mathlib documentation

topology.separation

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@[class]
structure t0_space (α : Type u) [topological_space α] :
Prop

A T₀ space, also known as a Kolmogorov space, is a topological space where for every pair x ≠ y, there is an open set containing one but not the other.

Instances
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theorem exists_open_singleton_of_open_finset {α : Type u} [topological_space α] [t0_space α] (s : finset α) :
s.nonempty → is_open ↑s → (∃ (x : α) (H : x ∈ s), is_open {x})

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theorem exists_open_singleton_of_fintype {α : Type u} [topological_space α] [t0_space α] [f : fintype α] [ha : nonempty α] :
∃ (x : α), is_open {x}

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@[instance]
def subtype.t0_space {α : Type u} [topological_space α] [t0_space α] {p : α → Prop} :
t0_space (subtype p)

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@[class]
structure t1_space (α : Type u) [topological_space α] :
Prop

A T₁ space, also known as a Fréchet space, is a topological space where every singleton set is closed. Equivalently, for every pair x ≠ y, there is an open set containing x and not y.

Instances
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theorem is_closed_singleton {α : Type u} [topological_space α] [t1_space α] {x : α} :
is_closed {x}

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theorem is_open_ne {α : Type u} [topological_space α] [t1_space α] {x : α} :
is_open {y : α | y ≠ x}

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@[instance]
def subtype.t1_space {α : Type u} [topological_space α] [t1_space α] {p : α → Prop} :
t1_space (subtype p)

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@[instance]
def t1_space.t0_space {α : Type u} [topological_space α] [t1_space α] :
t0_space α

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theorem compl_singleton_mem_nhds {α : Type u} [topological_space α] [t1_space α] {x y : α} :
y ≠ x → {x}ᶜ ∈ 𝓝 y

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@[simp]
theorem closure_singleton {α : Type u} [topological_space α] [t1_space α] {a : α} :
closure {a} = {a}

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theorem is_closed_map_const {α : Type u_1} {β : Type u_2} [topological_space α] [topological_space β] [t1_space β] {y : β} :
is_closed_map (function.const α y)

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@[class]
structure t2_space (α : Type u) [topological_space α] :
Prop

A T₂ space, also known as a Hausdorff space, is one in which for every x ≠ y there exists disjoint open sets around x and y. This is the most widely used of the separation axioms.

Instances
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theorem t2_separation {α : Type u} [topological_space α] [t2_space α] {x y : α} :
x ≠ y → (∃ (u v : set α), is_open u ∧ is_open v ∧ x ∈ u ∧ y ∈ v ∧ u ∩ v = ∅)

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@[instance]
def t2_space.t1_space {α : Type u} [topological_space α] [t2_space α] :
t1_space α

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theorem eq_of_nhds_ne_bot {α : Type u} [topological_space α] [ht : t2_space α] {x y : α} :
(𝓝 x ⊓ 𝓝 y).ne_bot → x = y

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theorem t2_iff_nhds {α : Type u} [topological_space α] :
t2_space α ↔ ∀ {x y : α}, (𝓝 x ⊓ 𝓝 y).ne_bot → x = y

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theorem t2_iff_ultrafilter {α : Type u} [topological_space α] :
t2_space α ↔ ∀ (f : filter α) {x y : α}, f.is_ultrafilter → f ≤ 𝓝 x → f ≤ 𝓝 y → x = y

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theorem is_closed_diagonal {α : Type u} [topological_space α] [t2_space α] :
is_closed (set.diagonal α)

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theorem t2_iff_is_closed_diagonal {α : Type u} [topological_space α] :
t2_space α ↔ is_closed (set.diagonal α)

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@[simp]
theorem nhds_eq_nhds_iff {α : Type u} [topological_space α] {a b : α} [t2_space α] :
𝓝 a = 𝓝 b ↔ a = b

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@[simp]
theorem nhds_le_nhds_iff {α : Type u} [topological_space α] {a b : α} [t2_space α] :
𝓝 a ≤ 𝓝 b ↔ a = b

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theorem tendsto_nhds_unique {α : Type u} {β : Type v} [topological_space α] [t2_space α] {f : β → α} {l : filter β} {a b : α} [l.ne_bot] :
filter.tendsto f l (𝓝 a) → filter.tendsto f l (𝓝 b) → a = b

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theorem tendsto_nhds_unique' {α : Type u} {β : Type v} [topological_space α] [t2_space α] {f : β → α} {l : filter β} {a b : α} :
l.ne_bot → filter.tendsto f l (𝓝 a) → filter.tendsto f l (𝓝 b) → a = b

Properties of Lim and lim

In this section we use explicit nonempty α instances for Lim and lim. This way the lemmas are useful without a nonempty α instance.

