subset_properties.lean

/-
Copyright (c) 2017 Johannes Hölzl. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Johannes Hölzl, Mario Carneiro, Yury Kudryashov
-/
import topology.continuous_on
import data.finset.order

/-!
# Properties of subsets of topological spaces

In this file we define various properties of subsets of a topological space, and some classes on
topological spaces.

## Main definitions

We define the following properties for sets in a topological space:

* `is_compact`: each open cover has a finite subcover. This is defined in mathlib using filters.
  The main property of a compact set is `is_compact.elim_finite_subcover`.
* `is_clopen`: a set that is both open and closed.
* `is_irreducible`: a nonempty set that has contains no non-trivial pair of disjoint opens.
  See also the section below in the module doc.
* `is_connected`: a nonempty set that has no non-trivial open partition.
  See also the section below in the module doc.
  `connected_component` is the connected component of an element in the space.
* `is_totally_disconnected`: all of its connected components are singletons.
* `is_totally_separated`: any two points can be separated by two disjoint opens that cover the set.

For each of these definitions (except for `is_clopen`), we also have a class stating that the whole
space satisfies that property:
`compact_space`, `irreducible_space`, `connected_space`, `totally_disconnected_space`,
`totally_separated_space`.

Furthermore, we have two more classes:
* `locally_compact_space`: for every point `x`, every open neighborhood of `x` contains a compact
  neighborhood of `x`. The definition is formulated in terms of the neighborhood filter.
* `sigma_compact_space`: a space that is the union of a countably many compact subspaces.

## On the definition of irreducible and connected sets/spaces

In informal mathematics, irreducible and connected spaces are assumed to be nonempty.
We formalise the predicate without that assumption
as `is_preirreducible` and `is_preconnected` respectively.
In other words, the only difference is whether the empty space
counts as irreducible and/or connected.
There are good reasons to consider the empty space to be “too simple to be simple”
See also https://ncatlab.org/nlab/show/too+simple+to+be+simple,
and in particular
https://ncatlab.org/nlab/show/too+simple+to+be+simple#relationship_to_biased_definitions.
-/

open set filter classical
open_locale classical topological_space filter

universes u v
variables {α : Type u} {β : Type v} [topological_space α] {s t : set α}

/- compact sets -/
section compact

/-- A set `s` is compact if for every filter `f` that contains `s`,
    every set of `f` also meets every neighborhood of some `a ∈ s`. -/
def is_compact (s : set α) := ∀ ⦃f⦄ [ne_bot f], f ≤ 𝓟 s → ∃a∈s, cluster_pt a f

/-- The complement to a compact set belongs to a filter `f` if it belongs to each filter
`𝓝 a ⊓ f`, `a ∈ s`. -/
lemma is_compact.compl_mem_sets (hs : is_compact s) {f : filter α} (hf : ∀ a ∈ s, sᶜ ∈ 𝓝 a ⊓ f) :
  sᶜ ∈ f :=
begin
  contrapose! hf,
  simp only [mem_iff_inf_principal_compl, compl_compl, inf_assoc, ← exists_prop] at hf ⊢,
  exact @hs _ hf inf_le_right
end

/-- The complement to a compact set belongs to a filter `f` if each `a ∈ s` has a neighborhood `t`
within `s` such that `tᶜ` belongs to `f`. -/
lemma is_compact.compl_mem_sets_of_nhds_within (hs : is_compact s) {f : filter α}
  (hf : ∀ a ∈ s, ∃ t ∈ 𝓝[s] a, tᶜ ∈ f) :
  sᶜ ∈ f :=
begin
  refine hs.compl_mem_sets (λ a ha, _),
  rcases hf a ha with ⟨t, ht, hst⟩,
  replace ht := mem_inf_principal.1 ht,
  refine mem_inf_sets.2 ⟨_, ht, _, hst, _⟩,
  rintros x ⟨h₁, h₂⟩ hs,
  exact h₂ (h₁ hs)
end

/-- If `p : set α → Prop` is stable under restriction and union, and each point `x`
  of a compact set `s` has a neighborhood `t` within `s` such that `p t`, then `p s` holds. -/
@[elab_as_eliminator]
lemma is_compact.induction_on {s : set α} (hs : is_compact s) {p : set α → Prop} (he : p ∅)
  (hmono : ∀ ⦃s t⦄, s ⊆ t → p t → p s) (hunion : ∀ ⦃s t⦄, p s → p t → p (s ∪ t))
  (hnhds : ∀ x ∈ s, ∃ t ∈ 𝓝[s] x, p t) :
  p s :=
let f : filter α :=
  { sets := {t | p tᶜ},
    univ_sets := by simpa,
    sets_of_superset := λ t₁ t₂ ht₁ ht, hmono (compl_subset_compl.2 ht) ht₁,
    inter_sets := λ t₁ t₂ ht₁ ht₂, by simp [compl_inter, hunion ht₁ ht₂] } in
have sᶜ ∈ f, from hs.compl_mem_sets_of_nhds_within (by simpa using hnhds),
by simpa

/-- The intersection of a compact set and a closed set is a compact set. -/
lemma is_compact.inter_right (hs : is_compact s) (ht : is_closed t) :
  is_compact (s ∩ t) :=
begin
  introsI f hnf hstf,
  obtain ⟨a, hsa, ha⟩ : ∃ a ∈ s, cluster_pt a f :=
    hs (le_trans hstf (le_principal_iff.2 (inter_subset_left _ _))),
  have : a ∈ t :=
    (ht.mem_of_nhds_within_ne_bot $ ha.mono $
      le_trans hstf (le_principal_iff.2 (inter_subset_right _ _))),
  exact ⟨a, ⟨hsa, this⟩, ha⟩
end

/-- The intersection of a closed set and a compact set is a compact set. -/
lemma is_compact.inter_left (ht : is_compact t) (hs : is_closed s) : is_compact (s ∩ t) :=
inter_comm t s ▸ ht.inter_right hs

/-- The set difference of a compact set and an open set is a compact set. -/
lemma compact_diff (hs : is_compact s) (ht : is_open t) : is_compact (s \ t) :=
hs.inter_right (is_closed_compl_iff.mpr ht)

/-- A closed subset of a compact set is a compact set. -/
lemma compact_of_is_closed_subset (hs : is_compact s) (ht : is_closed t) (h : t ⊆ s) :
  is_compact t :=
inter_eq_self_of_subset_right h ▸ hs.inter_right ht

lemma is_compact.adherence_nhdset {f : filter α}
  (hs : is_compact s) (hf₂ : f ≤ 𝓟 s) (ht₁ : is_open t) (ht₂ : ∀a∈s, cluster_pt a f → a ∈ t) :
  t ∈ f :=
classical.by_cases mem_sets_of_eq_bot $
  assume : f ⊓ 𝓟 tᶜ ≠ ⊥,
  let ⟨a, ha, (hfa : cluster_pt a $ f ⊓ 𝓟 tᶜ)⟩ := @@hs this $ inf_le_left_of_le hf₂ in
  have a ∈ t,
    from ht₂ a ha (hfa.of_inf_left),
  have tᶜ ∩ t ∈ 𝓝[tᶜ] a,
    from inter_mem_nhds_within _ (mem_nhds_sets ht₁ this),
  have A : 𝓝[tᶜ] a = ⊥,
    from empty_in_sets_eq_bot.1 $ compl_inter_self t ▸ this,
  have 𝓝[tᶜ] a ≠ ⊥,
    from hfa.of_inf_right,
  absurd A this

lemma compact_iff_ultrafilter_le_nhds :
  is_compact s ↔ (∀f : ultrafilter α, ↑f ≤ 𝓟 s → ∃a∈s, ↑f ≤ 𝓝 a) :=
begin
  refine (forall_ne_bot_le_iff _).trans _,
  { rintro f g hle ⟨a, has, haf⟩,
    exact ⟨a, has, haf.mono hle⟩ },
  { simp only [ultrafilter.cluster_pt_iff] }
end

alias compact_iff_ultrafilter_le_nhds ↔ is_compact.ultrafilter_le_nhds _

/-- For every open cover of a compact set, there exists a finite subcover. -/
lemma is_compact.elim_finite_subcover {ι : Type v} (hs : is_compact s)
  (U : ι → set α) (hUo : ∀i, is_open (U i)) (hsU : s ⊆ ⋃ i, U i) :
  ∃ t : finset ι, s ⊆ ⋃ i ∈ t, U i :=
is_compact.induction_on hs ⟨∅, empty_subset _⟩ (λ s₁ s₂ hs ⟨t, hs₂⟩, ⟨t, subset.trans hs hs₂⟩)
  (λ s₁ s₂ ⟨t₁, ht₁⟩ ⟨t₂, ht₂⟩,
    ⟨t₁ ∪ t₂, by { rw [finset.bUnion_union], exact union_subset_union ht₁ ht₂ }⟩)
  (λ x hx, let ⟨i, hi⟩ := mem_Union.1 (hsU hx) in
    ⟨U i, mem_nhds_within.2 ⟨U i, hUo i, hi, inter_subset_left _ _⟩, {i}, by simp⟩)

/-- For every family of closed sets whose intersection avoids a compact set,
there exists a finite subfamily whose intersection avoids this compact set. -/
lemma is_compact.elim_finite_subfamily_closed {s : set α} {ι : Type v} (hs : is_compact s)
  (Z : ι → set α) (hZc : ∀i, is_closed (Z i)) (hsZ : s ∩ (⋂ i, Z i) = ∅) :
  ∃ t : finset ι, s ∩ (⋂ i ∈ t, Z i) = ∅ :=
let ⟨t, ht⟩ := hs.elim_finite_subcover (λ i, (Z i)ᶜ) hZc
  (by simpa only [subset_def, not_forall, eq_empty_iff_forall_not_mem, mem_Union,
    exists_prop, mem_inter_eq, not_and, iff_self, mem_Inter, mem_compl_eq] using hsZ)
    in
⟨t, by simpa only [subset_def, not_forall, eq_empty_iff_forall_not_mem, mem_Union,
    exists_prop, mem_inter_eq, not_and, iff_self, mem_Inter, mem_compl_eq] using ht⟩

/-- To show that a compact set intersects the intersection of a family of closed sets,
  it is sufficient to show that it intersects every finite subfamily. -/
lemma is_compact.inter_Inter_nonempty {s : set α} {ι : Type v} (hs : is_compact s)
  (Z : ι → set α) (hZc : ∀i, is_closed (Z i)) (hsZ : ∀ t : finset ι, (s ∩ ⋂ i ∈ t, Z i).nonempty) :
  (s ∩ ⋂ i, Z i).nonempty :=
begin
  simp only [← ne_empty_iff_nonempty] at hsZ ⊢,
  apply mt (hs.elim_finite_subfamily_closed Z hZc), push_neg, exact hsZ
end

