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.ofBounded, IsBounded.union, IsBounded.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.IsCobounded: the predicate that a set is a member of the cobounded α filter. For s : Set α, one should prefer Bornology.IsCobounded s over s ∈ cobounded α.
• bornology.IsBounded: the predicate that states a set is bounded (i.e., the complement of a cobounded set). One should prefer Bornology.IsBounded s over sᶜ ∈ cobounded α.
• BoundedSpace α: a class extending Bornology α with the condition Bornology.IsBounded (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.

class Bornology (α : Type u_4) : Type u_4

• cobounded' : Filter α

  The filter of cobounded sets in a bornology. This is a field of the structure, but one should always prefer Bornology.cobounded because it makes the α argument explciit.

• le_cofinite' : Bornology.cobounded' ≤ Filter.cofinite

  The cobounded filter in a bornology is smaller than the cofinite filter. This is a field of the structure, but one should always prefer Bornology.le_cofinite because it makes the α argument explciit.

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.ofBounded and Bornology.ext_iff_isBounded

def Bornology.cobounded (α : Type u_4) [Bornology α] : Filter α

The filter of cobounded sets in a bornology.

def Bornology.Simps.cobounded (α : Type u_4) [Bornology α] : Filter α

Alias of Bornology.cobounded.

---

The filter of cobounded sets in a bornology.

theorem Bornology.le_cofinite (α : Type u_4) [Bornology α] : Bornology.cobounded α ≤ Filter.cofinite

theorem Bornology.ext {α : Type u_2} (t : Bornology α) (t' : Bornology α) (h_cobounded : Bornology.cobounded α = Bornology.cobounded α) : t = t'

theorem Bornology.ext_iff {α : Type u_2} (t : Bornology α) (t' : Bornology α) : t = t' ↔ Bornology.cobounded α = Bornology.cobounded α

@[simp]

theorem Bornology.ofBounded_cobounded_sets {α : Type u_4} (B : Set (Set α)) (empty_mem : ∅ ∈ B) (subset_mem : ∀ (s₁ : Set α), s₁ ∈ B → ∀ (s₂ : Set α), s₂ ⊆ s₁ → s₂ ∈ B) (union_mem : ∀ (s₁ : Set α), s₁ ∈ B → ∀ (s₂ : Set α), s₂ ∈ B → s₁ ∪ s₂ ∈ B) (singleton_mem : ∀ (x : α), {x} ∈ B) : (Bornology.cobounded α).sets = {s | sᶜ ∈ B}

def Bornology.ofBounded {α : Type u_4} (B : Set (Set α)) (empty_mem : ∅ ∈ B) (subset_mem : ∀ (s₁ : Set α), s₁ ∈ B → ∀ (s₂ : Set α), s₂ ⊆ s₁ → s₂ ∈ B) (union_mem : ∀ (s₁ : Set α), s₁ ∈ B → ∀ (s₂ : Set α), s₂ ∈ B → s₁ ∪ s₂ ∈ B) (singleton_mem : ∀ (x : α), {x} ∈ B) : Bornology α

A constructor for bornologies by specifying the bounded sets, and showing that they satisfy the appropriate conditions.

@[simp]

theorem Bornology.ofBounded'_cobounded_sets {α : Type u_4} (B : Set (Set α)) (empty_mem : ∅ ∈ B) (subset_mem : ∀ (s₁ : Set α), s₁ ∈ B → ∀ (s₂ : Set α), s₂ ⊆ s₁ → s₂ ∈ B) (union_mem : ∀ (s₁ : Set α), s₁ ∈ B → ∀ (s₂ : Set α), s₂ ∈ B → s₁ ∪ s₂ ∈ B) (sUnion_univ : ⋃₀ B = Set.univ) : (Bornology.cobounded α).sets = {s | sᶜ ∈ B}

def Bornology.ofBounded' {α : Type u_4} (B : Set (Set α)) (empty_mem : ∅ ∈ B) (subset_mem : ∀ (s₁ : Set α), s₁ ∈ B → ∀ (s₂ : Set α), s₂ ⊆ s₁ → s₂ ∈ B) (union_mem : ∀ (s₁ : Set α), s₁ ∈ B → ∀ (s₂ : Set α), s₂ ∈ B → s₁ ∪ s₂ ∈ B) (sUnion_univ : ⋃₀ B = Set.univ) : Bornology α

