open import Cat.Displayed.Cartesian
open import Cat.Displayed.Fibre
open import Cat.Displayed.Total
open import Cat.Displayed.Base
open import Cat.Prelude

import Cat.Functor.Reasoning
import Cat.Reasoning

module Cat.Displayed.Cartesian.Street where

Street fibrations🔗

In classical category theory, a fibration is defined to be a certain functor P:EBP : \mathcal{E} \to \mathcal{B}, the idea being that E\mathcal{E} is really the total space of a certain displayed category, and PP is really the first projection functor πf\pi^f, which sends each displayed object to the object it is displayed over. But can we go the other way? If we have a functor P:EBP : \mathcal{E} \to \mathcal{B}, can we create a category displayed E\mathcal{E}' over B\mathcal{B}, such that EE\int \mathcal{E}' \cong \mathcal{E}?

  functor→displayed : Displayed B (o  ℓ′) (  ℓ′)
  functor→displayed .Ob[_] x = Σ[ u  E.Ob ] (P.₀ u B.≅ x)

Following (Sterling and Angiuli 2022), we define such a category by defining the space of objects over xx to be “left corners”. Strictly speaking, a left corner is given by an object uu together with an isomorphism ϕ:P(u)x\phi : P(u) \cong x. But visually, we depict them as

whence the name “left corner”. The maps lying over ff consist of those maps ufvu \to_f v which commute with the mediating isomorphisms:

  functor→displayed .Hom[_] f (u , φ) (v , ψ) =
    Σ[ h  E.Hom u v ] (B.to ψ B.∘ P.₁ h  f B.∘ B.to φ)

This fits in a diagram like the one below. Note that the commutativity condition is for the lower shape, which is a distorted square.

The axioms for a displayed category are evident: all that matters are the maps in the total category E\mathcal{E}, since the rest of the data is property (rather than data).

  functor→displayed .Hom[_]-set f a b = hlevel 2
  functor→displayed .id′ = E.id , B.elimr P.F-id  B.introl refl
  functor→displayed ._∘′_ (f , φ) (g , ψ) = f E.∘ g ,
    ap₂ B._∘_ refl (P.F-∘ f g)  B.pulll φ  B.pullr ψ  B.assoc _ _ _
  functor→displayed .idr′ f′ = Σ-pathp-dep (E.idr _) $
    is-set→squarep  _ _  hlevel 2) _ _ _ _
  functor→displayed .idl′ f′ = Σ-pathp-dep (E.idl _) $
    is-set→squarep  _ _  hlevel 2) _ _ _ _
  functor→displayed .assoc′ f′ g′ h′ = Σ-pathp-dep (E.assoc _ _ _) $
    is-set→squarep  _ _  hlevel 2) _ _ _ _

We call a functor that gives rise to a Cartesian fibration through this process a Street fibration. It is routine to verify that our notion of Street fibration corresponds to.. well, Street’s.


References