Contents

Idea

The $n$-connected/$n$-truncated factorization system is an orthogonal factorization system in an (∞,1)-category, specifically in an (∞,1)-topos, that generalizes the relative Postnikov systems of ∞Grpd: it factors any morphism through its (n+2)-image by an (n+2)-epimorphism followed by an (n+2)-monomorphism.

As $n$ ranges through $(-1),0,1,2,3,\cdots$ these factorization systems form an ∞-ary factorization system.

Definitions

This appears as a remark in HTT, Example 5.2.8.16. A construction of the factorization in terms of a model category presentation is in (Rezk, prop. 8.5).

Properties

Stability

This appears as (Lurie, prop. 6.5.1.16(6)).

Examples

The case $n=-2$

A (-2)-truncated morphism is precisely an equivalence in an (∞,1)-category (see there or HTT, example 5.5.6.13).

Moreover, every morphism is (-2)-connected.

Therefore for $n=-2$ the $n$-connected/$n$-truncated factorization system says (only) that equivalences have inverses, unique up to coherent homotopy.

The case $n=-1$

A (-1)-truncated morphism is precisely a full and faithful morphism.

A (-1)-connected morphism is one whose homotopy fibers are inhabited.

In ∞Grpd a morphism between 0-truncated objects (sets)

• is full and faithful precisely if it is an injection;

• has non-empty fibers precisely if it is an epimorphism.

Therefore between 0-truncated objects the (-1)-connected/(-1)-truncated factorization system is the epi/mono factorization system and hence image factorization.

Generally, the (-1)-connected/(-1)-truncated factorization is through the $\mathrm{\infty }$-categorical 1-image, the homotopy image (see there for more details).

The case $n=0$

Let $X,Y$ be two groupoids (homotopy 1-types) in ∞Grpd.

A morphism $X\to Y$ is 0-truncated precisely if it is a faithful functor.

A morphism $X\to Y$ is 0-connected precisely if it is a full functor and an essentially surjective functor.

Therefore on homotopy 1-trypes the 0-connected/0-truncated factorization system is the (eso+full, faithful) factorization system.

References

The general abstract statement is in

• Jacob Lurie, Higher Topos Theory

A model category-theoretic discussion is in section 8 of

• Charles Rezk, Toposes and homotopy toposes (pdf)