/-- Cantor's intersection theorem:
the intersection of a directed family of nonempty compact closed sets is nonempty. -/
lemma is_compact.nonempty_Inter_of_directed_nonempty_compact_closed
  {ι : Type v} [hι : nonempty ι] (Z : ι → set α) (hZd : directed (⊇) Z)
  (hZn : ∀ i, (Z i).nonempty) (hZc : ∀ i, is_compact (Z i)) (hZcl : ∀ i, is_closed (Z i)) :
  (⋂ i, Z i).nonempty :=
begin
  apply hι.elim,
  intro i₀,
  let Z' := λ i, Z i ∩ Z i₀,
  suffices : (⋂ i, Z' i).nonempty,
  { exact nonempty.mono (Inter_subset_Inter $ assume i, inter_subset_left (Z i) (Z i₀)) this },
  rw ← ne_empty_iff_nonempty,
  intro H,
  obtain ⟨t, ht⟩ : ∃ (t : finset ι), ((Z i₀) ∩ ⋂ (i ∈ t), Z' i) = ∅,
    from (hZc i₀).elim_finite_subfamily_closed Z'
      (assume i, is_closed_inter (hZcl i) (hZcl i₀)) (by rw [H, inter_empty]),
  obtain ⟨i₁, hi₁⟩ : ∃ i₁ : ι, Z i₁ ⊆ Z i₀ ∧ ∀ i ∈ t, Z i₁ ⊆ Z' i,
  { rcases directed.finset_le hZd t with ⟨i, hi⟩,
    rcases hZd i i₀ with ⟨i₁, hi₁, hi₁₀⟩,
    use [i₁, hi₁₀],
    intros j hj,
    exact subset_inter (subset.trans hi₁ (hi j hj)) hi₁₀ },
  suffices : ((Z i₀) ∩ ⋂ (i ∈ t), Z' i).nonempty,
  { rw ← ne_empty_iff_nonempty at this, contradiction },
  refine nonempty.mono _ (hZn i₁),
  exact subset_inter hi₁.left (subset_bInter hi₁.right)
end

/-- Cantor's intersection theorem for sequences indexed by `ℕ`:
the intersection of a decreasing sequence of nonempty compact closed sets is nonempty. -/
lemma is_compact.nonempty_Inter_of_sequence_nonempty_compact_closed
  (Z : ℕ → set α) (hZd : ∀ i, Z (i+1) ⊆ Z i)
  (hZn : ∀ i, (Z i).nonempty) (hZ0 : is_compact (Z 0)) (hZcl : ∀ i, is_closed (Z i)) :
  (⋂ i, Z i).nonempty :=
have Zmono : _, from @monotone_of_monotone_nat (order_dual _) _ Z hZd,
have hZd : directed (⊇) Z, from directed_of_sup Zmono,
have ∀ i, Z i ⊆ Z 0, from assume i, Zmono $ zero_le i,
have hZc : ∀ i, is_compact (Z i), from assume i, compact_of_is_closed_subset hZ0 (hZcl i) (this i),
is_compact.nonempty_Inter_of_directed_nonempty_compact_closed Z hZd hZn hZc hZcl

/-- For every open cover of a compact set, there exists a finite subcover. -/
lemma is_compact.elim_finite_subcover_image {b : set β} {c : β → set α}
  (hs : is_compact s) (hc₁ : ∀i∈b, is_open (c i)) (hc₂ : s ⊆ ⋃i∈b, c i) :
  ∃b'⊆b, finite b' ∧ s ⊆ ⋃i∈b', c i :=
begin
  rcases hs.elim_finite_subcover (λ i, c i.1 : b → set α) _ _ with ⟨d, hd⟩,
  refine ⟨↑(d.image subtype.val), _, finset.finite_to_set _, _⟩,
  { intros i hi,
    erw finset.mem_image at hi,
    rcases hi with ⟨s, hsd, rfl⟩,
    exact s.property },
  { refine subset.trans hd _,
    rintros x ⟨_, ⟨s, rfl⟩, ⟨_, ⟨hsd, rfl⟩, H⟩⟩,
    refine ⟨c s.val, ⟨s.val, _⟩, H⟩,
    simp [finset.mem_image_of_mem subtype.val hsd] },
  { rintro ⟨i, hi⟩, exact hc₁ i hi },
  { refine subset.trans hc₂ _,
    rintros x ⟨_, ⟨i, rfl⟩, ⟨_, ⟨hib, rfl⟩, H⟩⟩,
    exact ⟨_, ⟨⟨i, hib⟩, rfl⟩, H⟩ },
end

/-- A set `s` is compact if for every family of closed sets whose intersection avoids `s`,
there exists a finite subfamily whose intersection avoids `s`. -/
theorem compact_of_finite_subfamily_closed
  (h : Π {ι : Type u} (Z : ι → (set α)), (∀ i, is_closed (Z i)) →
    s ∩ (⋂ i, Z i) = ∅ → (∃ (t : finset ι), s ∩ (⋂ i ∈ t, Z i) = ∅)) :
  is_compact s :=
assume f hfn hfs, classical.by_contradiction $ assume : ¬ (∃x∈s, cluster_pt x f),
  have hf : ∀x∈s, 𝓝 x ⊓ f = ⊥,
    by simpa only [cluster_pt, not_exists, not_not, ne_bot],
  have ¬ ∃x∈s, ∀t∈f.sets, x ∈ closure t,
    from assume ⟨x, hxs, hx⟩,
    have ∅ ∈ 𝓝 x ⊓ f, by rw [empty_in_sets_eq_bot, hf x hxs],
    let ⟨t₁, ht₁, t₂, ht₂, ht⟩ := by rw [mem_inf_sets] at this; exact this in
    have ∅ ∈ 𝓝[t₂] x,
      from (𝓝[t₂] x).sets_of_superset (inter_mem_inf_sets ht₁ (subset.refl t₂)) ht,
    have 𝓝[t₂] x = ⊥,
      by rwa [empty_in_sets_eq_bot] at this,
    by simp only [closure_eq_cluster_pts] at hx; exact hx t₂ ht₂ this,
  let ⟨t, ht⟩ := h (λ i : f.sets, closure i.1) (λ i, is_closed_closure)
    (by simpa [eq_empty_iff_forall_not_mem, not_exists]) in
  have (⋂i∈t, subtype.val i) ∈ f,
    from t.Inter_mem_sets.2 $ assume i hi, i.2,
  have s ∩ (⋂i∈t, subtype.val i) ∈ f,
    from inter_mem_sets (le_principal_iff.1 hfs) this,
  have ∅ ∈ f,
    from mem_sets_of_superset this $ assume x ⟨hxs, hx⟩,
    let ⟨i, hit, hxi⟩ := (show ∃i ∈ t, x ∉ closure (subtype.val i),
      by { rw [eq_empty_iff_forall_not_mem] at ht, simpa [hxs, not_forall] using ht x }) in
    have x ∈ closure i.val, from subset_closure (mem_bInter_iff.mp hx i hit),
    show false, from hxi this,
  hfn $ by rwa [empty_in_sets_eq_bot] at this

/-- A set `s` is compact if for every open cover of `s`, there exists a finite subcover. -/
lemma compact_of_finite_subcover
  (h : Π {ι : Type u} (U : ι → (set α)), (∀ i, is_open (U i)) →
    s ⊆ (⋃ i, U i) → (∃ (t : finset ι), s ⊆ (⋃ i ∈ t, U i))) :
  is_compact s :=
compact_of_finite_subfamily_closed $
  assume ι Z hZc hsZ,
  let ⟨t, ht⟩ := h (λ i, (Z i)ᶜ) (assume i, is_open_compl_iff.mpr $ hZc i)
    (by simpa only [subset_def, not_forall, eq_empty_iff_forall_not_mem, mem_Union,
      exists_prop, mem_inter_eq, not_and, iff_self, mem_Inter, mem_compl_eq] using hsZ)
      in
  ⟨t, by simpa only [subset_def, not_forall, eq_empty_iff_forall_not_mem, mem_Union,
      exists_prop, mem_inter_eq, not_and, iff_self, mem_Inter, mem_compl_eq] using ht⟩

/-- A set `s` is compact if and only if
for every open cover of `s`, there exists a finite subcover. -/
lemma compact_iff_finite_subcover :
  is_compact s ↔ (Π {ι : Type u} (U : ι → (set α)), (∀ i, is_open (U i)) →
    s ⊆ (⋃ i, U i) → (∃ (t : finset ι), s ⊆ (⋃ i ∈ t, U i))) :=
⟨assume hs ι, hs.elim_finite_subcover, compact_of_finite_subcover⟩

/-- A set `s` is compact if and only if
for every family of closed sets whose intersection avoids `s`,
there exists a finite subfamily whose intersection avoids `s`. -/
theorem compact_iff_finite_subfamily_closed :
  is_compact s ↔ (Π {ι : Type u} (Z : ι → (set α)), (∀ i, is_closed (Z i)) →
    s ∩ (⋂ i, Z i) = ∅ → (∃ (t : finset ι), s ∩ (⋂ i ∈ t, Z i) = ∅)) :=
⟨assume hs ι, hs.elim_finite_subfamily_closed, compact_of_finite_subfamily_closed⟩

@[simp]
lemma compact_empty : is_compact (∅ : set α) :=
assume f hnf hsf, not.elim hnf $
empty_in_sets_eq_bot.1 $ le_principal_iff.1 hsf

@[simp]
lemma compact_singleton {a : α} : is_compact ({a} : set α) :=
λ f hf hfa, ⟨a, rfl, cluster_pt.of_le_nhds'
  (hfa.trans $ by simpa only [principal_singleton] using pure_le_nhds a) hf⟩

lemma set.finite.compact_bUnion {s : set β} {f : β → set α} (hs : finite s)
  (hf : ∀i ∈ s, is_compact (f i)) :
  is_compact (⋃i ∈ s, f i) :=
compact_of_finite_subcover $ assume ι U hUo hsU,
  have ∀i : subtype s, ∃t : finset ι, f i ⊆ (⋃ j ∈ t, U j), from
    assume ⟨i, hi⟩, (hf i hi).elim_finite_subcover _ hUo
      (calc f i ⊆ ⋃i ∈ s, f i : subset_bUnion_of_mem hi
            ... ⊆ ⋃j, U j     : hsU),
  let ⟨finite_subcovers, h⟩ := axiom_of_choice this in
  by haveI : fintype (subtype s) := hs.fintype; exact
  let t := finset.bind finset.univ finite_subcovers in
  have (⋃i ∈ s, f i) ⊆ (⋃ i ∈ t, U i), from bUnion_subset $
    assume i hi, calc
    f i ⊆ (⋃ j ∈ finite_subcovers ⟨i, hi⟩, U j) : (h ⟨i, hi⟩)
    ... ⊆ (⋃ j ∈ t, U j) : bUnion_subset_bUnion_left $
      assume j hj, finset.mem_bind.mpr ⟨_, finset.mem_univ _, hj⟩,
  ⟨t, this⟩

lemma compact_Union {f : β → set α} [fintype β]
  (h : ∀i, is_compact (f i)) : is_compact (⋃i, f i) :=
by rw ← bUnion_univ; exact finite_univ.compact_bUnion (λ i _, h i)

lemma set.finite.is_compact (hs : finite s) : is_compact s :=
bUnion_of_singleton s ▸ hs.compact_bUnion (λ _ _, compact_singleton)

lemma is_compact.union (hs : is_compact s) (ht : is_compact t) : is_compact (s ∪ t) :=
by rw union_eq_Union; exact compact_Union (λ b, by cases b; assumption)

lemma is_compact.insert (hs : is_compact s) (a) : is_compact (insert a s) :=
compact_singleton.union hs

