nLab
filtered (infinity,1)-category

Idea

This is the analog of a filtered category in the context of (∞,1)-categories.

The main purpose of considering filtered (∞,1)-categories is to define filtered (∞,1)-colimits, which are the colimits that commute with finite (∞,1)-limits.

Definition

Definition

Let $\kappa$ be a regular cardinal, and let $C\in sSet$ be an (∞,1)-category, incarnated as a quasicategory.

$C$ is called $\kappa$-filtered if for all $\kappa$-small $K\in sSet$ and every morphism $f:K\to C$ there is a morphism $\hat{p}:rcone(K)\to C$ extending $f$, where $rcone(K)$ denotes the (right) cone of the simplicial set $K$. $C$ is called filtered if it is $\omega$-filtered.

Properties

Proposition

An (∞,1)-category $K$ is filtered precisely if (∞,1)-colimits of shape $K$ in ∞ Grpd commute with all finite (∞,1)-limits, hence if

$$\underrightarrow{\lim }:\mathrm{Func}(K,\mathrm{\infty }\mathrm{Grpd})\to \mathrm{\infty }\mathrm{Grpd}$${\lim_\to} : Func(K, \infty Grpd) \to \infty Grpd

is a left exact (∞,1)-functor.

This is HTT, prop. 5.3.3.3.

Proposition

A filtered $(\mathrm{\infty },1)$-category is in particular a sifted (∞,1)-category.

This appears as (Lurie, prop. 5.3.1.20). Since sifted (∞,1)-colimits are precisely those that commute with finite products, this is a direct reflection of the fact that finite products are a special kind of finite (∞,1)-limits.

Corollary

For $C$ a filtered $(\mathrm{\infty },1)$-category, the diagonal (∞,1)-functor $\Delta :C\to C\times C$ if a cofinal (∞,1)-functor.

Related concepts

• sifted category, sifted colimit, sifted (∞,1)-category, sifted (∞,1)-colimit

• directed set, filtered category, filtered (∞,1)-category

Reference

Section 5.3.1 of

• Jacob Lurie, Higher Topos Theory