formal/lean/mathlib/topology/sets/compacts.lean

/-
Copyright (c) 2020 Floris van Doorn. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Floris van Doorn, Yaël Dillies
-/
import topology.sets.closeds

/-!
# Compact sets

We define a few types of compact sets in a topological space.

## Main Definitions

For a topological space `α`,
* `compacts α`: The type of compact sets.
* `nonempty_compacts α`: The type of non-empty compact sets.
* `positive_compacts α`: The type of compact sets with non-empty interior.
* `compact_opens α`: The type of compact open sets. This is a central object in the study of
  spectral spaces.
-/

open set

variables {α β : Type*} [topological_space α] [topological_space β]

namespace topological_space

/-! ### Compact sets -/

/-- The type of compact sets of a topological space. -/
structure compacts (α : Type*) [topological_space α] :=
(carrier : set α)
(compact' : is_compact carrier)

namespace compacts
variables {α}

instance : set_like (compacts α) α :=
{ coe := compacts.carrier,
  coe_injective' := λ s t h, by { cases s, cases t, congr' } }

lemma compact (s : compacts α) : is_compact (s : set α) := s.compact'

instance (K : compacts α) : compact_space K := is_compact_iff_compact_space.1 K.compact

instance : can_lift (set α) (compacts α) :=
{ coe := coe,
  cond := is_compact,
  prf := λ K hK, ⟨⟨K, hK⟩, rfl⟩ }

@[ext] protected lemma ext {s t : compacts α} (h : (s : set α) = t) : s = t := set_like.ext' h

@[simp] lemma coe_mk (s : set α) (h) : (mk s h : set α) = s := rfl

@[simp] lemma carrier_eq_coe (s : compacts α) : s.carrier = s := rfl

instance : has_sup (compacts α) := ⟨λ s t, ⟨s ∪ t, s.compact.union t.compact⟩⟩
instance [t2_space α] : has_inf (compacts α) := ⟨λ s t, ⟨s ∩ t, s.compact.inter t.compact⟩⟩
instance [compact_space α] : has_top (compacts α) := ⟨⟨univ, compact_univ⟩⟩
instance : has_bot (compacts α) := ⟨⟨∅, is_compact_empty⟩⟩

instance : semilattice_sup (compacts α) := set_like.coe_injective.semilattice_sup _ (λ _ _, rfl)

instance [t2_space α] : distrib_lattice (compacts α) :=
set_like.coe_injective.distrib_lattice _ (λ _ _, rfl) (λ _ _, rfl)

instance : order_bot (compacts α) := order_bot.lift (coe : _ → set α) (λ _ _, id) rfl

instance [compact_space α] : bounded_order (compacts α) :=
bounded_order.lift (coe : _ → set α) (λ _ _, id) rfl rfl

/-- The type of compact sets is inhabited, with default element the empty set. -/
instance : inhabited (compacts α) := ⟨⊥⟩

@[simp] lemma coe_sup (s t : compacts α) : (↑(s ⊔ t) : set α) = s ∪ t := rfl
@[simp] lemma coe_inf [t2_space α] (s t : compacts α) : (↑(s ⊓ t) : set α) = s ∩ t := rfl
@[simp] lemma coe_top [compact_space α] : (↑(⊤ : compacts α) : set α) = univ := rfl
@[simp] lemma coe_bot : (↑(⊥ : compacts α) : set α) = ∅ := rfl

@[simp] lemma coe_finset_sup {ι : Type*} {s : finset ι} {f : ι → compacts α} :
  (↑(s.sup f) : set α) = s.sup (λ i, f i) :=
begin
  classical,
  refine finset.induction_on s rfl (λ a s _ h, _),
  simp_rw [finset.sup_insert, coe_sup, sup_eq_union],
  congr',
end

/-- The image of a compact set under a continuous function. -/
protected def map (f : α → β) (hf : continuous f) (K : compacts α) : compacts β :=
⟨f '' K.1, K.2.image hf⟩

@[simp] lemma coe_map {f : α → β} (hf : continuous f) (s : compacts α) :
  (s.map f hf : set β) = f '' s := rfl

/-- A homeomorphism induces an equivalence on compact sets, by taking the image. -/
@[simp] protected def equiv (f : α ≃ₜ β) : compacts α ≃ compacts β :=
{ to_fun := compacts.map f f.continuous,
  inv_fun := compacts.map _ f.symm.continuous,
  left_inv := λ s, by { ext1, simp only [coe_map, ← image_comp, f.symm_comp_self, image_id] },
  right_inv := λ s, by { ext1, simp only [coe_map, ← image_comp, f.self_comp_symm, image_id] } }

