This is an archived version pinned as of the submission of my master's thesis. An up-to-date version may be found online.

Sieves in categories

module category-theory.sieves-in-categories where

open import category-theory.categories

open import foundation.propositions
open import foundation.subtypes
open import foundation.universe-levels

Idea

A sieve S on an object X in a category C is a collection of morphisms into X which is closed under precomposition by arbitrary morphisms of C. In other words, for any morphism f : Y → X in S and any morphism g : Z → Y in C, the morphism f ∘ g : Z → X is in S.

The notion of sieve generalizes simultaneously the notion of right ideal in a monoid (a one-object category) and a lower set in a poset (a category with at most one morphism between any two objects).

Definition

module _
  {l1 l2 : Level} (C : Category l1 l2) (A : obj-Category C)
  where

  is-sieve-prop-Category :
    { l3 : Level}
    ( S : (X Y : obj-Category C) →
    subtype l3 (hom-Category C X Y)) → Prop (l1 ⊔ l2 ⊔ l3)
  is-sieve-prop-Category S =
    Π-Prop
      ( obj-Category C)
      ( λ X →
        Π-Prop
          ( obj-Category C)
          ( λ Y →
            Π-Prop
              ( obj-Category C)
              ( λ Z →
                Π-Prop
                  ( type-subtype (S Y X))
                  ( λ f →
                    Π-Prop
                      ( hom-Category C Z Y)
                      ( λ g →
                        S Z X
                          ( comp-hom-Category
                              C (inclusion-subtype (S Y X) f) g))))))

  is-sieve-Category :
    { l3 : Level}
    ( S : (X Y : obj-Category C) →
    subtype l3 (hom-Category C X Y)) → UU (l1 ⊔ l2 ⊔ l3)
  is-sieve-Category S = type-Prop (is-sieve-prop-Category S)

  is-prop-is-sieve-Category :
    { l3 : Level}
    ( S : (X Y : obj-Category C) → subtype l3 (hom-Category C X Y)) →
    is-prop (is-sieve-Category S)
  is-prop-is-sieve-Category S = is-prop-type-Prop (is-sieve-prop-Category S)