t0, ∃ y ∈ s n, edist x y ≤ 2 * B n,
  { /- For the (much easier) reverse inequality, we start from a point `x ∈ t0` and we want
        to find a point `y ∈ s n` which is close to `x`.
        `x` belongs to `t0`, the intersection of the closures. In particular, it is well
        approximated by a point `z` in `⋃m≥n, s m`, say in `s m`. Since `s m` and
        `s n` are close, this point is itself well approximated by a point `y` in `s n`,
        as required. -/
    assume n x xt0,
    have : x ∈ closure (⋃ m ≥ n, s m : set α), by apply mem_Inter.1 xt0 n,
    rcases mem_closure_iff.1 this (B n) (B_pos n) with ⟨z, hz, Dxz⟩,
    -- z : α,  Dxz : edist x z < B n,
    simp only [exists_prop, set.mem_Union] at hz,
    rcases hz with ⟨m, ⟨m_ge_n, hm⟩⟩,
    -- m : ℕ, m_ge_n : m ≥ n, hm : z ∈ s m
    have : Hausdorff_edist (s m : set α) (s n) < B n := hs n m n m_ge_n (le_refl n),
    rcases exists_edist_lt_of_Hausdorff_edist_lt hm this with ⟨y, hy, Dzy⟩,
    -- y : α,  hy : y ∈ s n,  Dzy : edist z y < B n
    exact ⟨y, hy, calc
      edist x y ≤ edist x z + edist z y : edist_triangle _ _ _
            ... ≤ B n + B n : add_le_add (le_of_lt Dxz) (le_of_lt Dzy)
            ... = 2 * B n : (two_mul _).symm ⟩ },
  -- Deduce from the above inequalities that the distance between `s n` and `t0` is at most `2 B n`.
  have main : ∀n:ℕ, edist (s n) t ≤ 2 * B n := λn, Hausdorff_edist_le_of_mem_edist (I1 n) (I2 n),
  -- from this, the convergence of `s n` to `t0` follows.
  refine tendsto_at_top.2 (λε εpos, _),
  have : tendsto (λn, 2 * B n) at_top (𝓝 (2 * 0)),
    from ennreal.tendsto.const_mul
      (ennreal.tendsto_pow_at_top_nhds_0_of_lt_1 $ by simp [ennreal.one_lt_two])
      (or.inr $ by simp),
  rw mul_zero at this,
  obtain ⟨N, hN⟩ : ∃ N, ∀ b ≥ N, ε > 2 * B b,
    from ((tendsto_order.1 this).2 ε εpos).exists_forall_of_at_top,
  exact ⟨N, λn hn, lt_of_le_of_lt (main n) (hN n hn)⟩
end

/-- In a compact space, the type of closed subsets is compact. -/
instance closeds.compact_space [compact_space α] : compact_space (closeds α) :=
⟨begin
  /- by completeness, it suffices to show that it is totally bounded,
    i.e., for all ε>0, there is a finite set which is ε-dense.
    start from a set `s` which is ε-dense in α. Then the subsets of `s`
    are finitely many, and ε-dense for the Hausdorff distance. -/
  refine compact_of_totally_bounded_is_closed (emetric.totally_bounded_iff.2 (λε εpos, _))
    is_closed_univ,
  rcases exists_between εpos with ⟨δ, δpos, δlt⟩,
  rcases emetric.totally_bounded_iff.1
    (compact_iff_totally_bounded_complete.1 (@compact_univ α _ _)).1 δ δpos with ⟨s, fs, hs⟩,
  -- s : set α,  fs : s.finite,  hs : univ ⊆ ⋃ (y : α) (H : y ∈ s), eball y δ
  -- we first show that any set is well approximated by a subset of `s`.
  have main : ∀ u : set α, ∃v ⊆ s, Hausdorff_edist u v ≤ δ,
  { assume u,
    let v := {x : α | x ∈ s ∧ ∃y∈u, edist x y < δ},
    existsi [v, ((λx hx, hx.1) : v ⊆ s)],
    refine Hausdorff_edist_le_of_mem_edist _ _,
    { assume x hx,
      have : x ∈ ⋃y ∈ s, ball y δ := hs (by simp),
      rcases mem_Union₂.1 this with ⟨y, ys, dy⟩,
      have : edist y x < δ := by simp at dy; rwa [edist_comm] at dy,
      exact ⟨y, ⟨ys, ⟨x, hx, this⟩⟩, le_of_lt dy⟩ },
    { rintros x ⟨hx1, ⟨y, yu, hy⟩⟩,
      exact ⟨y, yu, le_of_lt hy⟩ }},
  -- introduce the set F of all subsets of `s` (seen as members of `closeds α`).
  let F := {f : closeds α | (f : set α) ⊆ s},
  refine ⟨F, _, λ u _, _⟩,
  -- `F` is finite
  { apply @finite.of_finite_image _ _ F coe,
    { apply fs.finite_subsets.subset (λb, _),
      simp only [and_imp, set.mem_image, set.mem_set_of_eq, exists_imp_distrib],
      assume x hx hx',
      rwa hx' at hx },
    { exact set_like.coe_injective.inj_on F } },

