# Cavallo's trick

Content created by Egbert Rijke and Fredrik Bakke.

Created on 2023-05-03.

module synthetic-homotopy-theory.cavallos-trick where

Imports
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


## Idea

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.

## Theorem

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

cavallos-trick :
(f g : A →∗ B) → section (λ (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)))