Precategories

Content created by Fredrik Bakke, Egbert Rijke, Jonathan Prieto-Cubides, Elisabeth Stenholm, Julian KG, fernabnor and louismntnu.

Created on 2022-03-11.
Last modified on 2024-03-11.

module category-theory.precategories where
Imports 
open import category-theory.composition-operations-on-binary-families-of-sets
open import category-theory.nonunital-precategories
open import category-theory.set-magmoids

open import foundation.cartesian-product-types
open import foundation.dependent-pair-types
open import foundation.function-types
open import foundation.identity-types
open import foundation.propositions
open import foundation.sets
open import foundation.strictly-involutive-identity-types
open import foundation.truncated-types
open import foundation.truncation-levels
open import foundation.universe-levels

Idea

A precategory 𝒞 in Homotopy Type Theory is the structure of an associative and unital composition operation on a binary familiy of sets.

This means a precategory consists of:

  • Objects. A type Ob 𝒞 of objects.
  • Morphisms. For each pair of objects x y : Ob 𝒞, a set of morphisms hom 𝒞 x y : Set.
  • Composition. For every triple of objects x y z : Ob 𝒞 there is a composition operation on morphisms 
      _∘_ : hom 𝒞 y z → hom 𝒞 x y → hom 𝒞 x z.
  • Associativity. For every triple of composable morphisms, we have 
      (h ∘ g) ∘ f ＝ h ∘ (g ∘ f).
  • Identity morphisms. For every object x : Ob 𝒞, there is a distinguished identity morphism id_x : hom 𝒞 x x.
  • Unitality. The identity morphisms are two-sided units for the composition operation, meaning that for every f : hom 𝒞 x y we have 
      id_y ∘ f ＝ f   and   f ∘ id_x ＝ f.

Note. The reason this is called a precategory and not a category in Homotopy Type Theory is that we reserve that name for precategories where the identity types of the type of objects are characterized by the isomorphism sets.

Definitions

The predicate on composition operations on binary families of sets of being a precategory

module _
  {l1 l2 : Level} {A : UU l1}
  (hom-set : A → A → Set l2)
  (comp-hom : composition-operation-binary-family-Set hom-set)
  where

  is-precategory-prop-composition-operation-binary-family-Set : Prop (l1 ⊔ l2)
  is-precategory-prop-composition-operation-binary-family-Set =
    product-Prop
      ( is-unital-prop-composition-operation-binary-family-Set hom-set comp-hom)
      ( is-associative-prop-composition-operation-binary-family-Set
        ( hom-set)
        ( comp-hom))

  is-precategory-composition-operation-binary-family-Set : UU (l1 ⊔ l2)
  is-precategory-composition-operation-binary-family-Set =
    type-Prop is-precategory-prop-composition-operation-binary-family-Set

  is-prop-is-precategory-composition-operation-binary-family-Set :
    is-prop is-precategory-composition-operation-binary-family-Set
  is-prop-is-precategory-composition-operation-binary-family-Set =
    is-prop-type-Prop
      is-precategory-prop-composition-operation-binary-family-Set

The type of precategories

Precategory :
  (l1 l2 : Level) → UU (lsuc l1 ⊔ lsuc l2)
Precategory l1 l2 =
  Σ ( UU l1)
    ( λ A →
      Σ ( A → A → Set l2)
        ( λ hom-set →
          Σ ( associative-composition-operation-binary-family-Set hom-set)
            ( λ (comp-hom , assoc-comp) →
              is-unital-composition-operation-binary-family-Set
                ( hom-set)
                ( comp-hom))))

