Epimorphism in large precategories

Content created by Egbert Rijke and Fredrik Bakke.

Created on 2023-09-13.
Last modified on 2023-11-24.

module category-theory.epimorphisms-in-large-precategories where
Imports 
open import category-theory.isomorphisms-in-large-precategories
open import category-theory.large-precategories

open import foundation.embeddings
open import foundation.equivalences
open import foundation.propositions
open import foundation.universe-levels

Idea

A morphism f : x → y is a epimorphism if whenever we have two morphisms g h : y → w such that g ∘ f = h ∘ f, then in fact g = h. The way to state this in Homotopy Type Theory is to say that precomposition by f is an embedding.

Definition

module _
  {α : Level → Level} {β : Level → Level → Level}
  (C : Large-Precategory α β) {l1 l2 : Level} (l3 : Level)
  (X : obj-Large-Precategory C l1) (Y : obj-Large-Precategory C l2)
  (f : hom-Large-Precategory C X Y)
  where

  is-epi-prop-Large-Precategory : Prop (α l3 ⊔ β l1 l3 ⊔ β l2 l3)
  is-epi-prop-Large-Precategory =
    Π-Prop
      ( obj-Large-Precategory C l3)
      ( λ Z → is-emb-Prop (λ g → comp-hom-Large-Precategory C {Z = Z} g f))

  is-epi-Large-Precategory : UU (α l3 ⊔ β l1 l3 ⊔ β l2 l3)
  is-epi-Large-Precategory = type-Prop is-epi-prop-Large-Precategory

  is-prop-is-epi-Large-Precategory : is-prop is-epi-Large-Precategory
  is-prop-is-epi-Large-Precategory =
    is-prop-type-Prop is-epi-prop-Large-Precategory

Properties

Isomorphisms are epimorphisms

module _
  {α : Level → Level} {β : Level → Level → Level}
  (C : Large-Precategory α β) {l1 l2 : Level} (l3 : Level)
  (X : obj-Large-Precategory C l1) (Y : obj-Large-Precategory C l2)
  (f : iso-Large-Precategory C X Y)
  where

  is-epi-iso-Large-Precategory :
    is-epi-Large-Precategory C l3 X Y (hom-iso-Large-Precategory C f)
  is-epi-iso-Large-Precategory Z =
    is-emb-is-equiv (is-equiv-precomp-hom-iso-Large-Precategory C f Z)

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