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fredrik-bakke authored Nov 9, 2023
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4 changes: 2 additions & 2 deletions src/foundation-core/commuting-triangles-of-maps.lagda.md
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Expand Up @@ -106,7 +106,7 @@ then the triangle
g\ /f
\ /
V V
X,
X
```

commutes.
Expand Down Expand Up @@ -138,7 +138,7 @@ then the triangle
h\ /r
\ /
V V
B,
B
```

commutes.
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11 changes: 6 additions & 5 deletions src/foundation-core/equivalences.lagda.md
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Expand Up @@ -34,11 +34,12 @@ also called being **biinvertible**.

The condition of biinvertibility may look odd: Why not say that an equivalence
is a map that has a [2-sided inverse](foundation-core.invertible-maps.md)? The
reason is that the condition of invertibility is structure, whereas the
condition of being biinvertible is a
[property](foundation-core.propositions.md). To quickly see this: if `f` is an
equivalence, then it has up to [homotopy](foundation-core.homotopies.md) only
one section, and it has up to homotopy only one retraction.
reason is that the condition of invertibility is
[structure](foundation.structure.md), whereas the condition of being
biinvertible is a [property](foundation-core.propositions.md). To quickly see
this: if `f` is an equivalence, then it has up to
[homotopy](foundation-core.homotopies.md) only one section, and it has up to
homotopy only one retraction.

## Definition

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4 changes: 3 additions & 1 deletion src/foundation-core/pullbacks.lagda.md
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Expand Up @@ -780,7 +780,9 @@ module _
( is-fiberwise-equiv-map-fiber-cone-is-pullback
( j ∘ i)
( h)
( pasting-horizontal-cone i j h c d) is-pb-rect x))
( pasting-horizontal-cone i j h c d)
( is-pb-rect)
( x)))
```

### The vertical pullback pasting property
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12 changes: 6 additions & 6 deletions src/foundation-core/retractions.lagda.md
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Expand Up @@ -22,8 +22,8 @@ open import foundation-core.whiskering-homotopies
## Idea

A **retraction** of a map `f : A → B` consists of a map `r : B A` equipped
with a homotopy `r ∘ f ~ id`. In other words, a retraction of a map `f` is a
left inverse of `f`.
with a [homotopy](foundation-core.homotopies.md) `r ∘ f ~ id`. In other words, a
retraction of a map `f` is a left inverse of `f`.

## Definitions

Expand Down Expand Up @@ -70,7 +70,7 @@ i.e., that the concatenation

is identical to `p : x = y` for all `p : x = y`, we simply proceed by
identification elimination. Then it suffices to show that `(H x)⁻¹ ∙ (H x)` is
identical by `refl`, which is indeed the case by the left inverse law of
identical to `refl`, which is indeed the case by the left inverse law of
concatenation of identifications.

```agda
Expand Down Expand Up @@ -161,7 +161,7 @@ that would result in a cyclic module dependency.
```agda
module _
{l1 l2 l3 : Level} {A : UU l1} {B : UU l2} {X : UU l3}
(f : A X) (g : B X) (h : A B) (H : f ~ (g ∘ h))
(f : A X) (g : B X) (h : A B) (H : f ~ g ∘ h)
(r : retraction f)
where

Expand Down Expand Up @@ -195,8 +195,8 @@ In a commuting triangle of the form
X,
```

