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Tashi Walde committed Oct 13, 2023
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44 changes: 20 additions & 24 deletions src/hott/03-equivalences.rzk.md
Original file line number Diff line number Diff line change
Expand Up @@ -535,8 +535,8 @@ The retraction associated with an equivalence is an equivalence.

## Section-retraction pairs

A pair of maps `s : A → B` and `r : B → A'` is a
**section-retraction pair** if the composite `A → A'` is an equivalence.
A pair of maps `s : A → B` and `r : B → A'` is a **section-retraction pair** if
the composite `A → A'` is an equivalence.

```rzk
#def is-section-retraction-pair
Expand All @@ -547,9 +547,8 @@ A pair of maps `s : A → B` and `r : B → A'` is a
:= is-equiv A A' (comp A B A' r s)
```

When we have such a section-retraction pair `(s, r)`,
we say that `r` is an **external retraction** of `s`
and `s` is an **external section** of `r`.
When we have such a section-retraction pair `(s, r)`, we say that `r` is an
**external retraction** of `s` and `s` is an **external section** of `r`.

```rzk
#def has-external-retraction
Expand All @@ -569,14 +568,13 @@ and `s` is an **external section** of `r`.
, ( is-section-retraction-pair A B A' s r)
```

Note that exactly like `has-section` and `has-retraction`
these are not _properties_ of `s` or `r`, but structure.
Note that exactly like `has-section` and `has-retraction` these are not
_properties_ of `s` or `r`, but structure.

Without univalence, we cannot yet show that the types
`has-external-retraction` and `has-retraction`
(and their section counterparts, respectively)
are equivalent, so we content ourselves in showing that there is a
logical biimplication between them.
Without univalence, we cannot yet show that the types `has-external-retraction`
and `has-retraction` (and their section counterparts, respectively) are
equivalent, so we content ourselves in showing that there is a logical
biimplication between them.

```rzk
#def has-retraction-externalize
Expand Down Expand Up @@ -616,18 +614,16 @@ logical biimplication between them.
:= ( comp A' A B s sec-rs , ε-rs)
```

A consequence of the above is that in a section-retraction pair `(s, r)`,
the map `s` is a section and the map `r` is a retraction.
Note that not every composable pair `(s, r)` of such maps is a
section-retraction pair;
they have to (up to composition with an equivalence)
be section and retraction _of each other_.
A consequence of the above is that in a section-retraction pair `(s, r)`, the
map `s` is a section and the map `r` is a retraction. Note that not every
composable pair `(s, r)` of such maps is a section-retraction pair; they have to
(up to composition with an equivalence) be section and retraction _of each
other_.

In a section-retraction pair, if one of `s : A → B` and `r : B → A'` is an
equivalence, then so is the other.

This is just a rephrasing of `is-equiv-left-factor`
and `is-equiv-right-factor`.
This is just a rephrasing of `is-equiv-left-factor` and `is-equiv-right-factor`.

```rzk
#section is-equiv-is-section-retraction-pair
Expand Down Expand Up @@ -658,8 +654,8 @@ and `is-equiv-right-factor`.

## Retracts

We say that a type `A` is a retract of a type `B` if we have a map
`s : A → B` which has a retraction.
We say that a type `A` is a retract of a type `B` if we have a map `s : A → B`
which has a retraction.

```rzk title="The type of proofs that A is a retract of B"
#def is-retract-of
Expand All @@ -686,8 +682,8 @@ We say that a type `A` is a retract of a type `B` if we have a map
:= η
```

Section-retraction pairs `A → B → A'` are a convenient way to exhibit
both `A` and `A'` as retracts of `B`.
Section-retraction pairs `A → B → A'` are a convenient way to exhibit both `A`
and `A'` as retracts of `B`.

```rzk
#def is-retract-of-left-section-retraction-pair
Expand Down
2 changes: 1 addition & 1 deletion src/hott/06-contractible.rzk.md
Original file line number Diff line number Diff line change
Expand Up @@ -175,7 +175,6 @@ A type is contractible if and only if its terminal map is an equivalence.

A retract of contractible types is contractible.


```rzk title="If A is a retract of a contractible type it has a term"
#section is-contr-is-retract-of-is-contr
Expand Down Expand Up @@ -225,6 +224,7 @@ A retract of contractible types is contractible.
```

```rzk
```

## Functions between contractible types
Expand Down

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