semi-abelian category

Semiabelian categories


Homological algebra

homological algebra

(also nonabelian homological algebra)



Basic definitions

Stable homotopy theory notions



diagram chasing

Homology theories


Semiabelian categories


The axioms of a semi-abelian category are supposed to capture the properties of the categories of groups, rings without unit, associative algebras without unit, Lie algebras, as nicely as the axioms of an abelian category captures the properties of the categories of abelian groups and of modules.

It is important to take rings and algebras without unit, since otherwise there is no zero object. And, similarly, ideals are not subrings-with-unit. Note that the category of rings is still protomodular, however.


A category CC is semi-abelian if it

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

Equivalently, CC is semi-abelian if:


(split short five lemma)

Given a commutative diagram

L l F q C u w v K k E p B \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 }


then if uu and vv are isomorphisms so is ww.

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

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

can be computed as the pullback

P A×B (1,1,f) A×B (1,1,g) A×B×C\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.


  • 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 opSet^{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 *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 CC is exact and protomodular with finite colimits, then for any xCx\in C the over-under category (x/C/x)(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 * opSet_*^{op} is semi-abelian.

Urs: how can I understand that this (has to?) involve the opposite category?

Mike: Well, as the previous example shows, Set *Set_* itself is not semi-abelian. The way I’m thinking of it is that a surjection of pointed sets is not determined by its kernel, but an injection of pointed sets is determined by its cokernel.

  • 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 papers by Gran/Kadjo/Vercruysse and Gran/Sterck/Vercruyse




Dold–Kan correspondence

  • The analogue of a Dold–Kan correspondence? holds for simplicial objects in semi-abelian categories.


Last revised on October 13, 2020 at 19:29:58. See the history of this page for a list of all contributions to it.