if `r : X → A` is a section of the map `g` on the right and `t : B A` is a
section of the map `h` on top, then `ts` is a section of the map `f` on the
if `r : X → A` is a retraction of the map `g` on the right and `s : B A` is a
retraction of the map `h` on top, then `sr` is a retraction of the map `f` on the
left.

```agda
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6 changes: 4 additions & 2 deletions src/foundation/descent-coproduct-types.lagda.md
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Expand Up @@ -144,7 +144,8 @@ module _
is-pullback f i cone-A'
descent-coprod-inl (pair h (pair f' H)) (pair k (pair g' K)) is-pb-dsq =
is-pullback-is-fiberwise-equiv-map-fiber-cone f i (triple h f' H)
( λ a is-equiv-left-map-triangle
( λ a
is-equiv-left-map-triangle
( map-fiber-cone f i (triple h f' H) a)
( map-fiber-cone (ind-coprod _ f g) i
( cone-descent-coprod (triple h f' H) (triple k g' K))
Expand All @@ -165,7 +166,8 @@ module _
is-pullback g i cone-B'
descent-coprod-inr (pair h (pair f' H)) (pair k (pair g' K)) is-pb-dsq =
is-pullback-is-fiberwise-equiv-map-fiber-cone g i (triple k g' K)
( λ b is-equiv-left-map-triangle
( λ b
is-equiv-left-map-triangle
( map-fiber-cone g i (triple k g' K) b)
( map-fiber-cone (ind-coprod _ f g) i
( cone-descent-coprod (triple h f' H) (triple k g' K))
Expand Down
6 changes: 4 additions & 2 deletions src/foundation/descent-dependent-pair-types.lagda.md
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Expand Up @@ -56,7 +56,8 @@ module _
( h)
( cone-descent-Σ)
( ind-Σ
( λ i a is-equiv-right-map-triangle
( λ i a
is-equiv-right-map-triangle
( map-fiber-cone (f i) h (c i) a)
( map-fiber-cone (ind-Σ f) h cone-descent-Σ (pair i a))
( map-inv-compute-fiber-tot (λ i pr1 (c i)) (pair i a))
Expand All @@ -71,7 +72,8 @@ module _
((i : I) is-pullback (f i) h (c i))
descent-Σ' is-pb-dsq i =
is-pullback-is-fiberwise-equiv-map-fiber-cone (f i) h (c i)
( λ a is-equiv-left-map-triangle
( λ a
is-equiv-left-map-triangle
( map-fiber-cone (f i) h (c i) a)
( map-fiber-cone (ind-Σ f) h cone-descent-Σ (pair i a))
( map-inv-compute-fiber-tot (λ i pr1 (c i)) (pair i a))
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3 changes: 2 additions & 1 deletion src/foundation/descent-equivalences.lagda.md
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Expand Up @@ -57,7 +57,8 @@ module _
( map-inv-is-equiv-precomp-Π-is-equiv
( is-equiv-i)
( λ y is-equiv (map-fiber-cone j h c y))
( λ x is-equiv-right-map-triangle
( λ x
is-equiv-right-map-triangle
( map-fiber-cone (j ∘ i) h
( pasting-horizontal-cone i j h c d) x)
( map-fiber-cone j h c (i x))
Expand Down
8 changes: 4 additions & 4 deletions src/foundation/equivalences.lagda.md
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Expand Up @@ -345,9 +345,9 @@ along equivalences. Similarly, the map `j⁻¹ ∘ g` is a retraction of `h`, si
we have `(g ∘ h ~ j) (j⁻¹ ∘ g ∘ h ~ id)` by transposing along equivalences.
Since `h` therefore has a section and a retraction, it is an equivalence.

In fact, the above argument shows that if the top map has a section and the
bottom map has a retraction, then the diagonal filler, and hence all other maps
are equivalences.
In fact, the above argument shows that if the top map `i` has a section and the
bottom map `j` has a retraction, then the diagonal filler, and hence all other
maps are equivalences.

