# nLab orthogonal factorization system in an (infinity,1)-category

Contents

### Context

#### $(\infty,1)$-Category theory

(∞,1)-category theory

# Contents

## Definition

###### Definition

Let $C$ be an (∞,1)-category and $f : A \to B$ and $g : X \to Y$ two morphisms in $C$. Write $C_{A\sslash Y}$ for the under-over-(∞,1)-category.

We say that $f$ is left orthogonal to $g$ and that $g$ is right orthogonal to $f$ and write

$f \perp g$

if for every commuting diagram

$\array{ A &\to& X \\ {}^{\mathllap{f}}\downarrow &\swArrow_{\simeq}& \downarrow^{\mathrlap{g}} \\ B &\to& Y }$

in $C$ we have that $C_{A\sslash Y}(B,X) \simeq *$ is contractible.

Note that the notation $C_{A\sslash Y}(B,X)$ subtly includes the given commuting diagram, since $C_{A\sslash Y}$ is only defined relative to a particular given morphism $A\to Y$. Here we take that to be the common composite of the given commuting square, with $B$ and $X$ regarded as objects of $C_{A\sslash Y}$ via the resulting commuting triangles.

###### Definition

Let $C$ be an (∞,1)-category. An orthogonal factorization system on $C$ is a pair $(S_L, S_R)$ of classes of morphisms in $C$ that satisfy the following axioms.

1. Both classes are stable under retracts.

2. Every morphism in $S_L$ is left orthogonal to every morphism in $S_R$;

3. For every morphism $h : X \to Z$ in $C$ there exists a commuting triangle

$\array{ && Y \\ & {}^{\mathllap{f}}\nearrow && \searrow^{\mathrlap{g}} \\ X &&\stackrel{h}{\to}&& Z }$

with $f \in S_L$ and $g \in S_R$.

## Properties

### Closure properties

###### Proposition

For $(L,R)$ a factorization system in an (∞,1)-category $\mathcal{C}$, the full sub-(∞,1)-category of the arrow category $Func(\Delta^1, \mathcal{C})$ on the morphisms in $R$ is closed under (∞,1)-limits of shapes that exist in $\mathcal{C}$. Similarly the full subcategory on $L$ is closed under (∞,1)-colimits that exist in $\mathcal{C}$.

This is (Lurie, prop. 5.2.8.6 (7), (8)).

### Reflection

###### Definition

Let $(L,R)$ be an orthogonal factorization system on an $(\infty,1)$-category $\mathcal{C}$. Write $\mathcal{C}^I_R \hookrightarrow \mathcal{C}^I$ for the full sub-(∞,1)-category of the arrow category on the morphisms in $R$.

Then

1. this is a reflective sub-(∞,1)-category

$\mathcal{C}^I_R \stackrel{\stackrel{}{\leftarrow}}{\hookrightarrow} \mathcal{C}^I$
2. The adjunction units $\eta_f : f \to \bar f$ are of the form

$\array{ X &\stackrel{\in L}{\to}& \bar X \\ {}^{\mathllap{f}}\downarrow && \downarrow^{\mathrlap{\bar f \in R} } \\ Y &\stackrel{\simeq}{\to}& \bar Y } \,.$

(In words: the reflection into $\mathcal{C}^I_R$ is given by the factorization in $(L,R)$).

This is (Lurie, lemma 5.2.8.19).

## Examples

• In an (∞,1)-topos the classe of n-connected and that of n-truncated morphisms form an orthogonal factorization system, for all $(-2) \leq n \leq \infty$.

Section 5.2.8 of

Formalization in homotopy type theory is discussed in

• Egbert Rijke, Orthogonal factorization in HoTT, talk at IAS, January 24, 2013 (video)