# nLab effective epimorphism in an (infinity,1)-category

### Context

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

(∞,1)-category theory

# Contents

## Idea

The generalization of the notion of effective epimorphism from category theory to (∞,1)-category theory.

## Definition

###### Definition

A morphism $f : Y \to X$ in an (∞,1)-category is an effective epimorphism if it has a Cech nerve, of which it is the (∞,1)-colimit; in other words the augmented simplicial diagram

$\cdots Y \times_X Y \times_X Y \stackrel{\tl}{\stackrel{\to}{\to}} Y \times Y \times_X Y \stackrel{\to}{\to} Y \stackrel{f}{\to} X$

is a colimiting diagram.

This appears below HTT, cor. 6.2.3.5 for $C$ a (∞,1)-semitopos, but seems to be a good definition more generally.

## Properties

### Factorization

In an (∞,1)-topos the effective epis are the n-epimorphisms for $n = 1$ sitting in the (n-epi, n-mono) factorization system for $n = 1$ with the monomorphism in an (∞,1)-category, factoring every morphism through its 1-image.

### Stability

###### Proposition

In an (∞,1)-semitopos, effective epimorphisms are stable under (∞,1)-pullback.

This appears as (Lurie, prop. 6.2.3.15).

### Characterization

###### Proposition

For $C$ an (∞,1)-semitopos we have that $f : X \to Y$ is an effective epimorphism precisely if its (-1)-truncation is a terminal object in the over-(∞,1)-category $C/Y$.

This is HTT, cor. 6.2.3.5.

More generally,

###### Proposition

The effective epimorphisms in any (∞,1)-topos are precisely the (-1)-connected morphisms, and form a factorization system together with the monomorphisms (the (-1)-truncated morphisms).

See n-connected/n-truncated factorization system for more on this.

###### Proposition

For $C$ an (∞,1)-topos, a morphism $f : X \to Y$ in $C$ is effective epi precisely if the induced morphism on subobjects ((∞,1)-monos, they form actually a small set) by (∞,1)-pullback

$f^* : Sub(Y) \to Sub(X)$

is injective.

This appears as (Rezk, lemma 7.9).

Useful is also the following characterization:

###### Proposition

A morphism in an (∞,1)-topos is an effective epimorphism precisely if its 0-truncation is an effective epimorphism in the underlying 1-topos.

This is (Lurie, prop. 7.2.1.14).

###### Remark

In words this means that a map is an effective epimorphism if it induces an epimorphism on connected components.

This is true generally in the internal logic of the $(\infty,1)$-topos (i.e. in homotopy type theory, see at 1-epimorphism for more on this), but in ∞Grpd $\simeq L_{whe}$ sSet it is also true externally (prop. 6 below):

###### Example

A morphism of ∞-groupoids $f \colon X \to Y$ is an effective epimorphism precisely if it is a surjection on connected components, hence if

$\pi_0(f) \colon \pi_0(X) \to \pi_0(Y)$

is a surjection of sets.

## Examples

As a corollary of prop. 5 we have

###### Proposition

(effective epis of $\infty$-groupoids)

In $C =$ ∞Grpd a morphism $f : Y \to X$ is an effective epimorphism precisely if it induces an epimorphism $\pi_0 f : \pi_0 Y \to \pi_0 X$ in Set on connected components.

This appears as HTT, cor. 7.2.1.15.

## References

Section 7.7 of

Section 6.2.3 of

Revised on August 18, 2013 14:24:05 by Urs Schreiber (24.131.18.91)