/-- `filter.cocompact` is the filter generated by complements to compact sets. -/
def filter.cocompact (α : Type*) [topological_space α] : filter α :=
⨅ (s : set α) (hs : is_compact s), 𝓟 (sᶜ)
lemma filter.has_basis_cocompact : (filter.cocompact α).has_basis is_compact compl :=
has_basis_binfi_principal'
  (λ s hs t ht, ⟨s ∪ t, hs.union ht, compl_subset_compl.2 (subset_union_left s t),
    compl_subset_compl.2 (subset_union_right s t)⟩)
  ⟨∅, compact_empty⟩

lemma filter.mem_cocompact : s ∈ filter.cocompact α ↔ ∃ t, is_compact t ∧ tᶜ ⊆ s :=
filter.has_basis_cocompact.mem_iff.trans $ exists_congr $ λ t,  exists_prop

lemma filter.mem_cocompact' : s ∈ filter.cocompact α ↔ ∃ t, is_compact t ∧ sᶜ ⊆ t :=
filter.mem_cocompact.trans $ exists_congr $ λ t, and_congr_right $ λ ht, compl_subset_comm

lemma is_compact.compl_mem_cocompact (hs : is_compact s) : sᶜ ∈ filter.cocompact α :=
filter.has_basis_cocompact.mem_of_mem hs

section tube_lemma

variables [topological_space β]

/-- `nhds_contain_boxes s t` means that any open neighborhood of `s × t` in `α × β` includes
a product of an open neighborhood of `s` by an open neighborhood of `t`. -/
def nhds_contain_boxes (s : set α) (t : set β) : Prop :=
∀ (n : set (α × β)) (hn : is_open n) (hp : set.prod s t ⊆ n),
∃ (u : set α) (v : set β), is_open u ∧ is_open v ∧ s ⊆ u ∧ t ⊆ v ∧ set.prod u v ⊆ n

lemma nhds_contain_boxes.symm {s : set α} {t : set β} :
  nhds_contain_boxes s t → nhds_contain_boxes t s :=
assume H n hn hp,
  let ⟨u, v, uo, vo, su, tv, p⟩ :=
    H (prod.swap ⁻¹' n)
      (hn.preimage continuous_swap)
      (by rwa [←image_subset_iff, image_swap_prod]) in
  ⟨v, u, vo, uo, tv, su,
    by rwa [←image_subset_iff, image_swap_prod] at p⟩

lemma nhds_contain_boxes.comm {s : set α} {t : set β} :
  nhds_contain_boxes s t ↔ nhds_contain_boxes t s :=
iff.intro nhds_contain_boxes.symm nhds_contain_boxes.symm

lemma nhds_contain_boxes_of_singleton {x : α} {y : β} :
  nhds_contain_boxes ({x} : set α) ({y} : set β) :=
assume n hn hp,
  let ⟨u, v, uo, vo, xu, yv, hp'⟩ :=
    is_open_prod_iff.mp hn x y (hp $ by simp) in
  ⟨u, v, uo, vo, by simpa, by simpa, hp'⟩

lemma nhds_contain_boxes_of_compact {s : set α} (hs : is_compact s) (t : set β)
  (H : ∀ x ∈ s, nhds_contain_boxes ({x} : set α) t) : nhds_contain_boxes s t :=
assume n hn hp,
have ∀x : subtype s, ∃uv : set α × set β,
     is_open uv.1 ∧ is_open uv.2 ∧ {↑x} ⊆ uv.1 ∧ t ⊆ uv.2 ∧ set.prod uv.1 uv.2 ⊆ n,
  from assume ⟨x, hx⟩,
    have set.prod {x} t ⊆ n, from
      subset.trans (prod_mono (by simpa) (subset.refl _)) hp,
    let ⟨ux,vx,H1⟩ := H x hx n hn this in ⟨⟨ux,vx⟩,H1⟩,
let ⟨uvs, h⟩ := classical.axiom_of_choice this in
have us_cover : s ⊆ ⋃i, (uvs i).1, from
  assume x hx, subset_Union _ ⟨x,hx⟩ (by simpa using (h ⟨x,hx⟩).2.2.1),
let ⟨s0, s0_cover⟩ :=
  hs.elim_finite_subcover _ (λi, (h i).1) us_cover in
let u := ⋃(i ∈ s0), (uvs i).1 in
let v := ⋂(i ∈ s0), (uvs i).2 in
have is_open u, from is_open_bUnion (λi _, (h i).1),
have is_open v, from is_open_bInter s0.finite_to_set (λi _, (h i).2.1),
have t ⊆ v, from subset_bInter (λi _, (h i).2.2.2.1),
have set.prod u v ⊆ n, from assume ⟨x',y'⟩ ⟨hx',hy'⟩,
  have ∃i ∈ s0, x' ∈ (uvs i).1, by simpa using hx',
  let ⟨i,is0,hi⟩ := this in
  (h i).2.2.2.2 ⟨hi, (bInter_subset_of_mem is0 : v ⊆ (uvs i).2) hy'⟩,
⟨u, v, ‹is_open u›, ‹is_open v›, s0_cover, ‹t ⊆ v›, ‹set.prod u v ⊆ n›⟩

/-- If `s` and `t` are compact sets and `n` is an open neighborhood of `s × t`, then there exist
open neighborhoods `u ⊇ s` and `v ⊇ t` such that `u × v ⊆ n`. -/
lemma generalized_tube_lemma {s : set α} (hs : is_compact s) {t : set β} (ht : is_compact t)
  {n : set (α × β)} (hn : is_open n) (hp : set.prod s t ⊆ n) :
  ∃ (u : set α) (v : set β), is_open u ∧ is_open v ∧ s ⊆ u ∧ t ⊆ v ∧ set.prod u v ⊆ n :=
have _, from
  nhds_contain_boxes_of_compact hs t $ assume x _, nhds_contain_boxes.symm $
    nhds_contain_boxes_of_compact ht {x} $ assume y _, nhds_contain_boxes_of_singleton,
this n hn hp

end tube_lemma

/-- Type class for compact spaces. Separation is sometimes included in the definition, especially
in the French literature, but we do not include it here. -/
class compact_space (α : Type*) [topological_space α] : Prop :=
(compact_univ : is_compact (univ : set α))

lemma compact_univ [h : compact_space α] : is_compact (univ : set α) := h.compact_univ

lemma cluster_point_of_compact [compact_space α] (f : filter α) [ne_bot f] :
  ∃ x, cluster_pt x f :=
by simpa using compact_univ (show f ≤ 𝓟 univ, by simp)

theorem compact_space_of_finite_subfamily_closed {α : Type u} [topological_space α]
  (h : Π {ι : Type u} (Z : ι → (set α)), (∀ i, is_closed (Z i)) →
    (⋂ i, Z i) = ∅ → (∃ (t : finset ι), (⋂ i ∈ t, Z i) = ∅)) :
  compact_space α :=
{ compact_univ :=
  begin
    apply compact_of_finite_subfamily_closed,
    intros ι Z, specialize h Z,
    simpa using h
  end }

lemma is_closed.compact [compact_space α] {s : set α} (h : is_closed s) :
  is_compact s :=
compact_of_is_closed_subset compact_univ h (subset_univ _)

variables [topological_space β]

lemma is_compact.image_of_continuous_on {f : α → β} (hs : is_compact s) (hf : continuous_on f s) :
  is_compact (f '' s) :=
begin
  intros l lne ls,
  have : ne_bot (l.comap f ⊓ 𝓟 s) :=
    comap_inf_principal_ne_bot_of_image_mem lne (le_principal_iff.1 ls),
  obtain ⟨a, has, ha⟩ : ∃ a ∈ s, cluster_pt a (l.comap f ⊓ 𝓟 s) := @@hs this inf_le_right,
  use [f a, mem_image_of_mem f has],
  have : tendsto f (𝓝 a ⊓ (comap f l ⊓ 𝓟 s)) (𝓝 (f a) ⊓ l),
  { convert (hf a has).inf (@tendsto_comap _ _ f l) using 1,
    rw nhds_within,
    ac_refl },
  exact @@tendsto.ne_bot _ this ha,
end

lemma is_compact.image {f : α → β} (hs : is_compact s) (hf : continuous f) :
  is_compact (f '' s) :=
hs.image_of_continuous_on hf.continuous_on

lemma compact_range [compact_space α] {f : α → β} (hf : continuous f) :
  is_compact (range f) :=
by rw ← image_univ; exact compact_univ.image hf

/-- If X is is_compact then pr₂ : X × Y → Y is a closed map -/
theorem is_closed_proj_of_compact
  {X : Type*} [topological_space X] [compact_space X]
  {Y : Type*} [topological_space Y]  :
  is_closed_map (prod.snd : X × Y → Y) :=
begin
  set πX := (prod.fst : X × Y → X),
  set πY := (prod.snd : X × Y → Y),
  assume C (hC : is_closed C),
  rw is_closed_iff_cluster_pt at hC ⊢,
  assume y (y_closure : cluster_pt y $ 𝓟 (πY '' C)),
  have : ne_bot (map πX (comap πY (𝓝 y) ⊓ 𝓟 C)),
  { suffices : ne_bot (map πY (comap πY (𝓝 y) ⊓ 𝓟 C)),
      by simpa only [map_ne_bot_iff],
    calc map πY (comap πY (𝓝 y) ⊓ 𝓟 C) =
       𝓝 y ⊓ map πY (𝓟 C) : filter.push_pull' _ _ _
      ... = 𝓝 y ⊓ 𝓟 (πY '' C) : by rw map_principal
      ... ≠ ⊥ : y_closure },
  resetI,
  obtain ⟨x, hx⟩ : ∃ x, cluster_pt x (map πX (comap πY (𝓝 y) ⊓ 𝓟 C)),
    from cluster_point_of_compact _,
  refine ⟨⟨x, y⟩, _, by simp [πY]⟩,
  apply hC,
  rw [cluster_pt, ← filter.map_ne_bot_iff πX],
  calc map πX (𝓝 (x, y) ⊓ 𝓟 C)
      = map πX (comap πX (𝓝 x) ⊓ comap πY (𝓝 y) ⊓ 𝓟 C) : by rw [nhds_prod_eq, filter.prod]
  ... = map πX (comap πY (𝓝 y) ⊓ 𝓟 C ⊓ comap πX (𝓝 x)) : by ac_refl
  ... = map πX (comap πY (𝓝 y) ⊓ 𝓟 C) ⊓ 𝓝 x            : by rw filter.push_pull
  ... = 𝓝 x ⊓ map πX (comap πY (𝓝 y) ⊓ 𝓟 C)            : by rw inf_comm
  ... ≠ ⊥ : hx,
end

