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/- | |
Copyright (c) 2022 Jireh Loreaux. All rights reserved. | |
Released under Apache 2.0 license as described in the file LICENSE. | |
Authors: Jireh Loreaux | |
-/ | |
import order.filter.cofinite | |
/-! | |
# Basic theory of bornology | |
We develop the basic theory of bornologies. Instead of axiomatizing bounded sets and defining | |
bornologies in terms of those, we recognize that the cobounded sets form a filter and define a | |
bornology as a filter of cobounded sets which contains the cofinite filter. This allows us to make | |
use of the extensive library for filters, but we also provide the relevant connecting results for | |
bounded sets. | |
The specification of a bornology in terms of the cobounded filter is equivalent to the standard | |
one (e.g., see [Bourbaki, *Topological Vector Spaces*][bourbaki1987], **covering bornology**, now | |
often called simply **bornology**) in terms of bounded sets (see `bornology.of_bounded`, | |
`is_bounded.union`, `is_bounded.subset`), except that we do not allow the empty bornology (that is, | |
we require that *some* set must be bounded; equivalently, `∅` is bounded). In the literature the | |
cobounded filter is generally referred to as the *filter at infinity*. | |
## Main definitions | |
- `bornology α`: a class consisting of `cobounded : filter α` and a proof that this filter | |
contains the `cofinite` filter. | |
- `bornology.is_cobounded`: the predicate that a set is a member of the `cobounded α` filter. For | |
`s : set α`, one should prefer `bornology.is_cobounded s` over `s ∈ cobounded α`. | |
- `bornology.is_bounded`: the predicate that states a set is bounded (i.e., the complement of a | |
cobounded set). One should prefer `bornology.is_bounded s` over `sᶜ ∈ cobounded α`. | |
- `bounded_space α`: a class extending `bornology α` with the condition | |
`bornology.is_bounded (set.univ : set α)` | |
Although use of `cobounded α` is discouraged for indicating the (co)boundedness of individual sets, | |
it is intended for regular use as a filter on `α`. | |
-/ | |
open set filter | |
variables {ι α β : Type*} | |
/-- A **bornology** on a type `α` is a filter of cobounded sets which contains the cofinite filter. | |
Such spaces are equivalently specified by their bounded sets, see `bornology.of_bounded` | |
and `bornology.ext_iff_is_bounded`-/ | |
@[ext] | |
class bornology (α : Type*) := | |
(cobounded [] : filter α) | |
(le_cofinite [] : cobounded ≤ cofinite) | |
/-- A constructor for bornologies by specifying the bounded sets, | |
and showing that they satisfy the appropriate conditions. -/ | |
@[simps] | |
def bornology.of_bounded {α : Type*} (B : set (set α)) | |
(empty_mem : ∅ ∈ B) (subset_mem : ∀ s₁ ∈ B, ∀ s₂ : set α, s₂ ⊆ s₁ → s₂ ∈ B) | |
(union_mem : ∀ s₁ s₂ ∈ B, s₁ ∪ s₂ ∈ B) (singleton_mem : ∀ x, {x} ∈ B) : | |
bornology α := | |
{ cobounded := | |
{ sets := {s : set α | sᶜ ∈ B}, | |
univ_sets := by rwa ←compl_univ at empty_mem, | |
sets_of_superset := λ x y hx hy, subset_mem xᶜ hx yᶜ (compl_subset_compl.mpr hy), | |
inter_sets := λ x y hx hy, by simpa [compl_inter] using union_mem xᶜ hx yᶜ hy, }, | |
le_cofinite := | |
begin | |
rw le_cofinite_iff_compl_singleton_mem, | |
intros x, | |
change {x}ᶜᶜ ∈ B, | |
rw compl_compl, | |
exact singleton_mem x | |
end } | |
/-- A constructor for bornologies by specifying the bounded sets, | |