/-- The image of a compact set under a homeomorphism can also be expressed as a preimage. -/
lemma equiv_to_fun_val (f : α ≃ₜ β) (K : compacts α) :
  (compacts.equiv f K).1 = f.symm ⁻¹' K.1 :=
congr_fun (image_eq_preimage_of_inverse f.left_inv f.right_inv) K.1

/-- The product of two `compacts`, as a `compacts` in the product space. -/
protected def prod (K : compacts α) (L : compacts β) : compacts (α × β) :=
{ carrier := K ×ˢ L,
  compact' := is_compact.prod K.2 L.2 }

@[simp] lemma coe_prod (K : compacts α) (L : compacts β) : (K.prod L : set (α × β)) = K ×ˢ L := rfl

end compacts

/-! ### Nonempty compact sets -/

/-- The type of nonempty compact sets of a topological space. -/
structure nonempty_compacts (α : Type*) [topological_space α] extends compacts α :=
(nonempty' : carrier.nonempty)

namespace nonempty_compacts

instance : set_like (nonempty_compacts α) α :=
{ coe := λ s, s.carrier,
  coe_injective' := λ s t h, by { obtain ⟨⟨_, _⟩, _⟩ := s, obtain ⟨⟨_, _⟩, _⟩ := t, congr' } }

lemma compact (s : nonempty_compacts α) : is_compact (s : set α) := s.compact'
protected lemma nonempty (s : nonempty_compacts α) : (s : set α).nonempty := s.nonempty'

/-- Reinterpret a nonempty compact as a closed set. -/
def to_closeds [t2_space α] (s : nonempty_compacts α) : closeds α := ⟨s, s.compact.is_closed⟩

@[ext] protected lemma ext {s t : nonempty_compacts α} (h : (s : set α) = t) : s = t :=
set_like.ext' h

@[simp] lemma coe_mk (s : compacts α) (h) : (mk s h : set α) = s := rfl

@[simp] lemma carrier_eq_coe (s : nonempty_compacts α) : s.carrier = s := rfl

instance : has_sup (nonempty_compacts α) :=
⟨λ s t, ⟨s.to_compacts ⊔ t.to_compacts, s.nonempty.mono $ subset_union_left _ _⟩⟩
instance [compact_space α] [nonempty α] : has_top (nonempty_compacts α) := ⟨⟨⊤, univ_nonempty⟩⟩

instance : semilattice_sup (nonempty_compacts α) :=
set_like.coe_injective.semilattice_sup _ (λ _ _, rfl)

instance [compact_space α] [nonempty α] : order_top (nonempty_compacts α) :=
order_top.lift (coe : _ → set α) (λ _ _, id) rfl

@[simp] lemma coe_sup (s t : nonempty_compacts α) : (↑(s ⊔ t) : set α) = s ∪ t := rfl
@[simp] lemma coe_top [compact_space α] [nonempty α] :
  (↑(⊤ : nonempty_compacts α) : set α) = univ := rfl

/-- In an inhabited space, the type of nonempty compact subsets is also inhabited, with
default element the singleton set containing the default element. -/
instance [inhabited α] : inhabited (nonempty_compacts α) :=
⟨{ carrier := {default}, compact' := is_compact_singleton, nonempty' := singleton_nonempty _ }⟩

instance to_compact_space {s : nonempty_compacts α} : compact_space s :=
is_compact_iff_compact_space.1 s.compact

instance to_nonempty {s : nonempty_compacts α} : nonempty s := s.nonempty.to_subtype

/-- The product of two `nonempty_compacts`, as a `nonempty_compacts` in the product space. -/
protected def prod (K : nonempty_compacts α) (L : nonempty_compacts β) :
  nonempty_compacts (α × β) :=
{ nonempty' := K.nonempty.prod L.nonempty,
  .. K.to_compacts.prod L.to_compacts }