lemma embedding.compact_iff_compact_image {f : α → β} (hf : embedding f) :
  is_compact s ↔ is_compact (f '' s) :=
iff.intro (assume h, h.image hf.continuous) $ assume h, begin
  rw compact_iff_ultrafilter_le_nhds at ⊢ h,
  intros u us',
  have : ↑(u.map f) ≤ 𝓟 (f '' s), begin
    rw [ultrafilter.coe_map, map_le_iff_le_comap, comap_principal], convert us',
    exact preimage_image_eq _ hf.inj
  end,
  rcases h (u.map f) this with ⟨_, ⟨a, ha, ⟨⟩⟩, _⟩,
  refine ⟨a, ha, _⟩,
  rwa [hf.induced, nhds_induced, ←map_le_iff_le_comap]
end

lemma compact_iff_compact_in_subtype {p : α → Prop} {s : set {a // p a}} :
  is_compact s ↔ is_compact ((coe : _ → α) '' s) :=
embedding_subtype_coe.compact_iff_compact_image

lemma compact_iff_compact_univ {s : set α} : is_compact s ↔ is_compact (univ : set s) :=
by rw [compact_iff_compact_in_subtype, image_univ, subtype.range_coe]; refl

lemma compact_iff_compact_space {s : set α} : is_compact s ↔ compact_space s :=
compact_iff_compact_univ.trans ⟨λ h, ⟨h⟩, @compact_space.compact_univ _ _⟩

lemma is_compact.prod {s : set α} {t : set β} (hs : is_compact s) (ht : is_compact t) :
  is_compact (set.prod s t) :=
begin
  rw compact_iff_ultrafilter_le_nhds at hs ht ⊢,
  intros f hfs,
  rw le_principal_iff at hfs,
  obtain ⟨a : α, sa : a ∈ s, ha : map prod.fst ↑f ≤ 𝓝 a⟩ :=
    hs (f.map prod.fst) (le_principal_iff.2 $ mem_map.2 $ mem_sets_of_superset hfs (λ x, and.left)),
  obtain ⟨b : β, tb : b ∈ t, hb : map prod.snd ↑f ≤ 𝓝 b⟩ :=
    ht (f.map prod.snd) (le_principal_iff.2 $ mem_map.2 $
      mem_sets_of_superset hfs (λ x, and.right)),
  rw map_le_iff_le_comap at ha hb,
  refine ⟨⟨a, b⟩, ⟨sa, tb⟩, _⟩,
  rw nhds_prod_eq, exact le_inf ha hb
end

/-- Finite topological spaces are compact. -/
@[priority 100] instance fintype.compact_space [fintype α] : compact_space α :=
{ compact_univ := finite_univ.is_compact }

/-- The product of two compact spaces is compact. -/
instance [compact_space α] [compact_space β] : compact_space (α × β) :=
⟨by { rw ← univ_prod_univ, exact compact_univ.prod compact_univ }⟩

/-- The disjoint union of two compact spaces is compact. -/
instance [compact_space α] [compact_space β] : compact_space (α ⊕ β) :=
⟨begin
  rw ← range_inl_union_range_inr,
  exact (compact_range continuous_inl).union (compact_range continuous_inr)
end⟩

section tychonoff
variables {ι : Type*} {π : ι → Type*} [∀i, topological_space (π i)]

/-- Tychonoff's theorem -/
lemma compact_pi_infinite {s : Πi:ι, set (π i)} :
  (∀i, is_compact (s i)) → is_compact {x : Πi:ι, π i | ∀i, x i ∈ s i} :=
begin
  simp only [compact_iff_ultrafilter_le_nhds, nhds_pi, exists_prop, mem_set_of_eq, le_infi_iff,
    le_principal_iff],
  intros h f hfs,
  have : ∀i:ι, ∃a, a∈s i ∧ tendsto (λx:Πi:ι, π i, x i) f (𝓝 a),
  { refine λ i, h i (f.map _) (mem_map.2 _),
    exact mem_sets_of_superset hfs (λ x hx, hx i) },
  choose a ha,
  exact  ⟨a, assume i, (ha i).left, assume i, (ha i).right.le_comap⟩
end

/-- A version of Tychonoff's theorem that uses `set.pi`. -/
lemma compact_univ_pi {s : Πi:ι, set (π i)} (h : ∀i, is_compact (s i)) :
  is_compact (pi univ s) :=
by { convert compact_pi_infinite h, simp only [pi, forall_prop_of_true, mem_univ] }

instance pi.compact [∀i:ι, compact_space (π i)] : compact_space (Πi, π i) :=
⟨begin
  have A : is_compact {x : Πi:ι, π i | ∀i, x i ∈ (univ : set (π i))} :=
    compact_pi_infinite (λi, compact_univ),
  have : {x : Πi:ι, π i | ∀i, x i ∈ (univ : set (π i))} = univ := by ext; simp,
  rwa this at A,
end⟩

end tychonoff

instance quot.compact_space {r : α → α → Prop} [compact_space α] :
  compact_space (quot r) :=
⟨by { rw ← range_quot_mk, exact compact_range continuous_quot_mk }⟩

instance quotient.compact_space {s : setoid α} [compact_space α] :
  compact_space (quotient s) :=
quot.compact_space

/-- There are various definitions of "locally compact space" in the literature, which agree for
Hausdorff spaces but not in general. This one is the precise condition on X needed for the
evaluation `map C(X, Y) × X → Y` to be continuous for all `Y` when `C(X, Y)` is given the
compact-open topology. -/
class locally_compact_space (α : Type*) [topological_space α] : Prop :=
(local_compact_nhds : ∀ (x : α) (n ∈ 𝓝 x), ∃ s ∈ 𝓝 x, s ⊆ n ∧ is_compact s)

/-- A reformulation of the definition of locally compact space: In a locally compact space,
  every open set containing `x` has a compact subset containing `x` in its interior. -/
lemma exists_compact_subset [locally_compact_space α] {x : α} {U : set α}
  (hU : is_open U) (hx : x ∈ U) : ∃ (K : set α), is_compact K ∧ x ∈ interior K ∧ K ⊆ U :=
begin
  rcases locally_compact_space.local_compact_nhds x U _ with ⟨K, h1K, h2K, h3K⟩,
  { refine ⟨K, h3K, _, h2K⟩, rwa [ mem_interior_iff_mem_nhds] },
  rwa [← mem_interior_iff_mem_nhds, hU.interior_eq]
end

lemma ultrafilter.le_nhds_Lim [compact_space α] (F : ultrafilter α) :
  ↑F ≤ 𝓝 (@Lim _ _ (F : filter α).nonempty_of_ne_bot F) :=
begin
  rcases compact_univ.ultrafilter_le_nhds F (by simp) with ⟨x, -, h⟩,
  exact le_nhds_Lim ⟨x,h⟩,
end

variables (α)
/-- A σ-compact space is a space that is the union of a countable collection of compact subspaces.
  Note that a locally compact separable T₂ space need not be σ-compact.
  The sequence can be extracted using `topological_space.compact_covering`. -/
class sigma_compact_space (α : Type*) [topological_space α] : Prop :=
(exists_compact_covering : ∃ K : ℕ → set α, (∀ n, is_compact (K n)) ∧ (⋃ n, K n) = univ)

variables [sigma_compact_space α]
open sigma_compact_space

/-- An arbitrary compact covering of a σ-compact space. -/
def compact_covering : ℕ → set α :=
classical.some exists_compact_covering

lemma is_compact_compact_covering (n : ℕ) : is_compact (compact_covering α n) :=
(classical.some_spec sigma_compact_space.exists_compact_covering).1 n

lemma Union_compact_covering : (⋃ n, compact_covering α n) = univ :=
(classical.some_spec sigma_compact_space.exists_compact_covering).2

end compact

section clopen

/-- A set is clopen if it is both open and closed. -/
def is_clopen (s : set α) : Prop :=
is_open s ∧ is_closed s

theorem is_clopen_union {s t : set α} (hs : is_clopen s) (ht : is_clopen t) : is_clopen (s ∪ t) :=
⟨is_open_union hs.1 ht.1, is_closed_union hs.2 ht.2⟩

theorem is_clopen_inter {s t : set α} (hs : is_clopen s) (ht : is_clopen t) : is_clopen (s ∩ t) :=
⟨is_open_inter hs.1 ht.1, is_closed_inter hs.2 ht.2⟩

@[simp] theorem is_clopen_empty : is_clopen (∅ : set α) :=
⟨is_open_empty, is_closed_empty⟩

@[simp] theorem is_clopen_univ : is_clopen (univ : set α) :=
⟨is_open_univ, is_closed_univ⟩

theorem is_clopen_compl {s : set α} (hs : is_clopen s) : is_clopen sᶜ :=
⟨hs.2, is_closed_compl_iff.2 hs.1⟩

@[simp] theorem is_clopen_compl_iff {s : set α} : is_clopen sᶜ ↔ is_clopen s :=
⟨λ h, compl_compl s ▸ is_clopen_compl h, is_clopen_compl⟩

theorem is_clopen_diff {s t : set α} (hs : is_clopen s) (ht : is_clopen t) : is_clopen (s \ t) :=
is_clopen_inter hs (is_clopen_compl ht)

lemma is_clopen_Inter {β : Type*} [fintype β] {s : β → set α}
  (h : ∀ i, is_clopen (s i)) : is_clopen (⋂ i, s i) :=
⟨(is_open_Inter (forall_and_distrib.1 h).1), (is_closed_Inter (forall_and_distrib.1 h).2)⟩

lemma is_clopen_bInter {β : Type*} {s : finset β} {f : β → set α} (h : ∀i∈s, is_clopen (f i)) :
  is_clopen (⋂i∈s, f i) :=
⟨ is_open_bInter ⟨finset_coe.fintype s⟩ (λ i hi, (h i hi).1),
  by {show is_closed (⋂ (i : β) (H : i ∈ (↑s : set β)), f i), rw bInter_eq_Inter,
    apply is_closed_Inter, rintro ⟨i, hi⟩, exact (h i hi).2}⟩

lemma continuous_on.preimage_clopen_of_clopen {β: Type*} [topological_space β]
  {f : α → β} {s : set α} {t : set β} (hf : continuous_on f s) (hs : is_clopen s)
  (ht : is_clopen t) : is_clopen (s ∩ f⁻¹' t) :=
⟨continuous_on.preimage_open_of_open hf hs.1 ht.1,
  continuous_on.preimage_closed_of_closed hf hs.2 ht.2⟩

