Fiber inclusions

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

Created on 2022-01-27.
Last modified on 2024-03-02.

module foundation.fiber-inclusions where
Imports 
open import foundation.0-maps
open import foundation.cones-over-cospan-diagrams
open import foundation.dependent-pair-types
open import foundation.faithful-maps
open import foundation.fibers-of-maps
open import foundation.standard-pullbacks
open import foundation.transport-along-identifications
open import foundation.type-arithmetic-dependent-pair-types
open import foundation.unit-type
open import foundation.universe-levels

open import foundation-core.1-types
open import foundation-core.contractible-maps
open import foundation-core.embeddings
open import foundation-core.equality-dependent-pair-types
open import foundation-core.equivalences
open import foundation-core.functoriality-dependent-pair-types
open import foundation-core.homotopies
open import foundation-core.identity-types
open import foundation-core.propositional-maps
open import foundation-core.propositions
open import foundation-core.pullbacks
open import foundation-core.sets
open import foundation-core.truncated-maps
open import foundation-core.truncated-types
open import foundation-core.truncation-levels

Idea

Given a family B of types over A and an element a : A, then the fiber inclusion of B at a is a map B a → Σ A B.

Definition

module _
  {l1 l2 : Level} {A : UU l1} (B : A → UU l2)
  where

  fiber-inclusion : (x : A) → B x → Σ A B
  pr1 (fiber-inclusion x y) = x
  pr2 (fiber-inclusion x y) = y

  fiber-fiber-inclusion :
    (a : A) (t : Σ A B) → fiber (fiber-inclusion a) t ≃ (a ＝ pr1 t)
  fiber-fiber-inclusion a t =
    ( ( right-unit-law-Σ-is-contr
        ( λ p → is-contr-map-is-equiv (is-equiv-tr B p) (pr2 t))) ∘e
      ( equiv-left-swap-Σ)) ∘e
    ( equiv-tot (λ b → equiv-pair-eq-Σ (pair a b) t))

Properties

The fiber inclusions are truncated maps for any type family B if and only if A is truncated

module _
  {l1 l2 : Level} (k : 𝕋) {A : UU l1}
  where

  is-trunc-is-trunc-map-fiber-inclusion :
    ((B : A → UU l2) (a : A) → is-trunc-map k (fiber-inclusion B a)) →
    is-trunc (succ-𝕋 k) A
  is-trunc-is-trunc-map-fiber-inclusion H x y =
    is-trunc-equiv' k
      ( fiber (fiber-inclusion B x) (pair y raise-star))
      ( fiber-fiber-inclusion B x (pair y raise-star))
      ( H B x (pair y raise-star))
    where
    B : A → UU l2
    B a = raise-unit l2

  is-trunc-map-fiber-inclusion-is-trunc :
    (B : A → UU l2) (a : A) →
    is-trunc (succ-𝕋 k) A → is-trunc-map k (fiber-inclusion B a)
  is-trunc-map-fiber-inclusion-is-trunc B a H t =
    is-trunc-equiv k
      ( a ＝ pr1 t)
      ( fiber-fiber-inclusion B a t)
      ( H a (pr1 t))

module _
  {l1 l2 : Level} {A : UU l1} (B : A → UU l2)
  where

  is-contr-map-fiber-inclusion :
    (x : A) → is-prop A → is-contr-map (fiber-inclusion B x)
  is-contr-map-fiber-inclusion =
    is-trunc-map-fiber-inclusion-is-trunc neg-two-𝕋 B

  is-prop-map-fiber-inclusion :
    (x : A) → is-set A → is-prop-map (fiber-inclusion B x)
  is-prop-map-fiber-inclusion =
    is-trunc-map-fiber-inclusion-is-trunc neg-one-𝕋 B

  is-0-map-fiber-inclusion :
    (x : A) → is-1-type A → is-0-map (fiber-inclusion B x)
  is-0-map-fiber-inclusion =
    is-trunc-map-fiber-inclusion-is-trunc zero-𝕋 B

  is-emb-fiber-inclusion : is-set A → (x : A) → is-emb (fiber-inclusion B x)
  is-emb-fiber-inclusion H x =
    is-emb-is-prop-map (is-prop-map-fiber-inclusion x H)

  emb-fiber-inclusion : is-set A → (x : A) → B x ↪ Σ A B
  pr1 (emb-fiber-inclusion H x) = fiber-inclusion B x
  pr2 (emb-fiber-inclusion H x) = is-emb-fiber-inclusion H x

  is-faithful-fiber-inclusion :
    is-1-type A → (x : A) → is-faithful (fiber-inclusion B x)
  is-faithful-fiber-inclusion H x =
    is-faithful-is-0-map (is-0-map-fiber-inclusion x H)

fiber-inclusion-emb :
  {l1 l2 : Level} (A : Set l1) (B : type-Set A → UU l2) →
  (x : type-Set A) → B x ↪ Σ (type-Set A) B
pr1 (fiber-inclusion-emb A B x) = fiber-inclusion B x
pr2 (fiber-inclusion-emb A B x) = is-emb-fiber-inclusion B (is-set-type-Set A) x

fiber-inclusion-faithful-map :
  {l1 l2 : Level} (A : 1-Type l1) (B : type-1-Type A → UU l2) →
  (x : type-1-Type A) → faithful-map (B x) (Σ (type-1-Type A) B)
pr1 (fiber-inclusion-faithful-map A B x) = fiber-inclusion B x
pr2 (fiber-inclusion-faithful-map A B x) =
  is-faithful-fiber-inclusion B (is-1-type-type-1-Type A) x

Any fiber inclusion is a pullback of a point inclusion

module _
  {l1 l2 : Level} {A : UU l1} (B : A → UU l2) (a : A)
  where

  cone-fiber-fam : cone (pr1 {B = B}) (point a) (B a)
  pr1 cone-fiber-fam = fiber-inclusion B a
  pr1 (pr2 cone-fiber-fam) = terminal-map (B a)
  pr2 (pr2 cone-fiber-fam) = refl-htpy

  abstract
    is-pullback-cone-fiber-fam :
      is-pullback (pr1 {B = B}) (point a) cone-fiber-fam
    is-pullback-cone-fiber-fam =
      is-equiv-comp
        ( gap (pr1 {B = B}) (point a) (cone-fiber (pr1 {B = B}) a))
        ( map-inv-fiber-pr1 B a)
        ( is-equiv-map-inv-fiber-pr1 B a)
        ( is-pullback-cone-fiber pr1 a)

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