and showing that they satisfy the appropriate conditions. -/ | |
@[simps] | |
def bornology.of_bounded' {α : Type*} (B : set (set α)) | |
(empty_mem : ∅ ∈ B) (subset_mem : ∀ s₁ ∈ B, ∀ s₂ : set α, s₂ ⊆ s₁ → s₂ ∈ B) | |
(union_mem : ∀ s₁ s₂ ∈ B, s₁ ∪ s₂ ∈ B) (sUnion_univ : ⋃₀ B = univ) : | |
bornology α := | |
bornology.of_bounded B empty_mem subset_mem union_mem $ λ x, | |
begin | |
rw sUnion_eq_univ_iff at sUnion_univ, | |
rcases sUnion_univ x with ⟨s, hs, hxs⟩, | |
exact subset_mem s hs {x} (singleton_subset_iff.mpr hxs) | |
end | |
namespace bornology | |
section | |
variables [bornology α] {s t : set α} {x : α} | |
/-- `is_cobounded` is the predicate that `s` is in the filter of cobounded sets in the ambient | |
bornology on `α` -/ | |
def is_cobounded (s : set α) : Prop := s ∈ cobounded α | |
/-- `is_bounded` is the predicate that `s` is bounded relative to the ambient bornology on `α`. -/ | |
def is_bounded (s : set α) : Prop := is_cobounded sᶜ | |
lemma is_cobounded_def {s : set α} : is_cobounded s ↔ s ∈ cobounded α := iff.rfl | |
lemma is_bounded_def {s : set α} : is_bounded s ↔ sᶜ ∈ cobounded α := iff.rfl | |
@[simp] lemma is_bounded_compl_iff : is_bounded sᶜ ↔ is_cobounded s := | |
by rw [is_bounded_def, is_cobounded_def, compl_compl] | |
@[simp] lemma is_cobounded_compl_iff : is_cobounded sᶜ ↔ is_bounded s := iff.rfl | |
alias is_bounded_compl_iff ↔ is_bounded.of_compl is_cobounded.compl | |
alias is_cobounded_compl_iff ↔ is_cobounded.of_compl is_bounded.compl | |
@[simp] lemma is_bounded_empty : is_bounded (∅ : set α) := | |
by { rw [is_bounded_def, compl_empty], exact univ_mem} | |
@[simp] lemma is_bounded_singleton : is_bounded ({x} : set α) := | |
by {rw [is_bounded_def], exact le_cofinite _ (finite_singleton x).compl_mem_cofinite} | |
@[simp] lemma is_cobounded_univ : is_cobounded (univ : set α) := univ_mem | |
@[simp] lemma is_cobounded_inter : is_cobounded (s ∩ t) ↔ is_cobounded s ∧ is_cobounded t := | |
inter_mem_iff | |
lemma is_cobounded.inter (hs : is_cobounded s) (ht : is_cobounded t) : is_cobounded (s ∩ t) := | |
is_cobounded_inter.2 ⟨hs, ht⟩ | |
@[simp] lemma is_bounded_union : is_bounded (s ∪ t) ↔ is_bounded s ∧ is_bounded t := | |
by simp only [← is_cobounded_compl_iff, compl_union, is_cobounded_inter] | |
lemma is_bounded.union (hs : is_bounded s) (ht : is_bounded t) : is_bounded (s ∪ t) := | |
is_bounded_union.2 ⟨hs, ht⟩ | |
lemma is_cobounded.superset (hs : is_cobounded s) (ht : s ⊆ t) : is_cobounded t := | |
mem_of_superset hs ht | |
lemma is_bounded.subset (ht : is_bounded t) (hs : s ⊆ t) : is_bounded s := | |
ht.superset (compl_subset_compl.mpr hs) | |
@[simp] | |
lemma sUnion_bounded_univ : (⋃₀ {s : set α | is_bounded s}) = univ := | |
sUnion_eq_univ_iff.2 $ λ a, ⟨{a}, is_bounded_singleton, mem_singleton a⟩ | |
lemma comap_cobounded_le_iff [bornology β] {f : α → β} : | |
(cobounded β).comap f ≤ cobounded α ↔ ∀ ⦃s⦄, is_bounded s → is_bounded (f '' s) := | |
begin | |
refine ⟨λ h s hs, _, λ h t ht, | |
⟨(f '' tᶜ)ᶜ, h $ is_cobounded.compl ht, compl_subset_comm.1 $ subset_preimage_image _ _⟩⟩, | |
obtain ⟨t, ht, hts⟩ := h hs.compl, | |
rw [subset_compl_comm, ←preimage_compl] at hts, | |
exact (is_cobounded.compl ht).subset ((image_subset f hts).trans $ image_preimage_subset _ _), | |
end | |
end | |
lemma ext_iff' {t t' : bornology α} : | |
t = t' ↔ ∀ s, (@cobounded α t).sets s ↔ (@cobounded α t').sets s := | |
