This is an archived version pinned as of the submission of my master's thesis. An up-to-date version may be found online.

Lawvere's fixed point theorem

module foundation.lawveres-fixed-point-theorem where
Imports 
open import foundation.dependent-pair-types
open import foundation.existential-quantification
open import foundation.function-extensionality
open import foundation.propositional-truncations
open import foundation.surjective-maps
open import foundation.universe-levels

open import foundation-core.identity-types

Idea

Lawvere's fixed point theorem asserts that if there is a surjective map A → (A → B), then any map B → B must have a fixed point.

Theorem

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

  abstract
    fixed-point-theorem-Lawvere :
      is-surjective f → (h : B → B) → exists-structure B (λ b → h b ＝ b)
    fixed-point-theorem-Lawvere H h =
      apply-universal-property-trunc-Prop
        ( H g)
        ( exists-structure-Prop B (λ b → h b ＝ b))
        ( λ (x , p) → intro-exists (f x x) (inv (htpy-eq p x)))
      where
      g : A → B
      g a = h (f a a)