@[simp] lemma coe_prod (K : nonempty_compacts α) (L : nonempty_compacts β) :
  (K.prod L : set (α × β)) = K ×ˢ L := rfl

end nonempty_compacts

/-! ### Positive compact sets -/

/-- The type of compact sets with nonempty interior of a topological space.
See also `compacts` and `nonempty_compacts`. -/
structure positive_compacts (α : Type*) [topological_space α] extends compacts α :=
(interior_nonempty' : (interior carrier).nonempty)

namespace positive_compacts

instance : set_like (positive_compacts α) α :=
{ coe := λ s, s.carrier,
  coe_injective' := λ s t h, by { obtain ⟨⟨_, _⟩, _⟩ := s, obtain ⟨⟨_, _⟩, _⟩ := t, congr' } }

lemma compact (s : positive_compacts α) : is_compact (s : set α) := s.compact'
lemma interior_nonempty (s : positive_compacts α) : (interior (s : set α)).nonempty :=
s.interior_nonempty'

protected lemma nonempty (s : positive_compacts α) : (s : set α).nonempty :=
s.interior_nonempty.mono interior_subset

/-- Reinterpret a positive compact as a nonempty compact. -/
def to_nonempty_compacts (s : positive_compacts α) : nonempty_compacts α :=
⟨s.to_compacts, s.nonempty⟩

@[ext] protected lemma ext {s t : positive_compacts α} (h : (s : set α) = t) : s = t :=
set_like.ext' h

@[simp] lemma coe_mk (s : compacts α) (h) : (mk s h : set α) = s := rfl

@[simp] lemma carrier_eq_coe (s : positive_compacts α) : s.carrier = s := rfl

instance : has_sup (positive_compacts α) :=
⟨λ s t, ⟨s.to_compacts ⊔ t.to_compacts,
  s.interior_nonempty.mono $ interior_mono $ subset_union_left _ _⟩⟩

instance [compact_space α] [nonempty α] : has_top (positive_compacts α) :=
⟨⟨⊤, interior_univ.symm.subst univ_nonempty⟩⟩

instance : semilattice_sup (positive_compacts α) :=
set_like.coe_injective.semilattice_sup _ (λ _ _, rfl)

instance [compact_space α] [nonempty α] : order_top (positive_compacts α) :=
order_top.lift (coe : _ → set α) (λ _ _, id) rfl

@[simp] lemma coe_sup (s t : positive_compacts α) : (↑(s ⊔ t) : set α) = s ∪ t := rfl
@[simp] lemma coe_top [compact_space α] [nonempty α] :
  (↑(⊤ : positive_compacts α) : set α) = univ := rfl

lemma _root_.exists_positive_compacts_subset [locally_compact_space α] {U : set α} (ho : is_open U)
  (hn : U.nonempty) : ∃ K : positive_compacts α, ↑K ⊆ U :=
let ⟨x, hx⟩ := hn, ⟨K, hKc, hxK, hKU⟩ := exists_compact_subset ho hx in ⟨⟨⟨K, hKc⟩, ⟨x, hxK⟩⟩, hKU⟩

instance [compact_space α] [nonempty α] : inhabited (positive_compacts α) := ⟨⊤⟩

/-- In a nonempty locally compact space, there exists a compact set with nonempty interior. -/
instance nonempty' [locally_compact_space α] [nonempty α] : nonempty (positive_compacts α) :=
nonempty_of_exists $ exists_positive_compacts_subset is_open_univ univ_nonempty

/-- The product of two `positive_compacts`, as a `positive_compacts` in the product space. -/
protected def prod (K : positive_compacts α) (L : positive_compacts β) :
  positive_compacts (α × β) :=
{ interior_nonempty' :=
  begin
    simp only [compacts.carrier_eq_coe, compacts.coe_prod, interior_prod_eq],
    exact K.interior_nonempty.prod L.interior_nonempty,
  end,
  .. K.to_compacts.prod L.to_compacts }

@[simp] lemma coe_prod (K : positive_compacts α) (L : positive_compacts β) :
  (K.prod L : set (α × β)) = K ×ˢ L := rfl

end positive_compacts

/-! ### Compact open sets -/

/-- The type of compact open sets of a topological space. This is useful in non Hausdorff contexts,
in particular spectral spaces. -/
structure compact_opens (α : Type*) [topological_space α] extends compacts α :=
(open' : is_open carrier)

namespace compact_opens

instance : set_like (compact_opens α) α :=
{ coe := λ s, s.carrier,
  coe_injective' := λ s t h, by { obtain ⟨⟨_, _⟩, _⟩ := s, obtain ⟨⟨_, _⟩, _⟩ := t, congr' } }

lemma compact (s : compact_opens α) : is_compact (s : set α) := s.compact'
lemma «open» (s : compact_opens α) : is_open (s : set α) := s.open'