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theorem Lim_eq {α : Type u} [topological_space α] [t2_space α] {f : filter α} {a : α} [f.ne_bot] :
f ≤ 𝓝 a → Lim f = a

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theorem Lim_eq_iff {α : Type u} [topological_space α] [t2_space α] {f : filter α} [f.ne_bot] (h : ∃ (a : α), f ≤ 𝓝 a) {a : α} :
Lim f = a ↔ f ≤ 𝓝 a

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theorem is_ultrafilter.Lim_eq_iff_le_nhds {α : Type u} [topological_space α] [t2_space α] [compact_space α] (x : α) (F : filter.ultrafilter α) :
Lim F.val = x ↔ F.val ≤ 𝓝 x

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theorem is_open_iff_ultrafilter' {α : Type u} [topological_space α] [t2_space α] [compact_space α] (U : set α) :
is_open U ↔ ∀ (F : filter.ultrafilter α), F.Lim ∈ U → U ∈ F.val

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theorem filter.tendsto.lim_eq {α : Type u} {β : Type v} [topological_space α] [t2_space α] {a : α} {f : filter β} [f.ne_bot] {g : β → α} :
filter.tendsto g f (𝓝 a) → lim f g = a

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theorem filter.lim_eq_iff {α : Type u} {β : Type v} [topological_space α] [t2_space α] {f : filter β} [f.ne_bot] {g : β → α} (h : ∃ (a : α), filter.tendsto g f (𝓝 a)) {a : α} :
lim f g = a ↔ filter.tendsto g f (𝓝 a)

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theorem continuous.lim_eq {α : Type u} {β : Type v} [topological_space α] [t2_space α] [topological_space β] {f : β → α} (h : continuous f) (a : β) :
lim (𝓝 a) f = f a

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@[simp]
theorem Lim_nhds {α : Type u} [topological_space α] [t2_space α] (a : α) :
Lim (𝓝 a) = a

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@[simp]
theorem lim_nhds_id {α : Type u} [topological_space α] [t2_space α] (a : α) :
lim (𝓝 a) id = a

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@[simp]
theorem Lim_nhds_within {α : Type u} [topological_space α] [t2_space α] {a : α} {s : set α} :
a ∈ closure s → Lim (𝓝[s] a) = a

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@[simp]
theorem lim_nhds_within_id {α : Type u} [topological_space α] [t2_space α] {a : α} {s : set α} :
a ∈ closure s → lim (𝓝[s] a) id = a

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@[instance]
def t2_space_discrete {α : Type u_1} [topological_space α] [discrete_topology α] :
t2_space α

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@[instance]
def subtype.t2_space {α : Type u_1} {p : α → Prop} [t : topological_space α] [t2_space α] :
t2_space (subtype p)

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@[instance]
def prod.t2_space {α : Type u_1} {β : Type u_2} [t₁ : topological_space α] [t2_space α] [t₂ : topological_space β] [t2_space β] :
t2_space (α × β)

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@[instance]
def Pi.t2_space {α : Type u_1} {β : α → Type v} [t₂ : Π (a : α), topological_space (β a)] [∀ (a : α), t2_space (β a)] :
t2_space (Π (a : α), β a)

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theorem is_closed_eq {α : Type u} {β : Type v} [topological_space α] [topological_space β] [t2_space α] {f g : β → α} :
continuous f → continuous g → is_closed {x : β | f x = g x}

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theorem set.eq_on.closure {α : Type u} {β : Type v} [topological_space α] [topological_space β] [t2_space α] {s : set β} {f g : β → α} :
set.eq_on f g s → continuous f → continuous g → set.eq_on f g (closure s)

If two continuous maps are equal on s, then they are equal on the closure of s.

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theorem continuous.ext_on {α : Type u} {β : Type v} [topological_space α] [topological_space β] [t2_space α] {s : set β} (hs : dense s) {f g : β → α} :
continuous f → continuous g → set.eq_on f g s → f = g

If two continuous functions are equal on a dense set, then they are equal.

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theorem diagonal_eq_range_diagonal_map {α : Type u_1} :
{p : α × α | p.fst = p.snd} = set.range (λ (x : α), (x, x))

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theorem prod_subset_compl_diagonal_iff_disjoint {α : Type u_1} {s t : set α} :
s.prod t ⊆ {p : α × α | p.fst = p.snd}ᶜ ↔ s ∩ t = ∅

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theorem compact_compact_separated {α : Type u} [topological_space α] [t2_space α] {s t : set α} :
is_compact s → is_compact t → s ∩ t = ∅ → (∃ (u v : set α), is_open u ∧ is_open v ∧ s ⊆ u ∧ t ⊆ v ∧ u ∩ v = ∅)

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theorem is_compact.is_closed {α : Type u} [topological_space α] [t2_space α] {s : set α} :
is_compact s → is_closed s

In a t2_space, every compact set is closed.

