Pointed endofunctors on categories

Content created by Egbert Rijke.

Created on 2024-02-08.
Last modified on 2024-02-08.

module category-theory.pointed-endofunctors-categories where
open import category-theory.categories
open import category-theory.functors-categories
open import category-theory.natural-transformations-functors-categories
open import category-theory.pointed-endofunctors-precategories

open import foundation.dependent-pair-types
open import foundation.identity-types
open import foundation.universe-levels


An endofunctor F : C → C on a category C is said to be pointed if it comes equipped with a natural transformation id ⇒ F from the identity functor to F.

More explicitly, a pointing of an endofunctor F : C → C consists of a family of morphisms η X : X → F X such that for each morphism f : X → Y in C the diagram

       η X
    X -----> F X
    |         |
  f |         | F f
    ∨         ∨
    Y -----> F Y
       η Y



The structure of a pointing on an endofunctor on a category

module _
  {l1 l2 : Level} (C : Category l1 l2) (T : functor-Category C C)

  pointing-endofunctor-Category : UU (l1  l2)
  pointing-endofunctor-Category =
    pointing-endofunctor-Precategory (precategory-Category C) T

Pointed endofunctors on a category

module _
  {l1 l2 : Level} (C : Category l1 l2)

  pointed-endofunctor-Category : UU (l1  l2)
  pointed-endofunctor-Category =
    pointed-endofunctor-Precategory (precategory-Category C)

  module _
    (F : pointed-endofunctor-Category)

    functor-pointed-endofunctor-Category :
      functor-Category C C
    functor-pointed-endofunctor-Category =
      functor-pointed-endofunctor-Precategory (precategory-Category C) F

    obj-pointed-endofunctor-Category : obj-Category C  obj-Category C
    obj-pointed-endofunctor-Category =
      obj-pointed-endofunctor-Precategory (precategory-Category C) F

    hom-pointed-endofunctor-Category :
      {X Y : obj-Category C} 
      hom-Category C X Y 
      hom-Category C
        ( obj-pointed-endofunctor-Category X)
        ( obj-pointed-endofunctor-Category Y)
    hom-pointed-endofunctor-Category =
      hom-pointed-endofunctor-Precategory (precategory-Category C) F

    preserves-id-pointed-endofunctor-Category :
      (X : obj-Category C) 
      hom-pointed-endofunctor-Category (id-hom-Category C {X}) 
      id-hom-Category C
    preserves-id-pointed-endofunctor-Category =
      preserves-id-pointed-endofunctor-Precategory (precategory-Category C) F

    preserves-comp-pointed-endofunctor-Category :
      {X Y Z : obj-Category C}
      (g : hom-Category C Y Z) (f : hom-Category C X Y) 
        ( comp-hom-Category C g f) 
      comp-hom-Category C
        ( hom-pointed-endofunctor-Category g)
        ( hom-pointed-endofunctor-Category f)
    preserves-comp-pointed-endofunctor-Category =
      preserves-comp-pointed-endofunctor-Precategory (precategory-Category C) F

    pointing-pointed-endofunctor-Category :
      natural-transformation-Category C C
        ( id-functor-Category C)
        ( functor-pointed-endofunctor-Category)
    pointing-pointed-endofunctor-Category =
      pointing-pointed-endofunctor-Precategory (precategory-Category C) F

    hom-family-pointing-pointed-endofunctor-Category :
      hom-family-functor-Category C C
        ( id-functor-Category C)
        ( functor-pointed-endofunctor-Category)
    hom-family-pointing-pointed-endofunctor-Category =
        ( precategory-Category C)
        ( F)

    naturality-pointing-pointed-endofunctor-Category :
      is-natural-transformation-Category C C
        ( id-functor-Category C)
        ( functor-pointed-endofunctor-Category)
        ( hom-family-pointing-pointed-endofunctor-Category)
    naturality-pointing-pointed-endofunctor-Category =
        ( precategory-Category C) F

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