module Cat.Instances.Elements where

The category of elements🔗

The (contravariant¹) category of elements of a presheaf $P : C^{op} \to Sets$ is a means of unpacking the data of the presheaf. Its objects are pairs of an object $x,$ and a section $s : P x .$

  record Element : Type (o ⊔ s) where
    constructor elem
    field
      ob : Ob
      section : P ʻ ob

  open Element

We can think of this as taking an eraser to the data of $P .$ If $P (x) = {x_{0}, x_{1}, x_{2}},$ then the category of elements of $P$ will have three objects in place of the one: $(x, x_{0}),$ $(x, x_{1}),$ and $(x, x_{2}) .$ We’ve erased all the boundaries of each of the sets associated with $P,$ and are left with a big soup of points.

We do something similar for morphisms, and turn functions $P (f) : P (Y) \to P (X)$ into a huge collection of morphisms between points. We do this by defining a morphism $(x, x_{0}) \to (y, y_{0})$ to be a morphism $f : X \to Y$ in $C,$ as well as a proof that $P (f) (y_{0}) = x_{0}$ This too can be seen as erasing boundaries, just this time with the data associated with a function. Instead of having a bunch of data bundled together that describes the action of $P (f)$ on each point of $P (Y),$ we have a bunch of tiny bits of data that only describe the action of $P (f)$ on a single point.

  record Element-hom (x y : Element) : Type (ℓ ⊔ s) where
    constructor elem-hom
    no-eta-equality
    field
      hom : Hom (x .ob) (y .ob)
      commute : P.₁ hom (y .section) ≡ x .section

  open Element-hom

As per usual, we need to prove some helper lemmas that describe the path space of Element-hom.

  Element-hom-path : {x y : Element} {f g : Element-hom x y} → f .hom ≡ g .hom → f ≡ g
  Element-hom-path p i .hom = p i
  Element-hom-path {x = x} {y = y} {f = f} {g = g} p i .commute =
    is-prop→pathp (λ j → P.₀ (x .ob) .is-tr (P.₁ (p j) (y .section)) (x .section))
      (f .commute)
      (g .commute) i

One interesting fact is that morphisms $f : X \to Y$ in $C$ induce morphisms in the category of elements for each $p y : P (y) .$

  induce : ∀ {x y} (f : Hom x y) (py : P ʻ y)
        → Element-hom C P (elem x (P.₁ f py)) (elem y py)
  induce f _ = elem-hom f refl

Using all this machinery, we can now define the category of elements of $P!$

  ∫ : Precategory (o ⊔ s) (ℓ ⊔ s)
  ∫ .Precategory.Ob = Element C P
  ∫ .Precategory.Hom = Element-hom C P
  ∫ .Precategory.Hom-set _ _ = hlevel 2
  ∫ .Precategory.id {x = x} = elem-hom id λ i → P.F-id i (x .section)
  ∫ .Precategory._∘_ {x = x} {y = y} {z = z} f g = elem-hom (f .hom ∘ g .hom) comm where abstract
    comm : P.₁ (f .hom ∘ g .hom) (z .section) ≡ x .section
    comm =
      P.₁ (f .hom ∘ g .hom) (z .section)       ≡⟨ happly (P.F-∘ (g .hom) (f .hom)) (z .section) ⟩
      P.₁ (g .hom) (P.₁ (f .hom) (z .section)) ≡⟨ ap (P.F₁ (g .hom)) (f .commute)  ⟩
      P.₁ (g .hom) (y .section)                ≡⟨ g .commute ⟩
      x .section ∎
  ∫ .Precategory.idr f = ext (idr _)
  ∫ .Precategory.idl f = ext (idl _)
  ∫ .Precategory.assoc f g h = ext (assoc _ _ _)

Projection🔗

The category of elements is equipped with a canonical projection functor $π_{p} : \int P \to C,$ which just forgets all of the points and morphism actions.

  πₚ : Functor ∫ C
  πₚ .F₀ x = x .ob
  πₚ .F₁ f = f .hom
  πₚ .F-id = refl
  πₚ .F-∘ f g = refl

This functor makes it clear that we ought to think of the category of elements as something defined over $C .$ For instance, if we look at the fibre over each $X : C,$ we get back the set $P (X)!$

there is a separate covariant category of elements for covariant functors↩︎