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theorem is_compact.inter {α : Type u} [topological_space α] [t2_space α] {s t : set α} :
is_compact s → is_compact t → is_compact (s ∩ t)

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theorem is_compact.binary_compact_cover {α : Type u} [topological_space α] [t2_space α] {K U V : set α} :
is_compact K → is_open U → is_open V → K ⊆ U ∪ V → (∃ (K₁ K₂ : set α), is_compact K₁ ∧ is_compact K₂ ∧ K₁ ⊆ U ∧ K₂ ⊆ V ∧ K = K₁ ∪ K₂)

If a compact set is covered by two open sets, then we can cover it by two compact subsets.

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theorem is_compact.finite_compact_cover {α : Type u} [topological_space α] [t2_space α] {s : set α} (hs : is_compact s) {ι : Type u_1} (t : finset ι) (U : ι → set α) :
(∀ (i : ι), i ∈ t → is_open (U i)) → (s ⊆ ⋃ (i : ι) (H : i ∈ t), U i) → (∃ (K : ι → set α), (∀ (i : ι), is_compact (K i)) ∧ (∀ (i : ι), K i ⊆ U i) ∧ s = ⋃ (i : ι) (H : i ∈ t), K i)

For every finite open cover Uᵢ of a compact set, there exists a compact cover Kᵢ ⊆ Uᵢ.

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theorem locally_compact_of_compact_nhds {α : Type u} [topological_space α] [t2_space α] :
(∀ (x : α), ∃ (s : set α), s ∈ 𝓝 x ∧ is_compact s) → locally_compact_space α

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@[instance]
def locally_compact_of_compact {α : Type u} [topological_space α] [t2_space α] [compact_space α] :
locally_compact_space α

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theorem exists_open_with_compact_closure {α : Type u} [topological_space α] [locally_compact_space α] [t2_space α] (x : α) :
∃ (U : set α), is_open U ∧ x ∈ U ∧ is_compact (closure U)

In a locally compact T₂ space, every point has an open neighborhood with compact closure

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theorem exists_compact_superset {α : Type u} [topological_space α] [locally_compact_space α] [t2_space α] {K : set α} :
is_compact K → (∃ (K' : set α), is_compact K' ∧ K ⊆ interior K')

In a locally compact T₂ space, every compact set is contained in the interior of a compact set.

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@[class]
structure regular_space (α : Type u) [topological_space α] :
Prop

A T₃ space, also known as a regular space (although this condition sometimes omits T₂), is one in which for every closed C and x ∉ C, there exist disjoint open sets containing x and C respectively.

Instances
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theorem nhds_is_closed {α : Type u} [topological_space α] [regular_space α] {a : α} {s : set α} :
s ∈ 𝓝 a → (∃ (t : set α) (H : t ∈ 𝓝 a), t ⊆ s ∧ is_closed t)

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theorem closed_nhds_basis {α : Type u} [topological_space α] [regular_space α] (a : α) :
(𝓝 a).has_basis (λ (s : set α), s ∈ 𝓝 a ∧ is_closed s) id

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@[instance]
def subtype.regular_space {α : Type u} [topological_space α] [regular_space α] {p : α → Prop} :
regular_space (subtype p)

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@[instance]
def regular_space.t2_space (α : Type u) [topological_space α] [regular_space α] :
t2_space α

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theorem disjoint_nested_nhds {α : Type u} [topological_space α] [regular_space α] {x y : α} :
x ≠ y → (∃ (U₁ V₁ : set α) (H : U₁ ∈ 𝓝 x) (H : V₁ ∈ 𝓝 x) (U₂ V₂ : set α) (H : U₂ ∈ 𝓝 y) (H : V₂ ∈ 𝓝 y), is_closed V₁ ∧ is_closed V₂ ∧ is_open U₁ ∧ is_open U₂ ∧ V₁ ⊆ U₁ ∧ V₂ ⊆ U₂ ∧ U₁ ∩ U₂ = ∅)

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@[class]
structure normal_space (α : Type u) [topological_space α] :
Prop

A T₄ space, also known as a normal space (although this condition sometimes omits T₂), is one in which for every pair of disjoint closed sets C and D, there exist disjoint open sets containing C and D respectively.

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theorem normal_separation {α : Type u} [topological_space α] [normal_space α] (s t : set α) :
is_closed s → is_closed t → disjoint s t → (∃ (u v : set α), is_open u ∧ is_open v ∧ s ⊆ u ∧ t ⊆ v ∧ disjoint u v)

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@[instance]
def normal_space.regular_space {α : Type u} [topological_space α] [normal_space α] :
regular_space α

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theorem normal_of_compact_t2 {α : Type u} [topological_space α] [compact_space α] [t2_space α] :
normal_space α