# nLab semi-abelian category

Semiabelian categories

# Semiabelian categories

## Idea

The notion of semi-abelian category is supposed to capture the properties of categories such as that of groups, rings without unit, associative algebras without unit, Lie algebras, etc.; in generalization of how the notion of abelian categories captures the properties of the categories of abelian groups and of modules, etc.

###### Remark

Here it is important to consider rings and algebras without unit (really: not necessarily having a unit), since otherwise there is no zero object, and also to allow ideals to appear as subrings-without-unit.

Note that the category of rings with unit is still protomodular.

## Definition

A category $C$ is semi-abelian if it

In other words, it is a homological category which is Barr-exact and has finite coproducts.

Equivalently, $C$ is semi-abelian if:

###### Definition

(split short five lemma)

Given a commutative diagram

$\array{ L & \overset{l}{\to} & F & \overset{q}{\to} & C \\ {}^{\mathllap{u}}\downarrow && \downarrow^{\mathrlap{w}} && \downarrow^{\mathrlap{v}} \\ K & \underset{k}{\to} & E& \underset{p}{\to} & B }$

where

• $p$ and $q$ are split epimorphisms

• and $l$ and $k$ are their kernels,

then if $u$ and $v$ are isomorphisms so is $w$.

To see that the second list of axioms implies the existence of finite limits, observe that the pullback

$\array{P & \to & A\\ \downarrow && \downarrow^f\\ B& \underset{g}{\to} & C}$

can be computed as the pullback

$\array{P & \to & A\times B\\ \downarrow && \downarrow^{(1,1,f)}\\ A\times B& \underset{(1,1,g)}{\to} & A\times B\times C}$

in which both legs are split monics. Filling in one of the equivalent definitions of Barr-exactness, the equivalence of the two lists of axioms reduces to showing that in a Barr-exact category with coproducts and a zero object, protomodularity is equivalent to the Split Short Five Lemma; see the paper referenced below for a proof.

## Examples

• Every abelian category is semi-abelian. Conversely, a semi-abelian category is abelian if and only if it is additive (since any exact additive category is abelian), and if and only if its opposite is semi-abelian.

• The category Grp of not-necessarily-abelian groups is semi-abelian but not abelian. So are the categories of rings without units, algebras without units, Lie algebras, and many other sorts of algebras. (The category of rings with unit is not semi-abelian since it lacks a zero object.)

• More generally, the category of internal group objects in any exact category is semi-abelian as soon as it has finite coproducts. For instance, this applies to internal groups in any topos with a NNO.

• The opposite of any topos, such as $Set^{op}$, is Barr-exact and protomodular, but obviously lacks a zero object.

• The category of Heyting semilattices

• The category of (ordinary) Lie algebras

• The category $Set_*$ of pointed sets is Barr-exact with finite coproducts and a zero object, but is not semi-abelian: protomodularity and the Split Short Five Lemma fail to hold.

• If $C$ is exact and protomodular with finite colimits, then for any $x\in C$ the over-under category $(x/C/x)$ is semi-abelian. For example, the opposite of the category of pointed objects in a topos is semi-abelian, and in particular, $Set_*^{op}$ is semi-abelian.

• The categories of crossed modules, crossed complexes, and their friends are semi-abelian; see example 4.2.6 of the Van der Linden paper referenced below.

• the category of cocommutative Hopf algebras over a field. Was proven in the paper by Gran, Sterck and Vercruysse referenced below.

## References

Last revised on April 21, 2024 at 10:30:49. See the history of this page for a list of all contributions to it.