A constructor for bornologies by specifying the bounded sets, and showing that they satisfy the appropriate conditions.

def Bornology.IsCobounded {α : Type u_2} [Bornology α] (s : Set α) : Prop

IsCobounded is the predicate that s is in the filter of cobounded sets in the ambient bornology on α

def Bornology.IsBounded {α : Type u_2} [Bornology α] (s : Set α) : Prop

IsBounded is the predicate that s is bounded relative to the ambient bornology on α.

theorem Bornology.isCobounded_def {α : Type u_2} [Bornology α] {s : Set α} : Bornology.IsCobounded s ↔ s ∈ Bornology.cobounded α

theorem Bornology.isBounded_def {α : Type u_2} [Bornology α] {s : Set α} : Bornology.IsBounded s ↔ sᶜ ∈ Bornology.cobounded α

@[simp]

theorem Bornology.isBounded_compl_iff {α : Type u_2} [Bornology α] {s : Set α} : Bornology.IsBounded sᶜ ↔ Bornology.IsCobounded s

@[simp]

theorem Bornology.isCobounded_compl_iff {α : Type u_2} [Bornology α] {s : Set α} : Bornology.IsCobounded sᶜ ↔ Bornology.IsBounded s

theorem Bornology.IsBounded.of_compl {α : Type u_2} [Bornology α] {s : Set α} : Bornology.IsBounded sᶜ → Bornology.IsCobounded s

Alias of the forward direction of Bornology.isBounded_compl_iff.

theorem Bornology.IsCobounded.compl {α : Type u_2} [Bornology α] {s : Set α} : Bornology.IsCobounded s → Bornology.IsBounded sᶜ

Alias of the reverse direction of Bornology.isBounded_compl_iff.

theorem Bornology.IsCobounded.of_compl {α : Type u_2} [Bornology α] {s : Set α} : Bornology.IsCobounded sᶜ → Bornology.IsBounded s

Alias of the forward direction of Bornology.isCobounded_compl_iff.

theorem Bornology.IsBounded.compl {α : Type u_2} [Bornology α] {s : Set α} : Bornology.IsBounded s → Bornology.IsCobounded sᶜ

Alias of the reverse direction of Bornology.isCobounded_compl_iff.

@[simp]

theorem Bornology.isBounded_empty {α : Type u_2} [Bornology α] : Bornology.IsBounded ∅

@[simp]

theorem Bornology.isBounded_singleton {α : Type u_2} [Bornology α] {x : α} : Bornology.IsBounded {x}

theorem Bornology.isBounded_iff_forall_mem {α : Type u_2} [Bornology α] {s : Set α} : Bornology.IsBounded s ↔ ∀ (x : α), x ∈ s → Bornology.IsBounded s

@[simp]

theorem Bornology.isCobounded_univ {α : Type u_2} [Bornology α] : Bornology.IsCobounded Set.univ

@[simp]

theorem Bornology.isCobounded_inter {α : Type u_2} [Bornology α] {s : Set α} {t : Set α} : Bornology.IsCobounded (s ∩ t) ↔ Bornology.IsCobounded s ∧ Bornology.IsCobounded t

theorem Bornology.IsCobounded.inter {α : Type u_2} [Bornology α] {s : Set α} {t : Set α} (hs : Bornology.IsCobounded s) (ht : Bornology.IsCobounded t) : Bornology.IsCobounded (s ∩ t)

