Localizations at maps

Content created by Fredrik Bakke and Egbert Rijke.

Created on 2023-06-28.
Last modified on 2023-11-01.

module orthogonal-factorization-systems.localizations-maps where
open import foundation.cartesian-product-types
open import foundation.dependent-pair-types
open import foundation.universe-levels

open import orthogonal-factorization-systems.local-types
open import orthogonal-factorization-systems.localizations-subuniverses


Let f be a map of type A → B and let X be a type. The localization of X at f, or f-localization, is an f-local type Y together with a map η : X → Y with the property that every type that is f-local is also η-local.


The predicate of being a localization at a map

is-localization :
  {l1 l2 l3 l4 : Level} (l5 : Level)
  {A : UU l1} {B : UU l2} (f : A  B)
  (X : UU l3) (Y : UU l4) (η : X  Y) 
  UU (l1  l2  l3  l4  lsuc l5)
is-localization l5 f X Y η =
  ( is-local f Y) ×
  ( (Z : UU l5)  is-local f Z  is-local η Z)
module _
  {l1 l2 l3 l4 l5 : Level}
  {A : UU l1} {B : UU l2} {f : A  B}
  {X : UU l3} {Y : UU l4} {η : X  Y}
  (is-localization-Y : is-localization l5 f X Y η)

  is-local-is-localization : is-local f Y
  is-local-is-localization = pr1 is-localization-Y

The type of localizations of a type with respect to a map

localization :
  {l1 l2 l3 : Level} (l4 l5 : Level)
  {A : UU l1} {B : UU l2} (f : A  B)
  (X : UU l3)  UU (l1  l2  l3  lsuc l4  lsuc l5)
localization l4 l5 f X =
  Σ (UU l4)  Y  Σ (X  Y) (is-localization l5 f X Y))


Localizations at a map are localizations at a subuniverse

module _
  {l1 l2 l3 l4 : Level}
  {A : UU l1} {B : UU l2} (f : A  B)
  (X : UU l3) (Y : UU l4) (η : X  Y)

  is-subuniverse-localization-is-localization :
    is-localization l4 f X Y η 
    is-subuniverse-localization (is-local-Prop f) X Y
  pr1 (is-subuniverse-localization-is-localization is-localization-Y) =
    pr1 is-localization-Y
  pr1 (pr2 (is-subuniverse-localization-is-localization is-localization-Y)) = η
  pr2 (pr2 (is-subuniverse-localization-is-localization is-localization-Y))
    Z is-local-Z =
      pr2 is-localization-Y Z is-local-Z

It remains to construct a converse.


  1. Egbert Rijke, Michael Shulman, Bas Spitters, Modalities in homotopy type theory, Logical Methods in Computer Science, Volume 16, Issue 1, 2020 (arXiv:1706.07526, DOI:10.23638/LMCS-16(1:2)2020)
  2. Egbert Rijke, Classifying Types (arXiv:1906.09435)

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