/-- The intersection of a disjoint covering by two open sets of a clopen set will be clopen. -/
theorem is_clopen_inter_of_disjoint_cover_clopen {Z a b : set α} (h : is_clopen Z)
  (cover : Z ⊆ a ∪ b) (ha : is_open a) (hb : is_open b) (hab : a ∩ b = ∅) : is_clopen (Z ∩ a) :=
begin
  refine ⟨is_open_inter h.1 ha, _⟩,
  have : is_closed (Z ∩ bᶜ) := is_closed_inter h.2 (is_closed_compl_iff.2 hb),
  convert this using 1,
  apply subset.antisymm,
  { exact inter_subset_inter_right Z (subset_compl_iff_disjoint.2 hab) },
  { rintros x ⟨hx₁, hx₂⟩,
    exact ⟨hx₁, by simpa [not_mem_of_mem_compl hx₂] using cover hx₁⟩ }
end

end clopen

section preirreducible

/-- A preirreducible set `s` is one where there is no non-trivial pair of disjoint opens on `s`. -/
def is_preirreducible (s : set α) : Prop :=
∀ (u v : set α), is_open u → is_open v →
  (s ∩ u).nonempty → (s ∩ v).nonempty → (s ∩ (u ∩ v)).nonempty

/-- An irreducible set `s` is one that is nonempty and
where there is no non-trivial pair of disjoint opens on `s`. -/
def is_irreducible (s : set α) : Prop :=
s.nonempty ∧ is_preirreducible s

lemma is_irreducible.nonempty {s : set α} (h : is_irreducible s) :
  s.nonempty := h.1

lemma is_irreducible.is_preirreducible {s : set α} (h : is_irreducible s) :
  is_preirreducible s := h.2

theorem is_preirreducible_empty : is_preirreducible (∅ : set α) :=
λ _ _ _ _ _ ⟨x, h1, h2⟩, h1.elim

theorem is_irreducible_singleton {x} : is_irreducible ({x} : set α) :=
⟨singleton_nonempty x,
 λ u v _ _ ⟨y, h1, h2⟩ ⟨z, h3, h4⟩, by rw mem_singleton_iff at h1 h3;
 substs y z; exact ⟨x, rfl, h2, h4⟩⟩

theorem is_preirreducible.closure {s : set α} (H : is_preirreducible s) :
  is_preirreducible (closure s) :=
λ u v hu hv ⟨y, hycs, hyu⟩ ⟨z, hzcs, hzv⟩,
let ⟨p, hpu, hps⟩ := mem_closure_iff.1 hycs u hu hyu in
let ⟨q, hqv, hqs⟩ := mem_closure_iff.1 hzcs v hv hzv in
let ⟨r, hrs, hruv⟩ := H u v hu hv ⟨p, hps, hpu⟩ ⟨q, hqs, hqv⟩ in
⟨r, subset_closure hrs, hruv⟩

lemma is_irreducible.closure {s : set α} (h : is_irreducible s) :
  is_irreducible (closure s) :=
⟨h.nonempty.closure, h.is_preirreducible.closure⟩

theorem exists_preirreducible (s : set α) (H : is_preirreducible s) :
  ∃ t : set α, is_preirreducible t ∧ s ⊆ t ∧ ∀ u, is_preirreducible u → t ⊆ u → u = t :=
let ⟨m, hm, hsm, hmm⟩ := zorn.zorn_subset₀ {t : set α | is_preirreducible t}
  (λ c hc hcc hcn, let ⟨t, htc⟩ := hcn in
    ⟨⋃₀ c, λ u v hu hv ⟨y, hy, hyu⟩ ⟨z, hz, hzv⟩,
      let ⟨p, hpc, hyp⟩ := mem_sUnion.1 hy,
          ⟨q, hqc, hzq⟩ := mem_sUnion.1 hz in
      or.cases_on (zorn.chain.total hcc hpc hqc)
        (assume hpq : p ⊆ q, let ⟨x, hxp, hxuv⟩ := hc hqc u v hu hv
            ⟨y, hpq hyp, hyu⟩ ⟨z, hzq, hzv⟩ in
          ⟨x, mem_sUnion_of_mem hxp hqc, hxuv⟩)
        (assume hqp : q ⊆ p, let ⟨x, hxp, hxuv⟩ := hc hpc u v hu hv
            ⟨y, hyp, hyu⟩ ⟨z, hqp hzq, hzv⟩ in
          ⟨x, mem_sUnion_of_mem hxp hpc, hxuv⟩),
    λ x hxc, subset_sUnion_of_mem hxc⟩) s H in
⟨m, hm, hsm, λ u hu hmu, hmm _ hu hmu⟩

/-- A maximal irreducible set that contains a given point. -/
def irreducible_component (x : α) : set α :=
classical.some (exists_preirreducible {x} is_irreducible_singleton.is_preirreducible)

lemma irreducible_component_property (x : α) :
  is_preirreducible (irreducible_component x) ∧ {x} ⊆ (irreducible_component x) ∧
  ∀ u, is_preirreducible u → (irreducible_component x) ⊆ u → u = (irreducible_component x) :=
classical.some_spec (exists_preirreducible {x} is_irreducible_singleton.is_preirreducible)

theorem mem_irreducible_component {x : α} : x ∈ irreducible_component x :=
singleton_subset_iff.1 (irreducible_component_property x).2.1

theorem is_irreducible_irreducible_component {x : α} : is_irreducible (irreducible_component x) :=
⟨⟨x, mem_irreducible_component⟩, (irreducible_component_property x).1⟩

theorem eq_irreducible_component {x : α} :
  ∀ {s : set α}, is_preirreducible s → irreducible_component x ⊆ s → s = irreducible_component x :=
(irreducible_component_property x).2.2

theorem is_closed_irreducible_component {x : α} :
  is_closed (irreducible_component x) :=
closure_eq_iff_is_closed.1 $ eq_irreducible_component
  is_irreducible_irreducible_component.is_preirreducible.closure
  subset_closure

/-- A preirreducible space is one where there is no non-trivial pair of disjoint opens. -/
class preirreducible_space (α : Type u) [topological_space α] : Prop :=
(is_preirreducible_univ [] : is_preirreducible (univ : set α))

/-- An irreducible space is one that is nonempty
and where there is no non-trivial pair of disjoint opens. -/
class irreducible_space (α : Type u) [topological_space α] extends preirreducible_space α : Prop :=
(to_nonempty [] : nonempty α)

attribute [instance, priority 50] irreducible_space.to_nonempty -- see Note [lower instance priority]

theorem nonempty_preirreducible_inter [preirreducible_space α] {s t : set α} :
  is_open s → is_open t → s.nonempty → t.nonempty → (s ∩ t).nonempty :=
by simpa only [univ_inter, univ_subset_iff] using
  @preirreducible_space.is_preirreducible_univ α _ _ s t

theorem is_preirreducible.image [topological_space β] {s : set α} (H : is_preirreducible s)
  (f : α → β) (hf : continuous_on f s) : is_preirreducible (f '' s) :=
begin
  rintros u v hu hv ⟨_, ⟨⟨x, hx, rfl⟩, hxu⟩⟩ ⟨_, ⟨⟨y, hy, rfl⟩, hyv⟩⟩,
  rw ← mem_preimage at hxu hyv,
  rcases continuous_on_iff'.1 hf u hu with ⟨u', hu', u'_eq⟩,
  rcases continuous_on_iff'.1 hf v hv with ⟨v', hv', v'_eq⟩,
  have := H u' v' hu' hv',
  rw [inter_comm s u', ← u'_eq] at this,
  rw [inter_comm s v', ← v'_eq] at this,
  rcases this ⟨x, hxu, hx⟩ ⟨y, hyv, hy⟩ with ⟨z, hzs, hzu', hzv'⟩,
  refine ⟨f z, mem_image_of_mem f hzs, _, _⟩,
  all_goals
  { rw ← mem_preimage,
    apply mem_of_mem_inter_left,
    show z ∈ _ ∩ s,
    simp [*] }
end

theorem is_irreducible.image [topological_space β] {s : set α} (H : is_irreducible s)
  (f : α → β) (hf : continuous_on f s) : is_irreducible (f '' s) :=
⟨nonempty_image_iff.mpr H.nonempty, H.is_preirreducible.image f hf⟩

lemma subtype.preirreducible_space {s : set α} (h : is_preirreducible s) :
  preirreducible_space s :=
{ is_preirreducible_univ :=
  begin
    intros u v hu hv hsu hsv,
    rw is_open_induced_iff at hu hv,
    rcases hu with ⟨u, hu, rfl⟩,
    rcases hv with ⟨v, hv, rfl⟩,
    rcases hsu with ⟨⟨x, hxs⟩, hxs', hxu⟩,
    rcases hsv with ⟨⟨y, hys⟩, hys', hyv⟩,
    rcases h u v hu hv ⟨x, hxs, hxu⟩ ⟨y, hys, hyv⟩ with ⟨z, hzs, ⟨hzu, hzv⟩⟩,
    exact ⟨⟨z, hzs⟩, ⟨set.mem_univ _, ⟨hzu, hzv⟩⟩⟩
  end }

lemma subtype.irreducible_space {s : set α} (h : is_irreducible s) :
  irreducible_space s :=
{ is_preirreducible_univ :=
  (subtype.preirreducible_space h.is_preirreducible).is_preirreducible_univ,
  to_nonempty := h.nonempty.to_subtype }

/-- A set `s` is irreducible if and only if
for every finite collection of open sets all of whose members intersect `s`,
`s` also intersects the intersection of the entire collection
(i.e., there is an element of `s` contained in every member of the collection). -/
lemma is_irreducible_iff_sInter {s : set α} :
  is_irreducible s ↔
  ∀ (U : finset (set α)) (hU : ∀ u ∈ U, is_open u) (H : ∀ u ∈ U, (s ∩ u).nonempty),
  (s ∩ ⋂₀ ↑U).nonempty :=
begin
  split; intro h,
  { intro U, apply finset.induction_on U,
    { intros, simpa using h.nonempty },
    { intros u U hu IH hU H,
      rw [finset.coe_insert, sInter_insert],
      apply h.2,
      { solve_by_elim [finset.mem_insert_self] },
      { apply is_open_sInter (finset.finite_to_set U),
        intros, solve_by_elim [finset.mem_insert_of_mem] },
      { solve_by_elim [finset.mem_insert_self] },
      { apply IH,
        all_goals { intros, solve_by_elim [finset.mem_insert_of_mem] } } } },
  { split,
    { simpa using h ∅ _ _; intro u; simp },
    intros u v hu hv hu' hv',
    simpa using h {u,v} _ _,
    all_goals
    { intro t,
      rw [finset.mem_insert, finset.mem_singleton],
      rintro (rfl|rfl); assumption } }
end