make-Precategory :
  { l1 l2 : Level}
  ( obj : UU l1)
  ( hom-set : obj → obj → Set l2)
  ( _∘_ : composition-operation-binary-family-Set hom-set)
  ( id : (x : obj) → type-Set (hom-set x x))
  ( assoc-comp-hom :
    { x y z w : obj} →
    ( h : type-Set (hom-set z w))
    ( g : type-Set (hom-set y z))
    ( f : type-Set (hom-set x y)) →
    ( (h ∘ g) ∘ f ＝ h ∘ (g ∘ f)))
  ( left-unit-comp-hom :
    { x y : obj} (f : type-Set (hom-set x y)) → id y ∘ f ＝ f)
  ( right-unit-comp-hom :
    { x y : obj} (f : type-Set (hom-set x y)) → f ∘ id x ＝ f) →
  Precategory l1 l2
make-Precategory
  obj hom-set _∘_ id assoc-comp-hom left-unit-comp-hom right-unit-comp-hom =
  ( ( obj) ,
    ( hom-set) ,
    ( _∘_ , (λ h g f → involutive-eq-eq (assoc-comp-hom h g f))) ,
    ( id) ,
    ( left-unit-comp-hom) ,
    ( right-unit-comp-hom))

{-# INLINE make-Precategory #-}

module _
  {l1 l2 : Level} (C : Precategory l1 l2)
  where

  obj-Precategory : UU l1
  obj-Precategory = pr1 C

  hom-set-Precategory : (x y : obj-Precategory) → Set l2
  hom-set-Precategory = pr1 (pr2 C)

  hom-Precategory : (x y : obj-Precategory) → UU l2
  hom-Precategory x y = type-Set (hom-set-Precategory x y)

  is-set-hom-Precategory :
    (x y : obj-Precategory) → is-set (hom-Precategory x y)
  is-set-hom-Precategory x y = is-set-type-Set (hom-set-Precategory x y)

  associative-composition-operation-Precategory :
    associative-composition-operation-binary-family-Set hom-set-Precategory
  associative-composition-operation-Precategory = pr1 (pr2 (pr2 C))

  comp-hom-Precategory :
    {x y z : obj-Precategory} →
    hom-Precategory y z →
    hom-Precategory x y →
    hom-Precategory x z
  comp-hom-Precategory =
    comp-hom-associative-composition-operation-binary-family-Set
      ( hom-set-Precategory)
      ( associative-composition-operation-Precategory)

  comp-hom-Precategory' :
    {x y z : obj-Precategory} →
    hom-Precategory x y →
    hom-Precategory y z →
    hom-Precategory x z
  comp-hom-Precategory' f g = comp-hom-Precategory g f

  involutive-eq-associative-comp-hom-Precategory :
    {x y z w : obj-Precategory}
    (h : hom-Precategory z w)
    (g : hom-Precategory y z)
    (f : hom-Precategory x y) →
    ( comp-hom-Precategory (comp-hom-Precategory h g) f) ＝ⁱ
    ( comp-hom-Precategory h (comp-hom-Precategory g f))
  involutive-eq-associative-comp-hom-Precategory =
    involutive-eq-associative-composition-operation-binary-family-Set
      ( hom-set-Precategory)
      ( associative-composition-operation-Precategory)

  associative-comp-hom-Precategory :
    {x y z w : obj-Precategory}
    (h : hom-Precategory z w)
    (g : hom-Precategory y z)
    (f : hom-Precategory x y) →
    ( comp-hom-Precategory (comp-hom-Precategory h g) f) ＝
    ( comp-hom-Precategory h (comp-hom-Precategory g f))
  associative-comp-hom-Precategory =
    witness-associative-composition-operation-binary-family-Set
      ( hom-set-Precategory)
      ( associative-composition-operation-Precategory)

  is-unital-composition-operation-Precategory :
    is-unital-composition-operation-binary-family-Set
      ( hom-set-Precategory)
      ( comp-hom-Precategory)
  is-unital-composition-operation-Precategory = pr2 (pr2 (pr2 C))

  id-hom-Precategory : {x : obj-Precategory} → hom-Precategory x x
  id-hom-Precategory {x} = pr1 is-unital-composition-operation-Precategory x

  left-unit-law-comp-hom-Precategory :
    {x y : obj-Precategory} (f : hom-Precategory x y) →
    comp-hom-Precategory id-hom-Precategory f ＝ f
  left-unit-law-comp-hom-Precategory =
    pr1 (pr2 is-unital-composition-operation-Precategory)

  right-unit-law-comp-hom-Precategory :
    {x y : obj-Precategory} (f : hom-Precategory x y) →
    comp-hom-Precategory f id-hom-Precategory ＝ f
  right-unit-law-comp-hom-Precategory =
    pr2 (pr2 is-unital-composition-operation-Precategory)