  -- `F` is ε-dense
  { obtain ⟨t0, t0s, Dut0⟩ := main u,
    have : is_closed t0 := (fs.subset t0s).is_compact.is_closed,
    let t : closeds α := ⟨t0, this⟩,
    have : t ∈ F := t0s,
    have : edist u t < ε := lt_of_le_of_lt Dut0 δlt,
    apply mem_Union₂.2,
    exact ⟨t, ‹t ∈ F›, this⟩ }
end⟩

/-- In an emetric space, the type of non-empty compact subsets is an emetric space,
where the edistance is the Hausdorff edistance -/
instance nonempty_compacts.emetric_space : emetric_space (nonempty_compacts α) :=
{ edist               := λ s t, Hausdorff_edist (s : set α) t,
  edist_self          := λs, Hausdorff_edist_self,
  edist_comm          := λs t, Hausdorff_edist_comm,
  edist_triangle      := λs t u, Hausdorff_edist_triangle,
  eq_of_edist_eq_zero := λ s t h, nonempty_compacts.ext $ begin
    have : closure (s : set α) = closure t := Hausdorff_edist_zero_iff_closure_eq_closure.1 h,
    rwa [s.compact.is_closed.closure_eq, t.compact.is_closed.closure_eq] at this,
  end }

/-- `nonempty_compacts.to_closeds` is a uniform embedding (as it is an isometry) -/
lemma nonempty_compacts.to_closeds.uniform_embedding :
  uniform_embedding (@nonempty_compacts.to_closeds α _ _) :=
isometry.uniform_embedding $ λx y, rfl

/-- The range of `nonempty_compacts.to_closeds` is closed in a complete space -/
lemma nonempty_compacts.is_closed_in_closeds [complete_space α] :
  is_closed (range $ @nonempty_compacts.to_closeds α _ _) :=
begin
  have : range nonempty_compacts.to_closeds =
    {s : closeds α | (s : set α).nonempty ∧ is_compact (s : set α) },
  { ext s,
    refine ⟨_, λ h, ⟨⟨⟨s, h.2⟩, h.1⟩, closeds.ext rfl⟩⟩,
    rintro ⟨s, hs, rfl⟩,
    exact ⟨s.nonempty, s.compact⟩ },
  rw this,
  refine is_closed_of_closure_subset (λs hs, ⟨_, _⟩),
  { -- take a set set t which is nonempty and at a finite distance of s
    rcases mem_closure_iff.1 hs ⊤ ennreal.coe_lt_top with ⟨t, ht, Dst⟩,
    rw edist_comm at Dst,
    -- since `t` is nonempty, so is `s`
    exact nonempty_of_Hausdorff_edist_ne_top ht.1 (ne_of_lt Dst) },
  { refine compact_iff_totally_bounded_complete.2 ⟨_, s.closed.is_complete⟩,
    refine totally_bounded_iff.2 (λε (εpos : 0 < ε), _),
    -- we have to show that s is covered by finitely many eballs of radius ε
    -- pick a nonempty compact set t at distance at most ε/2 of s
    rcases mem_closure_iff.1 hs (ε/2) (ennreal.half_pos εpos.ne') with ⟨t, ht, Dst⟩,
    -- cover this space with finitely many balls of radius ε/2
    rcases totally_bounded_iff.1 (compact_iff_totally_bounded_complete.1 ht.2).1 (ε/2)
      (ennreal.half_pos εpos.ne') with ⟨u, fu, ut⟩,
    refine ⟨u, ⟨fu, λx hx, _⟩⟩,
    -- u : set α,  fu : u.finite,  ut : t ⊆ ⋃ (y : α) (H : y ∈ u), eball y (ε / 2)
    -- then s is covered by the union of the balls centered at u of radius ε
    rcases exists_edist_lt_of_Hausdorff_edist_lt hx Dst with ⟨z, hz, Dxz⟩,
    rcases mem_Union₂.1 (ut hz) with ⟨y, hy, Dzy⟩,
    have : edist x y < ε := calc
      edist x y ≤ edist x z + edist z y : edist_triangle _ _ _
      ... < ε/2 + ε/2 : ennreal.add_lt_add Dxz Dzy
      ... = ε : ennreal.add_halves _,
    exact mem_bUnion hy this },
end