/-- A set is preirreducible if and only if
for every cover by two closed sets, it is contained in one of the two covering sets. -/
lemma is_preirreducible_iff_closed_union_closed {s : set α} :
  is_preirreducible s ↔
  ∀ (z₁ z₂ : set α), is_closed z₁ → is_closed z₂ → s ⊆ z₁ ∪ z₂ → s ⊆ z₁ ∨ s ⊆ z₂ :=
begin
  split,
  all_goals
  { intros h t₁ t₂ ht₁ ht₂,
    specialize h t₁ᶜ t₂ᶜ,
    simp only [is_open_compl_iff, is_closed_compl_iff] at h,
    specialize h ht₁ ht₂ },
  { contrapose!, simp only [not_subset],
    rintro ⟨⟨x, hx, hx'⟩, ⟨y, hy, hy'⟩⟩,
    rcases h ⟨x, hx, hx'⟩ ⟨y, hy, hy'⟩ with ⟨z, hz, hz'⟩,
    rw ← compl_union at hz',
    exact ⟨z, hz, hz'⟩ },
  { rintro ⟨x, hx, hx'⟩ ⟨y, hy, hy'⟩,
    rw ← compl_inter at h,
    delta set.nonempty,
    rw imp_iff_not_or at h,
    contrapose! h,
    split,
    { intros z hz hz', exact h z ⟨hz, hz'⟩ },
    { split; intro H; refine H _ ‹_›; assumption } }
end

/-- A set is irreducible if and only if
for every cover by a finite collection of closed sets,
it is contained in one of the members of the collection. -/
lemma is_irreducible_iff_sUnion_closed {s : set α} :
  is_irreducible s ↔
  ∀ (Z : finset (set α)) (hZ : ∀ z ∈ Z, is_closed z) (H : s ⊆ ⋃₀ ↑Z),
  ∃ z ∈ Z, s ⊆ z :=
begin
  rw [is_irreducible, is_preirreducible_iff_closed_union_closed],
  split; intro h,
  { intro Z, apply finset.induction_on Z,
    { intros, rw [finset.coe_empty, sUnion_empty] at H,
      rcases h.1 with ⟨x, hx⟩,
      exfalso, tauto },
    { intros z Z hz IH hZ H,
      cases h.2 z (⋃₀ ↑Z) _ _ _
        with h' h',
      { exact ⟨z, finset.mem_insert_self _ _, h'⟩ },
      { rcases IH _ h' with ⟨z', hz', hsz'⟩,
        { exact ⟨z', finset.mem_insert_of_mem hz', hsz'⟩ },
        { intros, solve_by_elim [finset.mem_insert_of_mem] } },
      { solve_by_elim [finset.mem_insert_self] },
      { rw sUnion_eq_bUnion,
        apply is_closed_bUnion (finset.finite_to_set Z),
        { intros, solve_by_elim [finset.mem_insert_of_mem] } },
      { simpa using H } } },
  { split,
    { by_contradiction hs,
      simpa using h ∅ _ _,
      { intro z, simp },
      { simpa [set.nonempty] using hs } },
    intros z₁ z₂ hz₁ hz₂ H,
    have := h {z₁, z₂} _ _,
    simp only [exists_prop, finset.mem_insert, finset.mem_singleton] at this,
    { rcases this with ⟨z, rfl|rfl, hz⟩; tauto },
    { intro t,
      rw [finset.mem_insert, finset.mem_singleton],
      rintro (rfl|rfl); assumption },
    { simpa using H } }
end

end preirreducible

section preconnected

/-- A preconnected set is one where there is no non-trivial open partition. -/
def is_preconnected (s : set α) : Prop :=
∀ (u v : set α), is_open u → is_open v → s ⊆ u ∪ v →
  (s ∩ u).nonempty → (s ∩ v).nonempty → (s ∩ (u ∩ v)).nonempty

/-- A connected set is one that is nonempty and where there is no non-trivial open partition. -/
def is_connected (s : set α) : Prop :=
s.nonempty ∧ is_preconnected s

lemma is_connected.nonempty {s : set α} (h : is_connected s) :
  s.nonempty := h.1

lemma is_connected.is_preconnected {s : set α} (h : is_connected s) :
  is_preconnected s := h.2

theorem is_preirreducible.is_preconnected {s : set α} (H : is_preirreducible s) :
  is_preconnected s :=
λ _ _ hu hv _, H _ _ hu hv

theorem is_irreducible.is_connected {s : set α} (H : is_irreducible s) : is_connected s :=
⟨H.nonempty, H.is_preirreducible.is_preconnected⟩

theorem is_preconnected_empty : is_preconnected (∅ : set α) :=
is_preirreducible_empty.is_preconnected

theorem is_connected_singleton {x} : is_connected ({x} : set α) :=
is_irreducible_singleton.is_connected

/-- If any point of a set is joined to a fixed point by a preconnected subset,
then the original set is preconnected as well. -/
theorem is_preconnected_of_forall {s : set α} (x : α)
  (H : ∀ y ∈ s, ∃ t ⊆ s, x ∈ t ∧ y ∈ t ∧ is_preconnected t) :
  is_preconnected s :=
begin
  rintros u v hu hv hs ⟨z, zs, zu⟩ ⟨y, ys, yv⟩,
  have xs : x ∈ s, by { rcases H y ys with ⟨t, ts, xt, yt, ht⟩, exact ts xt },
  wlog xu : x ∈ u := hs xs using [u v y z, v u z y],
  rcases H y ys with ⟨t, ts, xt, yt, ht⟩,
  have := ht u v hu hv(subset.trans ts hs) ⟨x, xt, xu⟩ ⟨y, yt, yv⟩,
  exact this.imp (λ z hz, ⟨ts hz.1, hz.2⟩)
end

/-- If any two points of a set are contained in a preconnected subset,
then the original set is preconnected as well. -/
theorem is_preconnected_of_forall_pair {s : set α}
  (H : ∀ x y ∈ s, ∃ t ⊆ s, x ∈ t ∧ y ∈ t ∧ is_preconnected t) :
  is_preconnected s :=
begin
  rintros u v hu hv hs ⟨x, xs, xu⟩ ⟨y, ys, yv⟩,
  rcases H x y xs ys with ⟨t, ts, xt, yt, ht⟩,
  have := ht u v hu hv(subset.trans ts hs) ⟨x, xt, xu⟩ ⟨y, yt, yv⟩,
  exact this.imp (λ z hz, ⟨ts hz.1, hz.2⟩)
end

/-- A union of a family of preconnected sets with a common point is preconnected as well. -/
theorem is_preconnected_sUnion (x : α) (c : set (set α)) (H1 : ∀ s ∈ c, x ∈ s)
  (H2 : ∀ s ∈ c, is_preconnected s) : is_preconnected (⋃₀ c) :=
begin
  apply is_preconnected_of_forall x,
  rintros y ⟨s, sc, ys⟩,
  exact ⟨s, subset_sUnion_of_mem sc, H1 s sc, ys, H2 s sc⟩
end

theorem is_preconnected.union (x : α) {s t : set α} (H1 : x ∈ s) (H2 : x ∈ t)
  (H3 : is_preconnected s) (H4 : is_preconnected t) : is_preconnected (s ∪ t) :=
sUnion_pair s t ▸ is_preconnected_sUnion x {s, t}
  (by rintro r (rfl | rfl | h); assumption)
  (by rintro r (rfl | rfl | h); assumption)

theorem is_connected.union {s t : set α} (H : (s ∩ t).nonempty)
  (Hs : is_connected s) (Ht : is_connected t) : is_connected (s ∪ t) :=
begin
  rcases H with ⟨x, hx⟩,
  refine ⟨⟨x, mem_union_left t (mem_of_mem_inter_left hx)⟩, _⟩,
  exact is_preconnected.union x (mem_of_mem_inter_left hx) (mem_of_mem_inter_right hx)
    Hs.is_preconnected Ht.is_preconnected
end

theorem is_preconnected.closure {s : set α} (H : is_preconnected s) :
  is_preconnected (closure s) :=
λ u v hu hv hcsuv ⟨y, hycs, hyu⟩ ⟨z, hzcs, hzv⟩,
let ⟨p, hpu, hps⟩ := mem_closure_iff.1 hycs u hu hyu in
let ⟨q, hqv, hqs⟩ := mem_closure_iff.1 hzcs v hv hzv in
let ⟨r, hrs, hruv⟩ := H u v hu hv (subset.trans subset_closure hcsuv) ⟨p, hps, hpu⟩ ⟨q, hqs, hqv⟩ in
⟨r, subset_closure hrs, hruv⟩

theorem is_connected.closure {s : set α} (H : is_connected s) :
  is_connected (closure s) :=
⟨H.nonempty.closure, H.is_preconnected.closure⟩

theorem is_preconnected.image [topological_space β] {s : set α} (H : is_preconnected s)
  (f : α → β) (hf : continuous_on f s) : is_preconnected (f '' s) :=
begin
  -- Unfold/destruct definitions in hypotheses
  rintros u v hu hv huv ⟨_, ⟨x, xs, rfl⟩, xu⟩ ⟨_, ⟨y, ys, rfl⟩, yv⟩,
  rcases continuous_on_iff'.1 hf u hu with ⟨u', hu', u'_eq⟩,
  rcases continuous_on_iff'.1 hf v hv with ⟨v', hv', v'_eq⟩,
  -- Reformulate `huv : f '' s ⊆ u ∪ v` in terms of `u'` and `v'`
  replace huv : s ⊆ u' ∪ v',
  { rw [image_subset_iff, preimage_union] at huv,
    replace huv := subset_inter huv (subset.refl _),
    rw [inter_distrib_right, u'_eq, v'_eq, ← inter_distrib_right] at huv,
    exact (subset_inter_iff.1 huv).1 },
  -- Now `s ⊆ u' ∪ v'`, so we can apply `‹is_preconnected s›`
  obtain ⟨z, hz⟩ : (s ∩ (u' ∩ v')).nonempty,
  { refine H u' v' hu' hv' huv ⟨x, _⟩ ⟨y, _⟩; rw inter_comm,
    exacts [u'_eq ▸ ⟨xu, xs⟩, v'_eq ▸ ⟨yv, ys⟩] },
  rw [← inter_self s, inter_assoc, inter_left_comm s u', ← inter_assoc,
    inter_comm s, inter_comm s, ← u'_eq, ← v'_eq] at hz,
  exact ⟨f z, ⟨z, hz.1.2, rfl⟩, hz.1.1, hz.2.1⟩
end

theorem is_connected.image [topological_space β] {s : set α} (H : is_connected s)
  (f : α → β) (hf : continuous_on f s) : is_connected (f '' s) :=
⟨nonempty_image_iff.mpr H.nonempty, H.is_preconnected.image f hf⟩

theorem is_preconnected_closed_iff {s : set α} :
  is_preconnected s ↔ ∀ t t', is_closed t → is_closed t' → s ⊆ t ∪ t' →
    (s ∩ t).nonempty → (s ∩ t').nonempty → (s ∩ (t ∩ t')).nonempty :=
⟨begin
  rintros h t t' ht ht' htt' ⟨x, xs, xt⟩ ⟨y, ys, yt'⟩,
  by_contradiction h',
  rw [← ne_empty_iff_nonempty, ne.def, not_not, ← subset_compl_iff_disjoint, compl_inter] at h',
  have xt' : x ∉ t', from (h' xs).elim (absurd xt) id,
  have yt : y ∉ t, from (h' ys).elim id (absurd yt'),
  have := ne_empty_iff_nonempty.2 (h tᶜ t'ᶜ (is_open_compl_iff.2 ht)
    (is_open_compl_iff.2 ht') h' ⟨y, ys, yt⟩ ⟨x, xs, xt'⟩),
  rw [ne.def, ← compl_union, ← subset_compl_iff_disjoint, compl_compl] at this,
  contradiction
end,
begin
  rintros h u v hu hv huv ⟨x, xs, xu⟩ ⟨y, ys, yv⟩,
  by_contradiction h',
  rw [← ne_empty_iff_nonempty, ne.def, not_not,
    ← subset_compl_iff_disjoint, compl_inter] at h',
  have xv : x ∉ v, from (h' xs).elim (absurd xu) id,
  have yu : y ∉ u, from (h' ys).elim id (absurd yv),
  have := ne_empty_iff_nonempty.2 (h uᶜ vᶜ (is_closed_compl_iff.2 hu)
    (is_closed_compl_iff.2 hv) h' ⟨y, ys, yu⟩ ⟨x, xs, xv⟩),
  rw [ne.def, ← compl_union, ← subset_compl_iff_disjoint, compl_compl] at this,
  contradiction
end⟩