@[simp]

theorem Bornology.isBounded_union {α : Type u_2} [Bornology α] {s : Set α} {t : Set α} : Bornology.IsBounded (s ∪ t) ↔ Bornology.IsBounded s ∧ Bornology.IsBounded t

theorem Bornology.IsBounded.union {α : Type u_2} [Bornology α] {s : Set α} {t : Set α} (hs : Bornology.IsBounded s) (ht : Bornology.IsBounded t) : Bornology.IsBounded (s ∪ t)

theorem Bornology.IsCobounded.superset {α : Type u_2} [Bornology α] {s : Set α} {t : Set α} (hs : Bornology.IsCobounded s) (ht : s ⊆ t) : Bornology.IsCobounded t

theorem Bornology.IsBounded.subset {α : Type u_2} [Bornology α] {s : Set α} {t : Set α} (ht : Bornology.IsBounded t) (hs : s ⊆ t) : Bornology.IsBounded s

@[simp]

theorem Bornology.sUnion_bounded_univ {α : Type u_2} [Bornology α] : ⋃₀ {s | Bornology.IsBounded s} = Set.univ

theorem Bornology.IsBounded.insert {α : Type u_2} [Bornology α] {s : Set α} (h : Bornology.IsBounded s) (x : α) : Bornology.IsBounded (insert x s)

@[simp]

theorem Bornology.isBounded_insert {α : Type u_2} [Bornology α] {s : Set α} {x : α} : Bornology.IsBounded (insert x s) ↔ Bornology.IsBounded s

theorem Bornology.comap_cobounded_le_iff {α : Type u_2} {β : Type u_3} [Bornology α] [Bornology β] {f : α → β} : Filter.comap f (Bornology.cobounded β) ≤ Bornology.cobounded α ↔ ∀ ⦃s : Set α⦄, Bornology.IsBounded s → Bornology.IsBounded (f '' s)

theorem Bornology.ext_iff' {α : Type u_2} {t : Bornology α} {t' : Bornology α} : t = t' ↔ ∀ (s : Set α), s ∈ Bornology.cobounded α ↔ s ∈ Bornology.cobounded α

theorem Bornology.ext_iff_isBounded {α : Type u_2} {t : Bornology α} {t' : Bornology α} : t = t' ↔ ∀ (s : Set α), Bornology.IsBounded s ↔ Bornology.IsBounded s

theorem Bornology.isCobounded_ofBounded_iff {α : Type u_2} {s : Set α} (B : Set (Set α)) {empty_mem : ∅ ∈ B} {subset_mem : ∀ (s₁ : Set α), s₁ ∈ B → ∀ (s₂ : Set α), s₂ ⊆ s₁ → s₂ ∈ B} {union_mem : ∀ (s₁ : Set α), s₁ ∈ B → ∀ (s₂ : Set α), s₂ ∈ B → s₁ ∪ s₂ ∈ B} {sUnion_univ : ∀ (x : α), {x} ∈ B} : Bornology.IsCobounded s ↔ sᶜ ∈ B

theorem Bornology.isBounded_ofBounded_iff {α : Type u_2} {s : Set α} (B : Set (Set α)) {empty_mem : ∅ ∈ B} {subset_mem : ∀ (s₁ : Set α), s₁ ∈ B → ∀ (s₂ : Set α), s₂ ⊆ s₁ → s₂ ∈ B} {union_mem : ∀ (s₁ : Set α), s₁ ∈ B → ∀ (s₂ : Set α), s₂ ∈ B → s₁ ∪ s₂ ∈ B} {sUnion_univ : ∀ (x : α), {x} ∈ B} : Bornology.IsBounded s ↔ s ∈ B

theorem Bornology.isCobounded_biInter {ι : Type u_1} {α : Type u_2} [Bornology α] {s : Set ι} {f : ι → Set α} (hs : Set.Finite s) : Bornology.IsCobounded (⋂ (i : ι) (_ : i ∈ s), f i) ↔ ∀ (i : ι), i ∈ s → Bornology.IsCobounded (f i)