/-- In a complete space, the type of nonempty compact subsets is complete. This follows
from the same statement for closed subsets -/
instance nonempty_compacts.complete_space [complete_space α] :
  complete_space (nonempty_compacts α) :=
(complete_space_iff_is_complete_range
  nonempty_compacts.to_closeds.uniform_embedding.to_uniform_inducing).2 $
  nonempty_compacts.is_closed_in_closeds.is_complete

/-- In a compact space, the type of nonempty compact subsets is compact. This follows from
the same statement for closed subsets -/
instance nonempty_compacts.compact_space [compact_space α] : compact_space (nonempty_compacts α) :=
⟨begin
  rw nonempty_compacts.to_closeds.uniform_embedding.embedding.is_compact_iff_is_compact_image,
  rw [image_univ],
  exact nonempty_compacts.is_closed_in_closeds.is_compact
end⟩

/-- In a second countable space, the type of nonempty compact subsets is second countable -/
instance nonempty_compacts.second_countable_topology [second_countable_topology α] :
  second_countable_topology (nonempty_compacts α) :=
begin
  haveI : separable_space (nonempty_compacts α) :=
  begin
    /- To obtain a countable dense subset of `nonempty_compacts α`, start from
    a countable dense subset `s` of α, and then consider all its finite nonempty subsets.
    This set is countable and made of nonempty compact sets. It turns out to be dense:
    by total boundedness, any compact set `t` can be covered by finitely many small balls, and
    approximations in `s` of the centers of these balls give the required finite approximation
    of `t`. -/
    rcases exists_countable_dense α with ⟨s, cs, s_dense⟩,
    let v0 := {t : set α | t.finite ∧ t ⊆ s},
    let v : set (nonempty_compacts α) := {t : nonempty_compacts α | (t : set α) ∈ v0},
    refine  ⟨⟨v, _, _⟩⟩,
    { have : v0.countable, from countable_set_of_finite_subset cs,
      exact this.preimage set_like.coe_injective },
    { refine λt, mem_closure_iff.2 (λε εpos, _),
      -- t is a compact nonempty set, that we have to approximate uniformly by a a set in `v`.
      rcases exists_between εpos with ⟨δ, δpos, δlt⟩,
      have δpos' : 0 < δ / 2, from ennreal.half_pos δpos.ne',
      -- construct a map F associating to a point in α an approximating point in s, up to δ/2.
      have Exy : ∀x, ∃y, y ∈ s ∧ edist x y < δ/2,
      { assume x,
        rcases mem_closure_iff.1 (s_dense x) (δ/2) δpos' with ⟨y, ys, hy⟩,
        exact ⟨y, ⟨ys, hy⟩⟩ },
      let F := λx, some (Exy x),
      have Fspec : ∀x, F x ∈ s ∧ edist x (F x) < δ/2 := λx, some_spec (Exy x),

