# mathlibdocumentation

order.filter.countable_Inter

# Filters with countable intersection property

In this file we define countable_Inter_filter to be the class of filters with the following property: for any countable collection of sets s ∈ l their intersection belongs to l as well.

Two main examples are the residual filter defined in topology.metric_space.baire and the measure.ae filter defined in measure_theory.measure_space.

@[class]
structure countable_Inter_filter {α : Type u_2} :
→ Prop

A filter l has the countable intersection property if for any countable collection of sets s ∈ l their intersection belongs to l as well.

Instances
theorem countable_sInter_mem_sets {α : Type u_2} {l : filter α} {S : set (set α)} :
S.countable(⋂₀S l ∀ (s : set α), s Ss l)

theorem countable_Inter_mem_sets {ι : Type u_1} {α : Type u_2} {l : filter α} [encodable ι] {s : ι → set α} :
(⋂ (i : ι), s i) l ∀ (i : ι), s i l

theorem countable_bInter_mem_sets {ι : Type u_1} {α : Type u_2} {l : filter α} {S : set ι} (hS : S.countable) {s : Π (i : ι), i Sset α} :
(⋂ (i : ι) (H : i S), s i H) l ∀ (i : ι) (H : i S), s i H l

theorem eventually_countable_forall {ι : Type u_1} {α : Type u_2} {l : filter α} [encodable ι] {p : α → ι → Prop} :
(∀ᶠ (x : α) in l, ∀ (i : ι), p x i) ∀ (i : ι), ∀ᶠ (x : α) in l, p x i

theorem eventually_countable_ball {ι : Type u_1} {α : Type u_2} {l : filter α} {S : set ι} (hS : S.countable) {p : α → Π (i : ι), i S → Prop} :
(∀ᶠ (x : α) in l, ∀ (i : ι) (H : i S), p x i H) ∀ (i : ι) (H : i S), ∀ᶠ (x : α) in l, p x i H

@[instance]
def countable_Inter_filter_principal {α : Type u_2} (s : set α) :

@[instance]
def countable_Inter_filter_bot {α : Type u_2} :

@[instance]
def countable_Inter_filter_top {α : Type u_2} :

@[instance]
def countable_Inter_filter_inf {α : Type u_2} (l₁ l₂ : filter α)  :

Infimum of two countable_Inter_filters is a countable_Inter_filter. This is useful, e.g., to automatically get an instance for residual α ⊓ 𝓟 s.

@[instance]
def countable_Inter_filter_sup {α : Type u_2} (l₁ l₂ : filter α)  :

Supremum of two countable_Inter_filters is a countable_Inter_filter.