@[simp]

theorem Bornology.isCobounded_biInter_finset {ι : Type u_1} {α : Type u_2} [Bornology α] (s : Finset ι) {f : ι → Set α} : Bornology.IsCobounded (⋂ (i : ι) (_ : i ∈ s), f i) ↔ ∀ (i : ι), i ∈ s → Bornology.IsCobounded (f i)

@[simp]

theorem Bornology.isCobounded_iInter {ι : Type u_1} {α : Type u_2} [Bornology α] [Finite ι] {f : ι → Set α} : Bornology.IsCobounded (⋂ (i : ι), f i) ↔ ∀ (i : ι), Bornology.IsCobounded (f i)

theorem Bornology.isCobounded_sInter {α : Type u_2} [Bornology α] {S : Set (Set α)} (hs : Set.Finite S) : Bornology.IsCobounded (⋂₀ S) ↔ ∀ (s : Set α), s ∈ S → Bornology.IsCobounded s

theorem Bornology.isBounded_biUnion {ι : Type u_1} {α : Type u_2} [Bornology α] {s : Set ι} {f : ι → Set α} (hs : Set.Finite s) : Bornology.IsBounded (⋃ (i : ι) (_ : i ∈ s), f i) ↔ ∀ (i : ι), i ∈ s → Bornology.IsBounded (f i)

theorem Bornology.isBounded_biUnion_finset {ι : Type u_1} {α : Type u_2} [Bornology α] (s : Finset ι) {f : ι → Set α} : Bornology.IsBounded (⋃ (i : ι) (_ : i ∈ s), f i) ↔ ∀ (i : ι), i ∈ s → Bornology.IsBounded (f i)

theorem Bornology.isBounded_sUnion {α : Type u_2} [Bornology α] {S : Set (Set α)} (hs : Set.Finite S) : Bornology.IsBounded (⋃₀ S) ↔ ∀ (s : Set α), s ∈ S → Bornology.IsBounded s

@[simp]

theorem Bornology.isBounded_iUnion {ι : Type u_1} {α : Type u_2} [Bornology α] [Finite ι] {s : ι → Set α} : Bornology.IsBounded (⋃ (i : ι), s i) ↔ ∀ (i : ι), Bornology.IsBounded (s i)

theorem Filter.HasBasis.disjoint_cobounded_iff {α : Type u_2} [Bornology α] {ι : Sort u_4} {p : ι → Prop} {s : ι → Set α} {l : Filter α} (h : Filter.HasBasis l p s) : Disjoint l (Bornology.cobounded α) ↔ ∃ i, p i ∧ Bornology.IsBounded (s i)

theorem Set.Finite.isBounded {α : Type u_2} [Bornology α] {s : Set α} (hs : Set.Finite s) : Bornology.IsBounded s

instance instBornologyPUnit : Bornology PUnit.{u_4 + 1}

@[reducible]

def Bornology.cofinite {α : Type u_2} : Bornology α

The cofinite filter as a bornology

class BoundedSpace (α : Type u_4) [Bornology α] : Prop

• bounded_univ : Bornology.IsBounded Set.univ

  The Set.univ is bounded.

A space with a Bornology is a bounded space if Set.univ : Set α is bounded.

theorem Bornology.isBounded_univ {α : Type u_2} [Bornology α] : Bornology.IsBounded Set.univ ↔ BoundedSpace α

theorem Bornology.cobounded_eq_bot_iff {α : Type u_2} [Bornology α] : Bornology.cobounded α = ⊥ ↔ BoundedSpace α

theorem Bornology.IsBounded.all {α : Type u_2} [Bornology α] [BoundedSpace α] (s : Set α) : Bornology.IsBounded s

theorem Bornology.IsCobounded.all {α : Type u_2} [Bornology α] [BoundedSpace α] (s : Set α) : Bornology.IsCobounded s

@[simp]

theorem Bornology.cobounded_eq_bot (α : Type u_2) [Bornology α] [BoundedSpace α] : Bornology.cobounded α = ⊥