Singleton induction

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

Created on 2022-01-26.
Last modified on 2023-09-11.

module foundation.singleton-induction where
Imports 
open import foundation.action-on-identifications-functions
open import foundation.dependent-pair-types
open import foundation.universe-levels

open import foundation-core.contractible-types
open import foundation-core.function-types
open import foundation-core.homotopies
open import foundation-core.identity-types
open import foundation-core.sections
open import foundation-core.transport-along-identifications

Idea

Singleton induction on a type A equipped with a point a : A is a principle analogous to the induction principle of the unit type. A type satisfies singleton induction if and only if it is contractible.

Definition

is-singleton :
  (l1 : Level) {l2 : Level} (A : UU l2) → A → UU (lsuc l1 ⊔ l2)
is-singleton l A a = (B : A → UU l) → section (ev-point a {B})

ind-is-singleton :
  {l1 l2 : Level} {A : UU l1} (a : A) →
  ({l : Level} → is-singleton l A a) → (B : A → UU l2) →
  B a → (x : A) → B x
ind-is-singleton a is-sing-A B = pr1 (is-sing-A B)

compute-ind-is-singleton :
  {l1 l2 : Level} {A : UU l1} (a : A) (H : {l : Level} → is-singleton l A a) →
  (B : A → UU l2) → (ev-point a {B} ∘ ind-is-singleton a H B) ~ id
compute-ind-is-singleton a H B = pr2 (H B)

Properties

A type satisfies singleton induction if and only if it is contractible

abstract
  ind-singleton-is-contr :
    {l1 l2 : Level} {A : UU l1} (a : A) (is-contr-A : is-contr A)
    (B : A → UU l2) → B a → (x : A) → B x
  ind-singleton-is-contr a is-contr-A B b x =
    tr B ((inv (contraction is-contr-A a)) ∙ (contraction is-contr-A x)) b

  compute-ind-singleton-is-contr :
    {l1 l2 : Level} {A : UU l1}
    (a : A) (is-contr-A : is-contr A) (B : A → UU l2) →
    ((ev-point a {B}) ∘ (ind-singleton-is-contr a is-contr-A B)) ~ id
  compute-ind-singleton-is-contr a is-contr-A B b =
    ap (λ ω → tr B ω b) (left-inv (contraction is-contr-A a))

is-singleton-is-contr :
  {l1 l2 : Level} {A : UU l1} (a : A) → is-contr A → is-singleton l2 A a
pr1 (is-singleton-is-contr a is-contr-A B) =
  ind-singleton-is-contr a is-contr-A B
pr2 (is-singleton-is-contr a is-contr-A B) =
  compute-ind-singleton-is-contr a is-contr-A B

abstract
  is-contr-ind-singleton :
    {l1 : Level} (A : UU l1) (a : A) →
    ({l2 : Level} (P : A → UU l2) → P a → (x : A) → P x) → is-contr A
  pr1 (is-contr-ind-singleton A a S) = a
  pr2 (is-contr-ind-singleton A a S) = S (λ x → a ＝ x) refl

abstract
  is-contr-is-singleton :
    {l1 : Level} (A : UU l1) (a : A) →
    ({l2 : Level} → is-singleton l2 A a) → is-contr A
  is-contr-is-singleton A a S = is-contr-ind-singleton A a (λ P → pr1 (S P))

Examples

The total space of an identity type satisfies singleton induction

abstract
  is-singleton-total-path :
    {l1 l2 : Level} (A : UU l1) (a : A) →
    is-singleton l2 (Σ A (λ x → a ＝ x)) (pair a refl)
  pr1 (is-singleton-total-path A a B) = ind-Σ ∘ (ind-Id a _)
  pr2 (is-singleton-total-path A a B) = refl-htpy

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