The underlying nonunital precategory of a precategory

module _
  {l1 l2 : Level} (C : Precategory l1 l2)
  where

  nonunital-precategory-Precategory : Nonunital-Precategory l1 l2
  pr1 nonunital-precategory-Precategory = obj-Precategory C
  pr1 (pr2 nonunital-precategory-Precategory) = hom-set-Precategory C
  pr2 (pr2 nonunital-precategory-Precategory) =
    associative-composition-operation-Precategory C

The underlying set-magmoid of a precategory

module _
  {l1 l2 : Level} (C : Precategory l1 l2)
  where

  set-magmoid-Precategory : Set-Magmoid l1 l2
  set-magmoid-Precategory =
    set-magmoid-Nonunital-Precategory (nonunital-precategory-Precategory C)

The total hom-type of a precategory

total-hom-Precategory :
  {l1 l2 : Level} (C : Precategory l1 l2) → UU (l1 ⊔ l2)
total-hom-Precategory C =
  total-hom-Nonunital-Precategory (nonunital-precategory-Precategory C)

obj-total-hom-Precategory :
  {l1 l2 : Level} (C : Precategory l1 l2) →
  total-hom-Precategory C → obj-Precategory C × obj-Precategory C
obj-total-hom-Precategory C =
  obj-total-hom-Nonunital-Precategory (nonunital-precategory-Precategory C)

Equalities induce morphisms

module _
  {l1 l2 : Level} (C : Precategory l1 l2)
  where

  hom-eq-Precategory :
    (x y : obj-Precategory C) → x ＝ y → hom-Precategory C x y
  hom-eq-Precategory x .x refl = id-hom-Precategory C

  hom-inv-eq-Precategory :
    (x y : obj-Precategory C) → x ＝ y → hom-Precategory C y x
  hom-inv-eq-Precategory x y = hom-eq-Precategory y x ∘ inv

Pre- and postcomposition by a morphism

module _
  {l1 l2 : Level} (C : Precategory l1 l2)
  {x y : obj-Precategory C}
  (f : hom-Precategory C x y)
  (z : obj-Precategory C)
  where

  precomp-hom-Precategory : hom-Precategory C y z → hom-Precategory C x z
  precomp-hom-Precategory g = comp-hom-Precategory C g f

  postcomp-hom-Precategory : hom-Precategory C z x → hom-Precategory C z y
  postcomp-hom-Precategory = comp-hom-Precategory C f

Properties

If the objects of a precategory are k-truncated for nonnegative k, the total hom-type is k-truncated

module _
  {l1 l2 : Level} {k : 𝕋} (C : Precategory l1 l2)
  where

  is-trunc-total-hom-is-trunc-obj-Precategory :
    is-trunc (succ-𝕋 (succ-𝕋 k)) (obj-Precategory C) →
    is-trunc (succ-𝕋 (succ-𝕋 k)) (total-hom-Precategory C)
  is-trunc-total-hom-is-trunc-obj-Precategory =
    is-trunc-total-hom-is-trunc-obj-Nonunital-Precategory
      ( nonunital-precategory-Precategory C)

  total-hom-truncated-type-is-trunc-obj-Precategory :
    is-trunc (succ-𝕋 (succ-𝕋 k)) (obj-Precategory C) →
    Truncated-Type (l1 ⊔ l2) (succ-𝕋 (succ-𝕋 k))
  total-hom-truncated-type-is-trunc-obj-Precategory =
    total-hom-truncated-type-is-trunc-obj-Nonunital-Precategory
      ( nonunital-precategory-Precategory C)

See also

Recent changes