module Cat.Functor.FullSubcategory {o h} {C : Precategory o h} where

Full subcategories🔗

A full subcategory $D$ of some larger category $C$ is the category generated by some predicate $P$ on the objects of of $C :$ You keep only those objects for which $P$ holds, and all the morphisms between them. An example is the category of abelian groups, as a full subcategory of groups: being abelian is a proposition (there’s “at most one way for a group to be abelian”).

We can interpret full subcategories, by analogy, as being the “induced subgraphs” of the categorical world: Keep only some of the vertices (objects), but all of the arrows (arrows) between them.

Restrict : (P : C.Ob → Type ℓ) → Precategory (o ⊔ ℓ) h
Restrict P .Ob = Σ[ O ∈ C ] (P O)
Restrict P .Hom A B = C.Hom (A .fst) (B .fst)
Restrict P .Hom-set _ _ = C.Hom-set _ _
Restrict P .id    = C.id
Restrict P ._∘_   = C._∘_
Restrict P .idr   = C.idr
Restrict P .idl   = C.idl
Restrict P .assoc = C.assoc

A very important property of full subcategories (Restrictions) is that any full subcategory of a univalent category is univalent. The argument is roughly as follows: Since $C$ is univalent, an isomorphism $A ≅ B$ gives us a path $A \equiv B,$ so in particular if we know $A ≅ B$ and $P (A),$ then we have $P (B) .$ But, since the morphisms in the full subcategory coincide with those of $C,$ any iso in the subcategory is an iso in $C,$ thus a path!

module _ (P : C.Ob → Type ℓ) where
  import Cat.Reasoning (Restrict P) as R

We begin by translating between isomorphisms in the subcategory (called $R$ here) and in $C,$ which can be done by destructuring and reassembling:

  sub-iso→super-iso : ∀ {A B : Σ _ P} → (A R.≅ B) → (A .fst C.≅ B .fst)
  sub-iso→super-iso x = C.make-iso x.to x.from x.invl x.invr
    where module x = R._≅_ x

  super-iso→sub-iso : ∀ {A B : Σ _ P} → (A .fst C.≅ B .fst) → (A R.≅ B)
  super-iso→sub-iso y = R.make-iso y.to y.from y.invl y.invr
    where module y = C._≅_ y

module _ (P : C.Ob → Type ℓ) (pprop : ∀ x → is-prop (P x))
  where
  import Cat.Reasoning (Restrict P) as R

We then prove that object-isomorphism pairs in the subcategory (i.e. inhabitants of $\sum_{B : R} (A ≅ B))$ coincide with those in the supercategory; Hence, since $C$ is by assumption univalent, so is $R .$

  Restrict-is-category : is-category C → is-category (Restrict P)
  Restrict-is-category cids = λ where
    .to-path im i .fst → Univalent.iso→path cids (sub-iso→super-iso P im) i
    .to-path {a = a} {b = b} im i .snd → is-prop→pathp
      (λ i → pprop (cids .to-path (sub-iso→super-iso P im) i))
      (a .snd) (b .snd) i
    .to-path-over p → R.≅-pathp _ _ λ i → cids .to-path-over (sub-iso→super-iso P p) i .C.to

From full inclusions🔗

There is another way of representing full subcategories: By giving a full inclusion, i.e. a fully faithful functor $F : D \to C .$ Each full inclusion canonically determines a full subcategory of $C,$ namely that consisting of the objects in $C$ merely in the image of $F .$ This category is often referred to as the essential image of $F .$

module _ {o' h'} {D : Precategory o' h'} (F : Functor D C) where
  open Functor F

  Essential-image : Precategory _ _
  Essential-image =
    Restrict (λ x → ∃[ d ∈ Ob D ] (F₀ d C.≅ x))

There is a canonical inclusion of $D$ into the essential image of $F$ that is essentially surjective. Moreover, this inclusion is a weak equivalence if $F$ is fully faithful.

  Essential-inc : Functor D Essential-image
  Essential-inc .Functor.F₀ x = F₀ x , inc (x , C.id-iso)
  Essential-inc .Functor.F₁ = F₁
  Essential-inc .Functor.F-id = F-id
  Essential-inc .Functor.F-∘ = F-∘

  Essential-inc-eso : is-eso Essential-inc
  Essential-inc-eso yo =
    ∥-∥-map (λ (preimg , isom) → preimg , super-iso→sub-iso _ isom)
      (yo .snd)

  ff→Essential-inc-ff : is-fully-faithful F → is-fully-faithful Essential-inc
  ff→Essential-inc-ff ff = ff

Up to weak equivalence, admitting a full inclusion is equivalent to being a full subcategory: Every full subcategory admits a full inclusion, given on objects by projecting the first component and on morphisms by the identity function.

module _ {P : C.Ob → Type ℓ} where
  Forget-full-subcat : Functor (Restrict P) C
  Forget-full-subcat .Functor.F₀ = fst
  Forget-full-subcat .Functor.F₁ f = f
  Forget-full-subcat .Functor.F-id = refl
  Forget-full-subcat .Functor.F-∘ f g i = f C.∘ g

  Forget-full-subcat-is-ff : is-fully-faithful Forget-full-subcat
  Forget-full-subcat-is-ff = id-equiv

From families of objects🔗

Finally, we can construct a full subcategory by giving a family of objects $X_{i} : I \to O b$ of $C$ by forming a modified version of $C$ whose objects have been replaced by elements of $I .$

module _ {ℓi} {Idx : Type ℓi} (Xᵢ : Idx → C.Ob) where
  Family : Precategory ℓi h
  Family .Ob = Idx
  Family .Hom i j = C.Hom (Xᵢ i) (Xᵢ j)
  Family .Hom-set _ _ = hlevel 2
  Family .id = C.id
  Family ._∘_ = C._∘_
  Family .idr = C.idr
  Family .idl = C.idl
  Family .assoc = C.assoc

There is an evident functor from $X_{i} \to C$ that takes each $i$ to $X_{i} .$

  Forget-family : Functor Family C
  Forget-family .Functor.F₀ = Xᵢ
  Forget-family .Functor.F₁ f = f
  Forget-family .Functor.F-id = refl
  Forget-family .Functor.F-∘ _ _ = refl

  Forget-family-ff : is-fully-faithful Forget-family
  Forget-family-ff = id-equiv