# nLab final (infinity,1)-functor

### Context

#### Limits and colimits

limits and colimits

# Contents

## Idea

The notion of final $(\infty,1)$-functor (also called a cofinal $(\infty,1)$-functor) is the generalization of the notion of final functor from category theory to (∞,1)-category-theory.

An (∞,1)-functor $p : K' \to K$ is final precisely if precomposition with $p$ preserves colimits:

if $p$ is final then for for $F : K \to C$ any (∞,1)-functor we have

$\lim_\to (K \stackrel{F}{\to} C) \simeq \lim_{\to} ( K' \stackrel{p}{\to} K \stackrel{F}{\to} C)$

when either of these colimits exist.

## Definition

###### Definition

(final morphism of simplicial set)

A morphism $p : S \to T$ of simplicial sets is final if for every right fibration $X \to T$ the induced morphism of simplicial sets

$sSet_{/T}(T,X) \to sSet_{/T}(S,X)$

is a homotopy equivalence.

So in the overcategory $sSet/T$ a final morphism is an object such that morphisms out of it into any right fibration are the same as morphisms out of the terminal object into that right fibration.

$\left\{ \array{ T &&\to&& X \\ & {}_{\mathllap{=}}\searrow && \swarrow_{} \\ && T } \right\} \;\; \simeq \left\{ \array{ S &&\to&& X \\ & {}_{\mathllap{p}}\searrow && \swarrow_{} \\ && T } \right\} \,.$

This definition is originally due to Andre Joyal. It appears as HTT, def 4.1.1.1.

This is equivalent to the following definition, in terms of the model structure for right fibrations:

###### Proposition

The morphism $p : S \to T$ is final precisely if the terminal morphism $(p \to *) = \left( \array{ S &&\to&& T \\ & {}_{\mathllap{}}\searrow && \swarrow_{=} \\ && T } \right)$ in the overcategory $sSet_T$ is a weak equivalence in the model structure for right fibrations on $sSet_T$.

###### Proof

This is HTT, prop. 4.1.2.5.

###### Corollary

If $T$ is a Kan complex then $p : S \to T$ is final precisely if it is a weak equivalence in the standard model structure on simplicial sets.

###### Proof

This is HTT, cor. 4.1.2.6.

## Properties

###### Proposition

(preservation of undercategories and colimits)

A morphism $p : K' \to K$ of simplicial sets is final precisely if for every quasicategory $C$

• and for every morphism $\bar F : K^{\triangleright} \to C$ that exibits a colimit co-cone in $C$, also $(K')^\triangleright \stackrel{p}{\to} K^{\triangleright} \stackrel{\bar F}{\to} C$ is a colimit co-cone.

and equivalently precisely if

• … and for every $F : K \to C$ the morphism

$F/C \to (F\circ p)/C$

of under quasi-categories induced by composition with $p$ is an equivalence of (∞,1)-categories.

###### Proof

This is HTT, prop. 4.1.1.8.

The following result is the $(\infty,1)$-categorical analog of what is known as Quillen’s Theorem A.

###### Theorem

(recognition theorem for final $(\infty,1)$-functors)

A morphism $p : K \to C$ of simplicial sets with $C$ a quasi-category is final precisely if for each object $c \in C$ the comma-object $c/p := c/C \times_C K$ is weakly contractible.

More explicitly, the comma object in question here is the pullback

$\array{ c/p &\to& c/C \\ \downarrow && \downarrow \\ K &\stackrel{p}{\to}& C } \,,$

where $c/C$ is the under quasi-category under $c$.

###### Proof

This is due to Andre Joyal. A proof appears as HTT, prop. 4.1.3.1.

The following says that up to equivalence, the cofinal maps of simplicial sets are the right anodyne morphisms

###### Proposition

A map of simplicial sets is cofinal precisely if it factors as a right anodyne map followed by a trivial fibration.

This is (Lurie, cor. 4.1.1.12).

## Examples

### General

###### Example

The inclusion $\ast \to \mathcal{C}$ of a terminal object is final.

###### Proof

By theorem 1 the inclusion of the point is final precisely if for all $c \in \mathcal{C}$, the (∞,1)-categorical hom-space $\mathcal{C}(c,\ast)$ is contractible. This is the definition of $\ast$ being terminal.

### On categories of simplices

###### Definition

For $K \in$ sSet a simplicial set, write $\Delta_{/K}$ for its category of elements, called here the category of simplices of the simplicial set:

an object of $\Delta_{/K}$ is a morphism of simplicial sets of the form $\Delta^n \to K$ for some $n \in \mathbb{N}$ (hence an $n$-simplex of $K$) and a morphism is a commuting diagram

$\array{ \Delta^{n_1}&&\to&& \Delta^{n_2} \\ & \searrow && \swarrow \\ && K } \,.$

Moreover, write

$\Delta_{/K}^{nd} \hookrightarrow \Delta_{/K}$

for the full subcategory on the non-degenerate simplices.

###### Remark

The category $\Delta_{/K}^{nd}$ is also known as the barycentric subdivision of $K$.

###### Proposition

The inclusion

$N(\Delta_{/K}^{nd}) \hookrightarrow N(\Delta_{/K})$

is a cofinal morphism of quasi-categories.

This appears as (Lurie, variant 4.2.3.15).

###### Proposition

For every simplicial set $K$ there exists a cofinal map

$N(\Delta_{/K}) \to K \,.$

This is (Lurie, prop. 4.2.3.14).

## References

Section 4.1 of

Section 6 of

Revised on April 18, 2013 12:30:05 by Urs Schreiber (89.204.130.31)