(ext_iff _ _).trans filter.ext_iff | |
lemma ext_iff_is_bounded {t t' : bornology α} : | |
t = t' ↔ ∀ s, @is_bounded α t s ↔ @is_bounded α t' s := | |
⟨λ h s, h ▸ iff.rfl, λ h, by { ext, simpa only [is_bounded_def, compl_compl] using h sᶜ, }⟩ | |
variables {s : set α} | |
lemma is_cobounded_of_bounded_iff (B : set (set α)) {empty_mem subset_mem union_mem sUnion_univ} : | |
@is_cobounded _ (of_bounded B empty_mem subset_mem union_mem sUnion_univ) s ↔ sᶜ ∈ B := iff.rfl | |
lemma is_bounded_of_bounded_iff (B : set (set α)) {empty_mem subset_mem union_mem sUnion_univ} : | |
@is_bounded _ (of_bounded B empty_mem subset_mem union_mem sUnion_univ) s ↔ s ∈ B := | |
by rw [is_bounded_def, ←filter.mem_sets, of_bounded_cobounded_sets, set.mem_set_of_eq, compl_compl] | |
variables [bornology α] | |
lemma is_cobounded_bInter {s : set ι} {f : ι → set α} (hs : s.finite) : | |
is_cobounded (⋂ i ∈ s, f i) ↔ ∀ i ∈ s, is_cobounded (f i) := | |
bInter_mem hs | |
@[simp] lemma is_cobounded_bInter_finset (s : finset ι) {f : ι → set α} : | |
is_cobounded (⋂ i ∈ s, f i) ↔ ∀ i ∈ s, is_cobounded (f i) := | |
bInter_finset_mem s | |
@[simp] lemma is_cobounded_Inter [finite ι] {f : ι → set α} : | |
is_cobounded (⋂ i, f i) ↔ ∀ i, is_cobounded (f i) := | |
Inter_mem | |
lemma is_cobounded_sInter {S : set (set α)} (hs : S.finite) : | |
is_cobounded (⋂₀ S) ↔ ∀ s ∈ S, is_cobounded s := | |
sInter_mem hs | |
lemma is_bounded_bUnion {s : set ι} {f : ι → set α} (hs : s.finite) : | |
is_bounded (⋃ i ∈ s, f i) ↔ ∀ i ∈ s, is_bounded (f i) := | |
by simp only [← is_cobounded_compl_iff, compl_Union, is_cobounded_bInter hs] | |
lemma is_bounded_bUnion_finset (s : finset ι) {f : ι → set α} : | |
is_bounded (⋃ i ∈ s, f i) ↔ ∀ i ∈ s, is_bounded (f i) := | |
is_bounded_bUnion s.finite_to_set | |
lemma is_bounded_sUnion {S : set (set α)} (hs : S.finite) : | |
is_bounded (⋃₀ S) ↔ (∀ s ∈ S, is_bounded s) := | |
by rw [sUnion_eq_bUnion, is_bounded_bUnion hs] | |
@[simp] lemma is_bounded_Union [finite ι] {s : ι → set α} : | |
is_bounded (⋃ i, s i) ↔ ∀ i, is_bounded (s i) := | |
by rw [← sUnion_range, is_bounded_sUnion (finite_range s), forall_range_iff] | |
end bornology | |
open bornology | |
lemma set.finite.is_bounded [bornology α] {s : set α} (hs : s.finite) : is_bounded s := | |
bornology.le_cofinite α hs.compl_mem_cofinite | |
instance : bornology punit := ⟨⊥, bot_le⟩ | |
/-- The cofinite filter as a bornology -/ | |
@[reducible] def bornology.cofinite : bornology α := | |
{ cobounded := cofinite, | |
le_cofinite := le_rfl } | |
/-- A space with a `bornology` is a **bounded space** if `set.univ : set α` is bounded. -/ | |
class bounded_space (α : Type*) [bornology α] : Prop := | |
(bounded_univ : bornology.is_bounded (univ : set α)) | |
namespace bornology | |
variables [bornology α] | |
lemma is_bounded_univ : is_bounded (univ : set α) ↔ bounded_space α := | |
⟨λ h, ⟨h⟩, λ h, h.1⟩ | |
lemma cobounded_eq_bot_iff : cobounded α = ⊥ ↔ bounded_space α := | |
by rw [← is_bounded_univ, is_bounded_def, compl_univ, empty_mem_iff_bot] | |
variables [bounded_space α] | |
lemma is_bounded.all (s : set α) : is_bounded s := bounded_space.bounded_univ.subset s.subset_univ | |
lemma is_cobounded.all (s : set α) : is_cobounded s := compl_compl s ▸ is_bounded.all sᶜ | |
variable (α) | |
@[simp] lemma cobounded_eq_bot : cobounded α = ⊥ := cobounded_eq_bot_iff.2 ‹_› | |
end bornology | |