module Cat.Functor.Pullback
  {o ℓ} {C : Precategory o ℓ}
  where

Base change🔗

Let $\mathcal{C}$ be a category with all pullbacks, and $f : Y \to X$ a morphism in $\mathcal{C}$ . Then we have a functor $f* : \mathcal{C}/X \to \mathcal{C}/Y$ , called the base change, where the action on objects is given by pulling back along $f$ .

On objects, the functor maps as in the diagram below. It’s a bit busy, so look at it in parts: On the left we have the object $K \xrightarrow{g} X$ of $\mathcal{C}/X$ , and on the right we have the whole pullback diagram, showing how the parts fit together. The actual object of $\mathcal{C}/Y$ the functor gives is the vertical arrow $Y \times_X K \to Y$ .

module _ (pullbacks : ∀ {X Y Z} f g → Pullback C {X} {Y} {Z} f g) {X Y : Ob} (f : Hom Y X) where
  Base-change : Functor (Slice C X) (Slice C Y)
  Base-change .F₀ x = ob where
    ob : /-Obj Y
    ob .domain = pullbacks (x .map) f .apex
    ob .map    = pullbacks (x .map) f .p₂

On morphisms, we use the universal property of the pullback to obtain a map $Y \times_X K \to Y \times_X K'$ , by observing that the square diagram below is a cone over $K' \to X \leftarrow Y$ .

  Base-change .F₁ {x} {y} dh = dh' where
    module ypb = Pullback (pullbacks (y .map) f)
    module xpb = Pullback (pullbacks (x .map) f)
    dh' : /-Hom _ _
    dh' .map = ypb.universal {p₁' = dh .map ∘ xpb.p₁}
      (pulll (dh .commutes) ∙ xpb.square)
    dh' .commutes = ypb.p₂∘universal

The properties of pullbacks also guarantee that this operation is functorial, but the details are not particularly enlightening.

  Base-change .F-id {x} = ext (sym (xpb.unique id-comm (idr _)))
    where module xpb = Pullback (pullbacks (x .map) f)

  Base-change .F-∘ {x} {y} {z} am bm =
    ext (sym (zpb.unique
      (pulll zpb.p₁∘universal ∙ pullr ypb.p₁∘universal ∙ assoc _ _ _)
      (pulll zpb.p₂∘universal ∙ ypb.p₂∘universal)))
    where
      module ypb = Pullback (pullbacks (y .map) f)
      module zpb = Pullback (pullbacks (z .map) f)

Properties🔗

The base change functor is a right adjoint. We construct the left adjoint directly, then give the unit and counit, and finally prove the triangle identities.

The left adjoint, called dependent sum and written $\sum_f$ , is given on objects by precomposition with $f$ , and on morphisms by what is essentially the identity function — only the witness of commutativity must change.

module _ {X Y : Ob} (f : Hom Y X) where
  Σf : Functor (Slice C Y) (Slice C X)
  Σf .F₀ o = cut (f ∘ o .map)
  Σf .F₁ dh = record { map = dh .map ; commutes = pullr (dh .commutes) }
  Σf .F-id = trivial!
  Σf .F-∘ f g = trivial!

  open _⊣_
  open _=>_

The adjunction unit and counit are given by the universal properties of pullbacks. ⚠️ WIP ⚠️

module _ (pullbacks : ∀ {X Y Z} f g → Pullback C {X} {Y} {Z} f g) {X Y : Ob} (f : Hom Y X) where
  open _⊣_
  open _=>_

  Σf⊣f* : Σf f ⊣ Base-change pullbacks f
  Σf⊣f* .unit .η obj = dh where
    module pb = Pullback (pullbacks (f ∘ obj .map) f)
    dh : /-Hom _ _
    dh .map = pb.universal {p₁' = id} {p₂' = obj .map} (idr _)
    dh .commutes = pb.p₂∘universal
  Σf⊣f* .unit .is-natural x y g =
    ext (pb.unique₂
      {p = (f ∘ y .map) ∘ id ∘ g .map ≡⟨ cat! C ⟩ f ∘ y .map ∘ g .map ∎}
      (pulll pb.p₁∘universal)
      (pulll pb.p₂∘universal)
      (pulll pb.p₁∘universal ∙ pullr pb'.p₁∘universal ∙ id-comm)
      (pulll pb.p₂∘universal ∙ pb'.p₂∘universal ∙ sym (g .commutes)))
    where
      module pb = Pullback (pullbacks (f ∘ y .map) f)
      module pb' = Pullback (pullbacks (f ∘ x .map) f)

  Σf⊣f* .counit .η obj = dh where
    module pb = Pullback (pullbacks (obj .map) f)
    dh : /-Hom _ _
    dh .map = pb.p₁
    dh .commutes = pb.square
  Σf⊣f* .counit .is-natural x y g = ext pb.p₁∘universal
    where module pb = Pullback (pullbacks (y .map) f)

  Σf⊣f* .zig {A} = ext pb.p₁∘universal
    where module pb = Pullback (pullbacks (f ∘ A .map) f)

  Σf⊣f* .zag {B} = ext
    (sym (pb.unique₂ {p = pb.square}
      (idr _) (idr _)
      (pulll pb.p₁∘universal ∙ pullr pb'.p₁∘universal ∙ idr _)
      (pulll pb.p₂∘universal ∙ pb'.p₂∘universal))) where
    module pb = Pullback (pullbacks (B .map) f)
    module pb' = Pullback (pullbacks (f ∘ pb.p₂) f)