```agda
module _
Expand Down Expand Up @@ -691,4 +691,4 @@ equiv-fiberwise-equiv-fam-equiv B C = distributive-Π-Σ

- The
[2-out-of-6 property](https://ncatlab.org/nlab/show/two-out-of-six+property)
at the nLab.
at nlab
9 changes: 1 addition & 8 deletions src/foundation/functoriality-fibers-of-maps.lagda.md
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Expand Up @@ -325,7 +325,7 @@ constructed as a family of identifications of the form
eq-Eq-fiber g (γ₀ a) _,
```

it follows that when we left-whisker this homotopy with `inclusion-fiber g`, we
it follows that when we left whisker this homotopy with `inclusion-fiber g`, we
recover the homotopy `γ₀ ·r inclusion-fiber f`.

Now it remains to fill a coherence for the square of homotopies
Expand All @@ -344,13 +344,6 @@ Now it remains to fill a coherence for the square of homotopies
where `H` is the homotopy that we just constructed, witnessing that the upper
triangle commutes, and where we have written `i` for all fiber inclusions.

coherence-square-homotopies (pr1 (pr2 htpy-hom-arrow-fiber) ·r inclusion-fiber
f) (coh-hom-arrow (inclusion-fiber f) (inclusion-fiber g) (comp-hom-arrow
(inclusion-fiber f) (inclusion-fiber g) (inclusion-fiber g)
(tr-hom-arrow-inclusion-fiber g (htpy-codomain-htpy-hom-arrow f g α β γ b))
(hom-arrow-fiber f g α b))) (coh-hom-arrow (inclusion-fiber f) (inclusion-fiber
g) (hom-arrow-fiber f g β b)) (inclusion-fiber g ·l pr1 htpy-hom-arrow-fiber)

```agda
module _
{l1 l2 l3 l4 : Level} {A : UU l1} {B : UU l2} {X : UU l3} {Y : UU l4}
Expand Down
4 changes: 2 additions & 2 deletions src/foundation/morphisms-arrows.lagda.md
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Expand Up @@ -118,7 +118,7 @@ Consider a commuting diagram of the form
α₁ β₁
```

Then the outer rectangle commutes by horizontal pasing of commuting squares of
Then the outer rectangle commutes by horizontal pasting of commuting squares of
maps.

```agda
Expand Down Expand Up @@ -165,7 +165,7 @@ module _

### Homotopies of morphsims of arrows

A \*\*homotopy of morphisms of arrows from `(i , j , H)` to `(i' , j' , H')` is
A **homotopy of morphisms of arrows** from `(i , j , H)` to `(i' , j' , H')` is
a triple `(I , J , K)` consisting of homotopies `I : i ~ i'` and `J : j ~ j'`
and a homotopy `K` witnessing that the
[square of homotopies](foundation.commuting-squares-of-homotopies.md)
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3 changes: 3 additions & 0 deletions src/foundation/morphisms-twisted-arrows.lagda.md
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Expand Up @@ -38,6 +38,9 @@ A **morphism of twisted arrows** from `f : A → B` to `g : X → Y` is a triple

commutes.

Thus, a morphism of twisted arrows can also be understood as _a factorization of
`g` through `f`_.

## Definitions

```agda
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10 changes: 6 additions & 4 deletions src/foundation/retracts-of-maps.lagda.md
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Expand Up @@ -657,8 +657,9 @@ module _
( preserves-id-hom-arrow-fiber f b)))

retract-map-inclusion-fiber-retract-map :
( inclusion-fiber f {b}) retract-of-map
( inclusion-fiber g {map-codomain-inclusion-retract-map f g R b})
retract-map
( inclusion-fiber g {map-codomain-inclusion-retract-map f g R b})
( inclusion-fiber f {b})
pr1 retract-map-inclusion-fiber-retract-map =
inclusion-retract-map-inclusion-fiber-retract-map
pr1 (pr2 retract-map-inclusion-fiber-retract-map) =
Expand All @@ -676,8 +677,9 @@ module _
where

retract-fiber-retract-map :
( fiber f b) retract-of
( fiber g (map-codomain-inclusion-retract-map f g R b))
retract
( fiber g (map-codomain-inclusion-retract-map f g R b))
( fiber f b)
retract-fiber-retract-map =
retract-domain-retract-map
( inclusion-fiber f)
Expand Down

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