/-- The connected component of a point is the maximal connected set
that contains this point. -/
def connected_component (x : α) : set α :=
⋃₀ { s : set α | is_preconnected s ∧ x ∈ s }

/-- The connected component of a point inside a set. -/
def connected_component_in (F : set α) (x : F) : set α := coe '' (connected_component x)

theorem mem_connected_component {x : α} : x ∈ connected_component x :=
mem_sUnion_of_mem (mem_singleton x) ⟨is_connected_singleton.is_preconnected, mem_singleton x⟩

theorem is_connected_connected_component {x : α} : is_connected (connected_component x) :=
⟨⟨x, mem_connected_component⟩, is_preconnected_sUnion x _ (λ _, and.right) (λ _, and.left)⟩

theorem subset_connected_component {x : α} {s : set α} (H1 : is_preconnected s) (H2 : x ∈ s) :
  s ⊆ connected_component x :=
λ z hz, mem_sUnion_of_mem hz ⟨H1, H2⟩

theorem is_closed_connected_component {x : α} :
  is_closed (connected_component x) :=
closure_eq_iff_is_closed.1 $ subset.antisymm
  (subset_connected_component
    is_connected_connected_component.closure.is_preconnected
    (subset_closure mem_connected_component))
  subset_closure

theorem irreducible_component_subset_connected_component {x : α} :
  irreducible_component x ⊆ connected_component x :=
subset_connected_component
  is_irreducible_irreducible_component.is_connected.is_preconnected
  mem_irreducible_component

/-- A preconnected space is one where there is no non-trivial open partition. -/
class preconnected_space (α : Type u) [topological_space α] : Prop :=
(is_preconnected_univ : is_preconnected (univ : set α))

export preconnected_space (is_preconnected_univ)

/-- A connected space is a nonempty one where there is no non-trivial open partition. -/
class connected_space (α : Type u) [topological_space α] extends preconnected_space α : Prop :=
(to_nonempty : nonempty α)

attribute [instance, priority 50] connected_space.to_nonempty -- see Note [lower instance priority]

lemma is_connected_range [topological_space β] [connected_space α] {f : α → β} (h : continuous f) :
  is_connected (range f) :=
begin
  inhabit α,
  rw ← image_univ,
  exact ⟨⟨f (default α), mem_image_of_mem _ (mem_univ _)⟩,
         is_preconnected.image is_preconnected_univ _ h.continuous_on⟩
end

lemma connected_space_iff_connected_component :
  connected_space α ↔ ∃ x : α, connected_component x = univ :=
begin
  split,
  { rintros ⟨h, ⟨x⟩⟩,
    exactI ⟨x, eq_univ_of_univ_subset $
      subset_connected_component is_preconnected_univ (mem_univ x)⟩ },
  { rintros ⟨x, h⟩,
    haveI : preconnected_space α := ⟨by {rw ← h, exact is_connected_connected_component.2 }⟩,
    exact ⟨⟨x⟩⟩ }
end

@[priority 100] -- see Note [lower instance priority]
instance preirreducible_space.preconnected_space (α : Type u) [topological_space α]
  [preirreducible_space α] : preconnected_space α :=
⟨(preirreducible_space.is_preirreducible_univ α).is_preconnected⟩

@[priority 100] -- see Note [lower instance priority]
instance irreducible_space.connected_space (α : Type u) [topological_space α]
  [irreducible_space α] : connected_space α :=
{ to_nonempty := irreducible_space.to_nonempty α }

theorem nonempty_inter [preconnected_space α] {s t : set α} :
  is_open s → is_open t → s ∪ t = univ → s.nonempty → t.nonempty → (s ∩ t).nonempty :=
by simpa only [univ_inter, univ_subset_iff] using
  @preconnected_space.is_preconnected_univ α _ _ s t

theorem is_clopen_iff [preconnected_space α] {s : set α} : is_clopen s ↔ s = ∅ ∨ s = univ :=
⟨λ hs, classical.by_contradiction $ λ h,
  have h1 : s ≠ ∅ ∧ sᶜ ≠ ∅, from ⟨mt or.inl h,
    mt (λ h2, or.inr $ (by rw [← compl_compl s, h2, compl_empty] : s = univ)) h⟩,
  let ⟨_, h2, h3⟩ := nonempty_inter hs.1 hs.2 (union_compl_self s)
    (ne_empty_iff_nonempty.1 h1.1) (ne_empty_iff_nonempty.1 h1.2) in
  h3 h2,
by rintro (rfl | rfl); [exact is_clopen_empty, exact is_clopen_univ]⟩

lemma eq_univ_of_nonempty_clopen [preconnected_space α] {s : set α}
  (h : s.nonempty) (h' : is_clopen s) : s = univ :=
by { rw is_clopen_iff at h', finish [h.ne_empty] }

lemma subtype.preconnected_space {s : set α} (h : is_preconnected s) :
  preconnected_space s :=
{ is_preconnected_univ :=
  begin
    intros u v hu hv hs hsu hsv,
    rw is_open_induced_iff at hu hv,
    rcases hu with ⟨u, hu, rfl⟩,
    rcases hv with ⟨v, hv, rfl⟩,
    rcases hsu with ⟨⟨x, hxs⟩, hxs', hxu⟩,
    rcases hsv with ⟨⟨y, hys⟩, hys', hyv⟩,
    rcases h u v hu hv _ ⟨x, hxs, hxu⟩ ⟨y, hys, hyv⟩ with ⟨z, hzs, ⟨hzu, hzv⟩⟩,
    exact ⟨⟨z, hzs⟩, ⟨set.mem_univ _, ⟨hzu, hzv⟩⟩⟩,
    intros z hz,
    rcases hs (mem_univ ⟨z, hz⟩) with hzu|hzv,
    { left, assumption },
    { right, assumption }
  end }

lemma subtype.connected_space {s : set α} (h : is_connected s) :
  connected_space s :=
{ is_preconnected_univ :=
  (subtype.preconnected_space h.is_preconnected).is_preconnected_univ,
  to_nonempty := h.nonempty.to_subtype }

lemma is_preconnected_iff_preconnected_space {s : set α} :
  is_preconnected s ↔ preconnected_space s :=
⟨subtype.preconnected_space,
 begin
   introI,
   simpa using is_preconnected_univ.image (coe : s → α) continuous_subtype_coe.continuous_on
 end⟩

lemma is_connected_iff_connected_space {s : set α} : is_connected s ↔ connected_space s :=
⟨subtype.connected_space,
 λ h, ⟨nonempty_subtype.mp h.2, is_preconnected_iff_preconnected_space.mpr h.1⟩⟩

/-- A set `s` is preconnected if and only if
for every cover by two open sets that are disjoint on `s`,
it is contained in one of the two covering sets. -/
lemma is_preconnected_iff_subset_of_disjoint {s : set α} :
  is_preconnected s ↔
  ∀ (u v : set α) (hu : is_open u) (hv : is_open v) (hs : s ⊆ u ∪ v) (huv : s ∩ (u ∩ v) = ∅),
  s ⊆ u ∨ s ⊆ v :=
begin
  split; intro h,
  { intros u v hu hv hs huv,
    specialize h u v hu hv hs,
    contrapose! huv,
    rw ne_empty_iff_nonempty,
    simp [not_subset] at huv,
    rcases huv with ⟨⟨x, hxs, hxu⟩, ⟨y, hys, hyv⟩⟩,
    have hxv : x ∈ v := or_iff_not_imp_left.mp (hs hxs) hxu,
    have hyu : y ∈ u := or_iff_not_imp_right.mp (hs hys) hyv,
    exact h ⟨y, hys, hyu⟩ ⟨x, hxs, hxv⟩ },
  { intros u v hu hv hs hsu hsv,
    rw ← ne_empty_iff_nonempty,
    intro H,
    specialize h u v hu hv hs H,
    contrapose H,
    apply ne_empty_iff_nonempty.mpr,
    cases h,
    { rcases hsv with ⟨x, hxs, hxv⟩, exact ⟨x, hxs, ⟨h hxs, hxv⟩⟩ },
    { rcases hsu with ⟨x, hxs, hxu⟩, exact ⟨x, hxs, ⟨hxu, h hxs⟩⟩ } }
end

/-- A set `s` is connected if and only if
for every cover by a finite collection of open sets that are pairwise disjoint on `s`,
it is contained in one of the members of the collection. -/
lemma is_connected_iff_sUnion_disjoint_open {s : set α} :
  is_connected s ↔
  ∀ (U : finset (set α)) (H : ∀ (u v : set α), u ∈ U → v ∈ U → (s ∩ (u ∩ v)).nonempty → u = v)
  (hU : ∀ u ∈ U, is_open u) (hs : s ⊆ ⋃₀ ↑U),
  ∃ u ∈ U, s ⊆ u :=
begin
  rw [is_connected, is_preconnected_iff_subset_of_disjoint],
  split; intro h,
  { intro U, apply finset.induction_on U,
    { rcases h.left,
      suffices : s ⊆ ∅ → false, { simpa },
      intro, solve_by_elim },
    { intros u U hu IH hs hU H,
      rw [finset.coe_insert, sUnion_insert] at H,
      cases h.2 u (⋃₀ ↑U) _ _ H _ with hsu hsU,
      { exact ⟨u, finset.mem_insert_self _ _, hsu⟩ },
      { rcases IH _ _ hsU with ⟨v, hvU, hsv⟩,
        { exact ⟨v, finset.mem_insert_of_mem hvU, hsv⟩ },
        { intros, apply hs; solve_by_elim [finset.mem_insert_of_mem] },
        { intros, solve_by_elim [finset.mem_insert_of_mem] } },
      { solve_by_elim [finset.mem_insert_self] },
      { apply is_open_sUnion,
        intros, solve_by_elim [finset.mem_insert_of_mem] },
      { apply eq_empty_of_subset_empty,
        rintro x ⟨hxs, hxu, hxU⟩,
        rw mem_sUnion at hxU,
        rcases hxU with ⟨v, hvU, hxv⟩,
        rcases hs u v (finset.mem_insert_self _ _) (finset.mem_insert_of_mem hvU) _ with rfl,
        { contradiction },
        { exact ⟨x, hxs, hxu, hxv⟩ } } } },
  { split,
    { rw ← ne_empty_iff_nonempty,
      by_contradiction hs, push_neg at hs, subst hs,
      simpa using h ∅ _ _ _; simp },
    intros u v hu hv hs hsuv,
    rcases h {u, v} _ _ _ with ⟨t, ht, ht'⟩,
    { rw [finset.mem_insert, finset.mem_singleton] at ht,
      rcases ht with rfl|rfl; tauto },
    { intros t₁ t₂ ht₁ ht₂ hst,
      rw ← ne_empty_iff_nonempty at hst,
      rw [finset.mem_insert, finset.mem_singleton] at ht₁ ht₂,
      rcases ht₁ with rfl|rfl; rcases ht₂ with rfl|rfl,
      all_goals { refl <|> contradiction <|> skip },
      rw inter_comm t₁ at hst, contradiction },
    { intro t,
      rw [finset.mem_insert, finset.mem_singleton],
      rintro (rfl|rfl); assumption },
    { simpa using hs } }
end

