# nLab n-localic (infinity,1)-topos

### Context

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

(∞,1)-topos theory

## Constructions

structures in a cohesive (∞,1)-topos

# Contents

## Idea

An (∞,1)-topos is $n$-localic if

More precisely: if (∞,1)-geometric morphisms into it are fixed by their restriction to the underlying (n,1)-toposes of $(n-1)$-truncated objects.

To the tower of (n,1)-toposes of $(n-1)$-truncated objects

$\cdots \to \tau_{\leq 3-1} \mathcal{X} \to \tau_{\leq 2-1} \mathcal{X} \to \tau_{\leq 1-1} \mathcal{X} \to \tau_{\leq 0-1} \mathcal{X} \to *$

of a given (∞,1)-topos $\mathcal{X}$ corresponds a tower of $n$-localic toposes $\mathcal{X}_n$ such that $\tau_{\leq n -1} \mathcal{X} \simeq \tau_{\leq n-1} \mathcal{X}_n$. We may think of the $n$-localic $\mathcal{X}_n$ as being $n$th stage in the Postnikov tower decomposition of $\mathcal{X}$.

A 0-localic $(1,1)$-topos is a localic topos from ordinary topos theory.

## Definition

We write (∞,1)Topos for the (∞,1)-category of (∞,1)-toposes and (∞,1)-geometric morphisms between them.

For $\mathcal{X}$ an (∞,1)-topos we denote by

$\tau_{\leq n-1} \mathcal{X} \hookrightarrow \mathcal{X}$

the (n,1)-topos of $(n-1)$-truncated objects of $\mathcal{X}$.

We write $(n,1)Topos$ for the (n+1,1)-category of (n,1)-toposes and $(n,1)$-geometric morphisms between them.

###### Definition

($n$-localic $(\infty,1)$-topos)

An (∞,1)-topos $\mathcal{X}$ is $n$-localic if for any other $(\infty,1)$-topos $\mathcal{Y}$ the canonical morphism

$(\infty,1)Topos(\mathcal{Y},\mathcal{X}) \to (n,1)Topos(\tau_{\leq n-1} \mathcal{Y}, \tau_{\leq n-1}\mathcal{Y})$

is an equivalence of (∞,1)-categories (of ∞-groupoids).

More generally,

a (k,1)-topos $\mathcal{X}$ is $n$-localic for $0 \leq n \leq k \leq \infty$ if for any other $(k,1)$-topos $\mathcal{Y}$ the canonical morphism

$(k,1)Topos(\mathcal{Y},\mathcal{X}) \to (n,1)Topos(\tau_{\leq n-1} \mathcal{Y}, \tau_{\leq n-1}\mathcal{Y})$

is an equivalence of (∞,1)-categories (of ∞-groupoids).

This is (HTT, def. 6.4.5.8).

###### Remark

This implies that an $n$-localic $(\infty,1)$-topos is also $(n+1)$-localic and generally $k$-localic for all $k \geq n$.

## Examples

###### Proposition

The (∞,1)-category of (∞,1)-sheaves over an (∞,1)-site $C$ with finite limits which is an (n,1)-category is $n$-localic.

This is (HTT, lemma 6.4.5.6).

###### Remark

For $n = 0$ this implies the familiar statement from ordinary topos theory: a category of sheaves over a posite=(0,1)-site is a localic topos (= 0-localic $(1,1)$-topos).

This is (LurieStructured, lemma 2.3.16).

###### Proposition

For $n \in \mathbb{N}$ and $\mathcal{X}$ an $n$-localic $(\infty,1)$-topos, the over-(∞,1)-topos $\mathcal{X}/U$ is $n$-localic precisely if the object $U$ is $n$-truncated.

This is (StrSp, lemma 2.3.14).

###### Proposition

For $\mathcal{X}$ an $n$-localic $(\infty,1)$-topos let $U \in \mathcal{X}$ be an object. Then the following are equivalent

1. the restriction of the inverse image $U^* : \mathcal{X} \to \mathcal{X}/U$ (of the etale geometric morphism from the over-(∞,1)-topos) to $(n-1)$-truncated objects is an equivalence of (∞,1)-categories;

2. the object $U$ is $n$-connected.

This is (StrSp, lemma 2.3.14).

## Properties

###### Proposition

Every (n,1)-topos $\mathcal{Y}$ is the (n,1)-category of $(n-1)$-truncated objects in an $n$-localic $(\infty,1)$-topos $\mathcal{X}_n$

$\tau_{n-1} X_n \stackrel{\simeq}{\to} \mathcal{Y} \,.$

This is (HTT, prop. 6.4.5.7).

Let $\mathcal{G}$ be a geometry (for structured (∞,1)-toposes).

###### Proposition

If $\mathcal{G}$ is an (∞,n)-category then a $n$-localic $\mathcal{G}$-structured (∞,1)-topos is an $n$-truncated object in the (∞,1)-category $Topos(\mathcal{G})$.

This is StrSp, lemma 2.6.17

## References

The general noion is the topic of section 6.4.5 of

Remarks on the application of $n$-localic $(\infty,1)$-toposes in higher geometry are in

Revised on November 19, 2013 08:39:27 by Urs Schreiber (145.116.130.92)