Full subtypes of types

Content created by Fredrik Bakke, Egbert Rijke and Jonathan Prieto-Cubides.

Created on 2022-08-20.
Last modified on 2023-09-11.

module foundation.full-subtypes where
open import foundation.decidable-subtypes
open import foundation.dependent-pair-types
open import foundation.type-arithmetic-dependent-pair-types
open import foundation.unit-type
open import foundation.universe-levels

open import foundation-core.equivalences
open import foundation-core.propositions
open import foundation-core.subtypes
open import foundation-core.transport-along-identifications


The full subtype of a type contains every element. We define a full subtype at each universe level.


Full subtypes

module _
  {l1 l2 : Level} {A : UU l1} (P : subtype l2 A)

  is-full-subtype-Prop : Prop (l1  l2)
  is-full-subtype-Prop = Π-Prop A  x  P x)

  is-full-subtype : UU (l1  l2)
  is-full-subtype = type-Prop is-full-subtype-Prop

  is-prop-is-full-subtype : is-prop is-full-subtype
  is-prop-is-full-subtype = is-prop-type-Prop is-full-subtype-Prop

Full decidable subtypes

is-full-decidable-subtype :
  {l1 l2 : Level} {A : UU l1}  decidable-subtype l2 A  UU (l1  l2)
is-full-decidable-subtype P =
  is-full-subtype (subtype-decidable-subtype P)

The full subtype at a universe level

full-subtype : {l1 : Level} (l2 : Level) (A : UU l1)  subtype l2 A
full-subtype l2 A x = raise-unit-Prop l2

type-full-subtype : {l1 : Level} (l2 : Level) (A : UU l1)  UU (l1  l2)
type-full-subtype l2 A = type-subtype (full-subtype l2 A)

module _
  {l1 l2 : Level} {A : UU l1}

  is-in-full-subtype : (x : A)  is-in-subtype (full-subtype l2 A) x
  is-in-full-subtype x = raise-star

  inclusion-full-subtype : type-full-subtype l2 A  A
  inclusion-full-subtype = inclusion-subtype (full-subtype l2 A)

  is-equiv-inclusion-full-subtype : is-equiv inclusion-full-subtype
  is-equiv-inclusion-full-subtype =
    is-equiv-pr1-is-contr  a  is-contr-raise-unit)


A subtype is full if and only if the inclusion is an equivalence

module _
  {l1 l2 : Level} {A : UU l1} (P : subtype l2 A)

  is-equiv-inclusion-is-full-subtype :
    is-full-subtype P  is-equiv (inclusion-subtype P)
  is-equiv-inclusion-is-full-subtype H =
      ( λ x  is-proof-irrelevant-is-prop (is-prop-is-in-subtype P x) (H x))

  equiv-inclusion-is-full-subtype :
    is-full-subtype P  type-subtype P  A
  pr1 (equiv-inclusion-is-full-subtype H) = inclusion-subtype P
  pr2 (equiv-inclusion-is-full-subtype H) = is-equiv-inclusion-is-full-subtype H

  is-full-is-equiv-inclusion-subtype :
    is-equiv (inclusion-subtype P)  is-full-subtype P
  is-full-is-equiv-inclusion-subtype H x =
      ( is-in-subtype P)
      ( is-section-map-inv-is-equiv H x)
      ( pr2 (map-inv-is-equiv H x))

Recent changes