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maybemabeline committed Oct 18, 2023
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Expand Up @@ -305,43 +305,43 @@ abstract

#### The type of invertible equivalences is equivalent to the type of invertible maps

**Proof:** Since every invertible map is an equivalence, the Sigma type of
**Proof:** Since every invertible map is an equivalence, the `Σ`-type of
invertible maps which are equivalences forms a full subtype of the type of
invertible maps. Swapping the order of Sigma types then shows that the Sigma
type of invertible maps which are equivalences is equivalent to the Sigma type
of equivalences which are invertible.
invertible maps. Swapping the order of `Σ`-types then shows that the `Σ`-type of
invertible maps which are equivalences is equivalent to the `Σ`-type of
equivalences which are invertible.

```agda
module _
{l1 l2 : Level} {A : UU l1} {B : UU l2}
where

tot-is-equiv-is-invertible : (invertible-map A B) Prop (l1 ⊔ l2)
tot-is-equiv-is-invertible = is-equiv-Prop ∘ map-invertible-map
is-equiv-prop-is-invertible : (invertible-map A B) Prop (l1 ⊔ l2)
is-equiv-prop-is-invertible = is-equiv-Prop ∘ map-invertible-map

is-full-subtype-tot-is-equiv-is-invertible :
is-full-subtype tot-is-equiv-is-invertible
is-full-subtype-tot-is-equiv-is-invertible =
is-full-subtype-is-equiv-prop-is-invertible :
is-full-subtype is-equiv-prop-is-invertible
is-full-subtype-is-equiv-prop-is-invertible =
is-equiv-is-invertible' ∘ is-invertible-map-invertible-map

equiv-invertible-equivalence-invertible-map :
Σ (A ≃ B) (is-invertible ∘ map-equiv) ≃ invertible-map A B
equiv-invertible-equivalence-invertible-map =
( equiv-inclusion-is-full-subtype
( tot-is-equiv-is-invertible)
( is-full-subtype-tot-is-equiv-is-invertible)) ∘e
( is-equiv-prop-is-invertible)
( is-full-subtype-is-equiv-prop-is-invertible)) ∘e
( equiv-right-swap-Σ)
```

#### The type of free loops on equivalences is equivalent to the type of invertible equivalences

**Proof:** Firstly, associating the order of Sigma types shows that a function
being invertible is equivalent to it having a section, such that this section is
also its retraction. Now, since equivalences have a contractible type of
sections, a proof of invertibility of the underlying map `f` of an equivalence
contracts to just a single homotopy `g ∘ f ~ id`, showing that a section `g` of
`f` is also its retraction. As `g` is a section, composing on the left with `f`
and canceling `f ∘ g` yields a loop `f ~ f`. By equivalence extensionality, this
**Proof:** First, associating the order of `Σ`-types shows that a function being
invertible is equivalent to it having a section, such that this section is also
its retraction. Now, since equivalences have a contractible type of sections, a
proof of invertibility of the underlying map `f` of an equivalence contracts to
just a single homotopy `g ∘ f ~ id`, showing that a section `g` of `f` is also
its retraction. As `g` is a section, composing on the left with `f` and
canceling `f ∘ g` yields a loop `f ~ f`. By equivalence extensionality, this
loop may be lifted to a loop on the entire equivalence.

```agda
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