      -- cover `t` with finitely many balls. Their centers form a set `a`
      have : totally_bounded (t : set α) := t.compact.totally_bounded,
      rcases totally_bounded_iff.1 this (δ/2) δpos' with ⟨a, af, ta⟩,
      -- a : set α,  af : a.finite,  ta : t ⊆ ⋃ (y : α) (H : y ∈ a), eball y (δ / 2)
      -- replace each center by a nearby approximation in `s`, giving a new set `b`
      let b := F '' a,
      have : b.finite := af.image _,
      have tb : ∀ x ∈ t, ∃ y ∈ b, edist x y < δ,
      { assume x hx,
        rcases mem_Union₂.1 (ta hx) with ⟨z, za, Dxz⟩,
        existsi [F z, mem_image_of_mem _ za],
        calc edist x (F z) ≤ edist x z + edist z (F z) : edist_triangle _ _ _
             ... < δ/2 + δ/2 : ennreal.add_lt_add Dxz (Fspec z).2
             ... = δ : ennreal.add_halves _ },
      -- keep only the points in `b` that are close to point in `t`, yielding a new set `c`
      let c := {y ∈ b | ∃ x ∈ t, edist x y < δ},
      have : c.finite := ‹b.finite›.subset (λx hx, hx.1),
      -- points in `t` are well approximated by points in `c`
      have tc : ∀ x ∈ t, ∃ y ∈ c, edist x y ≤ δ,
      { assume x hx,
        rcases tb x hx with ⟨y, yv, Dxy⟩,
        have : y ∈ c := by simp [c, -mem_image]; exact ⟨yv, ⟨x, hx, Dxy⟩⟩,
        exact ⟨y, this, le_of_lt Dxy⟩ },
      -- points in `c` are well approximated by points in `t`
      have ct : ∀ y ∈ c, ∃ x ∈ t, edist y x ≤ δ,
      { rintro y ⟨hy1, x, xt, Dyx⟩,
        have : edist y x ≤ δ := calc
          edist y x = edist x y : edist_comm _ _
          ... ≤ δ : le_of_lt Dyx,
        exact ⟨x, xt, this⟩ },
      -- it follows that their Hausdorff distance is small
      have : Hausdorff_edist (t :set α) c ≤ δ :=
        Hausdorff_edist_le_of_mem_edist tc ct,
      have Dtc : Hausdorff_edist (t : set α) c < ε := this.trans_lt δlt,
      -- the set `c` is not empty, as it is well approximated by a nonempty set
      have hc : c.nonempty,
        from nonempty_of_Hausdorff_edist_ne_top t.nonempty (ne_top_of_lt Dtc),
      -- let `d` be the version of `c` in the type `nonempty_compacts α`
      let d : nonempty_compacts α := ⟨⟨c, ‹c.finite›.is_compact⟩, hc⟩,
      have : c ⊆ s,
      { assume x hx,
        rcases (mem_image _ _ _).1 hx.1 with ⟨y, ⟨ya, yx⟩⟩,
        rw ← yx,
        exact (Fspec y).1 },
      have : d ∈ v := ⟨‹c.finite›, this⟩,
      -- we have proved that `d` is a good approximation of `t` as requested
      exact ⟨d, ‹d ∈ v›, Dtc⟩ },
  end,
  apply uniform_space.second_countable_of_separable,
end

end --section
end emetric --namespace

namespace metric
section

variables {α : Type u} [metric_space α]

/-- `nonempty_compacts α` inherits a metric space structure, as the Hausdorff
edistance between two such sets is finite. -/
instance nonempty_compacts.metric_space : metric_space (nonempty_compacts α) :=
emetric_space.to_metric_space $ λ x y, Hausdorff_edist_ne_top_of_nonempty_of_bounded
  x.nonempty y.nonempty x.compact.bounded y.compact.bounded

/-- The distance on `nonempty_compacts α` is the Hausdorff distance, by construction -/
lemma nonempty_compacts.dist_eq {x y : nonempty_compacts α} :
  dist x y = Hausdorff_dist (x : set α) y := rfl

lemma lipschitz_inf_dist_set (x : α) : lipschitz_with 1 (λ s : nonempty_compacts α, inf_dist x s) :=
lipschitz_with.of_le_add $ assume s t,
by { rw dist_comm,
  exact inf_dist_le_inf_dist_add_Hausdorff_dist (edist_ne_top t s) }

lemma lipschitz_inf_dist : lipschitz_with 2 (λ p : α × (nonempty_compacts α), inf_dist p.1 p.2) :=
@lipschitz_with.uncurry _ _ _ _ _ _ (λ (x : α) (s : nonempty_compacts α), inf_dist x s) 1 1
  (λ s, lipschitz_inf_dist_pt s) lipschitz_inf_dist_set

lemma uniform_continuous_inf_dist_Hausdorff_dist :
  uniform_continuous (λ p : α × (nonempty_compacts α), inf_dist p.1 p.2) :=
lipschitz_inf_dist.uniform_continuous

end --section
end metric --namespace