Cavallo's trick

Content created by Egbert Rijke and Fredrik Bakke.

Created on 2023-05-03.
Last modified on 2023-10-22.

module synthetic-homotopy-theory.cavallos-trick where
open import foundation.action-on-identifications-functions
open import foundation.dependent-pair-types
open import foundation.function-types
open import foundation.homotopies
open import foundation.identity-types
open import foundation.sections
open import foundation.universe-levels

open import structured-types.pointed-homotopies
open import structured-types.pointed-maps
open import structured-types.pointed-types


Cavallo's trick is a way of upgrading an unpointed homotopy between pointed maps to a pointed homotopy.

Originally, this trick was formulated by Evan Cavallo for homogeneous spaces, but it works as soon as the evaluation map (id ~ id) → Ω B has a section.


module _
  {l1 l2 : Level} {A : Pointed-Type l1} {B : Pointed-Type l2}

  cavallos-trick :
    (f g : A →∗ B)  section  (H : id ~ id)  H (point-Pointed-Type B)) 
    (map-pointed-map f ~ map-pointed-map g)  f ~∗ g
  pr1 (cavallos-trick (f , refl) (g , q) (K , α) H) a =
    K (inv q  inv (H (point-Pointed-Type A))) (f a)  H a
  pr2 (cavallos-trick (f , refl) (g , q) (K , α) H) =
    ( ap
      ( concat' (f (point-Pointed-Type A)) (H (point-Pointed-Type A)))
      ( α (inv q  inv (H (point-Pointed-Type A))))) 
    ( ( assoc
        ( inv q)
        ( inv (H (point-Pointed-Type A)))
        ( H (point-Pointed-Type A))) 
      ( ( ap
          ( concat (inv q) (g (point-Pointed-Type A)))
          ( left-inv (H (point-Pointed-Type A)))) 
        ( right-unit)))


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