# nLab Postnikov tower in an (infinity,1)-category

### Context

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

(∞,1)-category theory

## Models

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

(∞,1)-topos theory

## Constructions

structures in a cohesive (∞,1)-topos

# Contents

## Idea

The notion of Postnikov tower in an $(\infty,1)$-category is the generalization of the notion of Postnikov tower from the archetypical (∞,1)-category Top$\simeq$ ∞Grpd to more general $(\infty,1)$-categories.

## Definition

###### Proposition

For $C$ a presentable (∞,1)-category the subcategory $C_{\leq n}$ of n-truncated objects is a reflective (∞,1)-subcategory

$C_{\leq n} \stackrel{\overset{\tau_{\leq n}}{\leftarrow}}{\hookrightarrow} C \,.$

This is (Lurie, prop. 5.5.6.18).

We write

$\mathbf{\tau}_{\leq n} : C \stackrel{\tau_{\leq n}}{\to} C_{\leq n} \hookrightarrow C$

for the corresponding localization. For $X \in C$, we say that $\mathbf{\tau}_{\leq n} X$ is the $n$-truncation of $X$.

The reflector of the reflective embedding provides morphisms

$X \to \mathbf{\tau}_{\leq n} X$

from each object to its $n$-truncation.

###### Definition

A Postnikov tower for $X \in C$ is a diagram

$X \to \cdots \to X_2 \to X_1 \to X_0$

such that each $X \to X_n$ exhibits $X_n$ as the $n$-truncation of $X$.

This is HTT, def. 5.5.6.23.

###### Definition

A Postnikov pretower is a pre-tower

$\cdots \to X_2 \to X_1 \to X_0$

(no initial $X$ on the left!) which exhibits each $X_n$ as the $n$-truncation of $X_{n+1}$.

We say Postnikov towers converge in the ambient (∞,1)-category if the forgetful (∞,1)-functor from Postnikov towers to Postnikov pretowers is an equivalence of (∞,1)-categories.

This is (Lurie, def. 5.5.6.23).

## Examples

### In $\infty Grpd$

When the archetypical (∞,1)-topos ∞Grpd is presented by the model structure on simplicial sets, truncation is given by the the coskeleton endofunctor $\mathbf{cosk}_{n+1}$ on sSet.

The unit of the adjunction $(tr_n \dashv cosk_n)$

$\mathbf{\tau}_n : \mathbf{}X \to \mathbf{cosk}_n(X)$

sends an $\infty$-groupoid modeled as a Kan complex simplicial set to its $n$-truncation.

Discussion of this can be found for instance in

• William Dwyer, Dan Kan, An obstruction theory for diagrams of simplicial sets (pdf)

• John Duskin Simplicial matrices and the nerves of weak $n$-categories I: Nerves of bicategories , TAC 9 no. 2, (2002). (web)

## Properties

### Criteria for convergence

We discuss conditions that ensure that Postnikov towers converge.

###### Proposition

In an (∞,1)-topos which is locally of finite homotopy dimension, Postnikov towers converge.

This is (Lurie, prop. 7.2.1.10).

## References

Section 6.5

Revised on November 18, 2013 01:01:17 by Fosco Loregian (147.162.114.245)