/-- Preconnected sets are either contained in or disjoint to any given clopen set. -/
theorem subset_or_disjoint_of_clopen {α : Type*} [topological_space α] {s t : set α}
  (h : is_preconnected t) (h1 : is_clopen s) : s ∩ t = ∅ ∨ t ⊆ s :=
begin
  by_contradiction h2,
  have h3 : (s ∩ t).nonempty := ne_empty_iff_nonempty.mp (mt or.inl h2),
  have h4 : (t ∩ sᶜ).nonempty,
  { apply inter_compl_nonempty_iff.2,
    push_neg at h2,
    exact h2.2 },
  rw [inter_comm] at h3,
  apply ne_empty_iff_nonempty.2 (h s sᶜ h1.1 (is_open_compl_iff.2 h1.2) _ h3 h4),
  { rw [inter_compl_self, inter_empty] },
  { rw [union_compl_self],
    exact subset_univ t },
end

/-- A set `s` is preconnected if and only if
for every cover by two closed sets that are disjoint on `s`,
it is contained in one of the two covering sets. -/
theorem is_preconnected_iff_subset_of_disjoint_closed {α : Type*} {s : set α}
  [topological_space α] :
  is_preconnected s ↔
    ∀ (u v : set α) (hu : is_closed u) (hv : is_closed v) (hs : s ⊆ u ∪ v) (huv : s ∩ (u ∩ v) = ∅),
      s ⊆ u ∨ s ⊆ v :=
begin
  split; intro h,
  { intros u v hu hv hs huv,
    rw is_preconnected_closed_iff at h,
    specialize h u v hu hv hs,
    contrapose! huv,
    rw ne_empty_iff_nonempty,
    simp [not_subset] at huv,
    rcases huv with ⟨⟨x, hxs, hxu⟩, ⟨y, hys, hyv⟩⟩,
    have hxv : x ∈ v := or_iff_not_imp_left.mp (hs hxs) hxu,
    have hyu : y ∈ u := or_iff_not_imp_right.mp (hs hys) hyv,
    exact h ⟨y, hys, hyu⟩ ⟨x, hxs, hxv⟩ },
  { rw is_preconnected_closed_iff,
    intros u v hu hv hs hsu hsv,
    rw ← ne_empty_iff_nonempty,
    intro H,
    specialize h u v hu hv hs H,
    contrapose H,
    apply ne_empty_iff_nonempty.mpr,
    cases h,
    { rcases hsv with ⟨x, hxs, hxv⟩, exact ⟨x, hxs, ⟨h hxs, hxv⟩⟩ },
    { rcases hsu with ⟨x, hxs, hxu⟩, exact ⟨x, hxs, ⟨hxu, h hxs⟩⟩ } }
end

/-- A closed set `s` is preconnected if and only if
for every cover by two closed sets that are disjoint,
it is contained in one of the two covering sets. -/
theorem is_preconnected_iff_subset_of_fully_disjoint_closed {s : set α} (hs : is_closed s) :
  is_preconnected s ↔
  ∀ (u v : set α) (hu : is_closed u) (hv : is_closed v) (hss : s ⊆ u ∪ v) (huv : u ∩ v = ∅),
  s ⊆ u ∨ s ⊆ v :=
begin
  split,
  { intros h u v hu hv hss huv,
    apply is_preconnected_iff_subset_of_disjoint_closed.1 h u v hu hv hss,
    rw huv,
    exact inter_empty s },
  intro H,
  rw is_preconnected_iff_subset_of_disjoint_closed,
  intros u v hu hv hss huv,
  have H1 := H (u ∩ s) (v ∩ s),
  rw [subset_inter_iff, subset_inter_iff] at H1,
  simp only [subset.refl, and_true] at H1,
  apply H1 (is_closed_inter hu hs) (is_closed_inter hv hs),
  { rw ←inter_distrib_right,
    apply subset_inter_iff.2,
    exact ⟨hss, subset.refl s⟩ },
  { rw [inter_comm v s, inter_assoc, ←inter_assoc s, inter_self s,
        inter_comm, inter_assoc, inter_comm v u, huv] }
end

/-- The connected component of a point is always a subset of the intersection of all its clopen
neighbourhoods. -/
lemma connected_component_subset_Inter_clopen {x : α} :
  connected_component x ⊆ ⋂ Z : {Z : set α // is_clopen Z ∧ x ∈ Z}, Z :=
begin
  apply subset_Inter (λ Z, _),
  cases (subset_or_disjoint_of_clopen (@is_connected_connected_component _ _ x).2 Z.2.1),
  { exfalso,
    apply nonempty.ne_empty
      (nonempty_of_mem (mem_inter (@mem_connected_component _ _ x) Z.2.2)),
    rw inter_comm,
    exact h  },
  exact h,
end

end preconnected

section totally_disconnected

/-- A set is called totally disconnected if all of its connected components are singletons. -/
def is_totally_disconnected (s : set α) : Prop :=
∀ t, t ⊆ s → is_preconnected t → subsingleton t

theorem is_totally_disconnected_empty : is_totally_disconnected (∅ : set α) :=
λ t ht _, ⟨λ ⟨_, h⟩, (ht h).elim⟩

theorem is_totally_disconnected_singleton {x} : is_totally_disconnected ({x} : set α) :=
λ t ht _, ⟨λ ⟨p, hp⟩ ⟨q, hq⟩, subtype.eq $ show p = q,
from (eq_of_mem_singleton (ht hp)).symm ▸ (eq_of_mem_singleton (ht hq)).symm⟩

/-- A space is totally disconnected if all of its connected components are singletons. -/
class totally_disconnected_space (α : Type u) [topological_space α] : Prop :=
(is_totally_disconnected_univ : is_totally_disconnected (univ : set α))

instance pi.totally_disconnected_space {α : Type*} {β : α → Type*}
  [t₂ : Πa, topological_space (β a)] [∀a, totally_disconnected_space (β a)] :
  totally_disconnected_space (Π (a : α), β a) :=
⟨λ t h1 h2, ⟨λ a b, subtype.ext $ funext $ λ x, subtype.mk_eq_mk.1 $
  (totally_disconnected_space.is_totally_disconnected_univ
    ((λ (c : Π (a : α), β a), c x) '' t) (set.subset_univ _)
    (is_preconnected.image h2 _ (continuous.continuous_on (continuous_apply _)))).cases_on
  (λ h3, h3
    ⟨(a.1 x), by {simp only [set.mem_image, subtype.val_eq_coe], use a, split,
      simp only [subtype.coe_prop]}⟩
    ⟨(b.1 x), by {simp only [set.mem_image, subtype.val_eq_coe], use b, split,
      simp only [subtype.coe_prop]}⟩)⟩⟩

instance subtype.totally_disconnected_space {α : Type*} {p : α → Prop} [topological_space α]
  [totally_disconnected_space α] : totally_disconnected_space (subtype p) :=
⟨λ s h1 h2,
  set.subsingleton_of_image subtype.val_injective s (
    totally_disconnected_space.is_totally_disconnected_univ (subtype.val '' s) (set.subset_univ _)
      ((is_preconnected.image h2 _) (continuous.continuous_on (@continuous_subtype_val _ _ p))))⟩

end totally_disconnected

section totally_separated

/-- A set `s` is called totally separated if any two points of this set can be separated
by two disjoint open sets covering `s`. -/
def is_totally_separated (s : set α) : Prop :=
∀ x ∈ s, ∀ y ∈ s, x ≠ y → ∃ u v : set α, is_open u ∧ is_open v ∧
  x ∈ u ∧ y ∈ v ∧ s ⊆ u ∪ v ∧ u ∩ v = ∅

theorem is_totally_separated_empty : is_totally_separated (∅ : set α) :=
λ x, false.elim

theorem is_totally_separated_singleton {x} : is_totally_separated ({x} : set α) :=
λ p hp q hq hpq, (hpq $ (eq_of_mem_singleton hp).symm ▸ (eq_of_mem_singleton hq).symm).elim

theorem is_totally_disconnected_of_is_totally_separated {s : set α}
  (H : is_totally_separated s) : is_totally_disconnected s :=
λ t hts ht, ⟨λ ⟨x, hxt⟩ ⟨y, hyt⟩, subtype.eq $ classical.by_contradiction $
assume hxy : x ≠ y, let ⟨u, v, hu, hv, hxu, hyv, hsuv, huv⟩ := H x (hts hxt) y (hts hyt) hxy in
let ⟨r, hrt, hruv⟩ := ht u v hu hv (subset.trans hts hsuv) ⟨x, hxt, hxu⟩ ⟨y, hyt, hyv⟩ in
(ext_iff.1 huv r).1 hruv⟩

/-- A space is totally separated if any two points can be separated by two disjoint open sets
covering the whole space. -/
class totally_separated_space (α : Type u) [topological_space α] : Prop :=
(is_totally_separated_univ [] : is_totally_separated (univ : set α))

@[priority 100] -- see Note [lower instance priority]
instance totally_separated_space.totally_disconnected_space (α : Type u) [topological_space α]
  [totally_separated_space α] : totally_disconnected_space α :=
⟨is_totally_disconnected_of_is_totally_separated $
  totally_separated_space.is_totally_separated_univ α⟩

@[priority 100] -- see Note [lower instance priority]
instance totally_separated_space.of_discrete
  (α : Type*) [topological_space α] [discrete_topology α] : totally_separated_space α :=
⟨λ a _ b _ h, ⟨{b}ᶜ, {b}, is_open_discrete _, is_open_discrete _, by simpa⟩⟩

end totally_separated