/-- Reinterpret a compact open as an open. -/
@[simps] def to_opens (s : compact_opens α) : opens α := ⟨s, s.open⟩

/-- Reinterpret a compact open as a clopen. -/
@[simps] def to_clopens [t2_space α] (s : compact_opens α) : clopens α :=
⟨s, s.open, s.compact.is_closed⟩

@[ext] protected lemma ext {s t : compact_opens α} (h : (s : set α) = t) : s = t := set_like.ext' h

@[simp] lemma coe_mk (s : compacts α) (h) : (mk s h : set α) = s := rfl

instance : has_sup (compact_opens α) :=
⟨λ s t, ⟨s.to_compacts ⊔ t.to_compacts, s.open.union t.open⟩⟩
instance [t2_space α] : has_inf (compact_opens α) :=
⟨λ s t, ⟨s.to_compacts ⊓ t.to_compacts, s.open.inter t.open⟩⟩
instance [compact_space α] : has_top (compact_opens α) := ⟨⟨⊤, is_open_univ⟩⟩
instance : has_bot (compact_opens α) := ⟨⟨⊥, is_open_empty⟩⟩
instance [t2_space α] : has_sdiff (compact_opens α) :=
⟨λ s t, ⟨⟨s \ t, s.compact.diff t.open⟩, s.open.sdiff t.compact.is_closed⟩⟩
instance [t2_space α] [compact_space α] : has_compl (compact_opens α) :=
⟨λ s, ⟨⟨sᶜ, s.open.is_closed_compl.is_compact⟩, s.compact.is_closed.is_open_compl⟩⟩

instance : semilattice_sup (compact_opens α) :=
set_like.coe_injective.semilattice_sup _ (λ _ _, rfl)

instance : order_bot (compact_opens α) := order_bot.lift (coe : _ → set α) (λ _ _, id) rfl

instance [t2_space α] : generalized_boolean_algebra (compact_opens α) :=
set_like.coe_injective.generalized_boolean_algebra _ (λ _ _, rfl) (λ _ _, rfl) rfl (λ _ _, rfl)

instance [compact_space α] : bounded_order (compact_opens α) :=
bounded_order.lift (coe : _ → set α) (λ _ _, id) rfl rfl

instance [t2_space α] [compact_space α] : boolean_algebra (compact_opens α) :=
set_like.coe_injective.boolean_algebra _ (λ _ _, rfl) (λ _ _, rfl) rfl rfl (λ _, rfl) (λ _ _, rfl)

@[simp] lemma coe_sup (s t : compact_opens α) : (↑(s ⊔ t) : set α) = s ∪ t := rfl
@[simp] lemma coe_inf [t2_space α] (s t : compact_opens α) : (↑(s ⊓ t) : set α) = s ∩ t := rfl
@[simp] lemma coe_top [compact_space α] : (↑(⊤ : compact_opens α) : set α) = univ := rfl
@[simp] lemma coe_bot : (↑(⊥ : compact_opens α) : set α) = ∅ := rfl
@[simp] lemma coe_sdiff [t2_space α] (s t : compact_opens α) : (↑(s \ t) : set α) = s \ t := rfl
@[simp] lemma coe_compl [t2_space α] [compact_space α] (s : compact_opens α) : (↑sᶜ : set α) = sᶜ :=
rfl

instance : inhabited (compact_opens α) := ⟨⊥⟩

/-- The image of a compact open under a continuous open map. -/
@[simps] def map (f : α → β) (hf : continuous f) (hf' : is_open_map f) (s : compact_opens α) :
  compact_opens β :=
⟨s.to_compacts.map f hf, hf' _ s.open⟩

@[simp] lemma coe_map {f : α → β} (hf : continuous f) (hf' : is_open_map f) (s : compact_opens α) :
  (s.map f hf hf' : set β) = f '' s := rfl

/-- The product of two `compact_opens`, as a `compact_opens` in the product space. -/
protected def prod (K : compact_opens α) (L : compact_opens β) :
  compact_opens (α × β) :=
{ open' := K.open.prod L.open,
  .. K.to_compacts.prod L.to_compacts }

@[simp] lemma coe_prod (K : compact_opens α) (L : compact_opens β) :
  (K.prod L : set (α × β)) = K ×ˢ L := rfl

end compact_opens
end topological_space