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ISSN: 1736-4337
Journal of Generalized Lie Theory and Applications
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Some Aspects of Semi-Abelian Homology and Protoadditive Functors

Everaert T and Gran M*

Catholic University of Louvain, Place de l'Université 1, 1348, Belgium

Corresponding Author:
Gran M
President of Research Institute of Mathematics and Physics
Catholic University of Louvain, Place de l'Université 1
1348, Belgium
Tel: +3210472111
E-mail: [email protected]

Received date October 11, 2015; Accepted date November 20, 2015; Published date November 27, 2015

Citation: Everaert T, Gran M (2015) Some Aspects of Semi-Abelian Homology and Protoadditive Functors. J Generalized Lie Theory Appl 9:238. doi:10.4172/1736-4337.1000238

Copyright: © 2015 Everaert T, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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In this note some recent developments in the study of homology in semi-abelian categories are briefly presented. In particular the role of protoadditive functors in the study of Hopf formulae for homology is explained.


Semiabelian homology; Centralisation functor; Protoadditive functors


The discovery of higher Hopf formulae for the homology of a group, due to Ronald Brown and Graham Ellis [1], has naturally led to some new perspectives in non-abelian homological algebra. An important advance in this area was made by George Janelidze, who found a connection between group homology and categorical Galois theory [2-5], the latter being a wide extension of Alexander Grothendieck’s theory [6]. Among other things, this cleared the path for the discovery of higher Hopf formulae for the homology of general algebraic structures [7-14]. Here, a crucial role is played by the so-called higher order central extensions, which are the covering morphisms with respect to certain Galois structures induced by a reflection

equation (1)

whose left adjoint equation is sometimes called the “coefficient functor”. Here, equation could, for instance, be the variety of groups, equation its subvariety of abelian groups, and I the abelianisation functor. In this case, the induced higher order central extensions are related to the Brown- Ellis Hopf-formulae, as explained below. More generally, higher order central extensions can be defined for any semi-abelian category equation ([15] e.g., the varieties of groups, rings, Lie algebras, (pre)crossed modules, compact groups, or any abelian category) and any Birkhoff subcategory (i.e., a reflective subcategory closed under subobjects and regular quotients) equation of equation. When, moreover, equation has enough projectives, one obtains higher Hopf formulae for the homology induced by the reflector (or, coefficient functor) equation.

In order to explicitly determine the Hopf-Brown-Ellis formulae for homology in specific algebraic contexts, it is crucial to find suitable descriptions of the higher central extensions, as for instance in terms of algebraic conditions using “generalised commutators”. In general, this is a non-trivial problem, about which we are going to say more in what follows.

Note that, in fact, different approaches to obtaining higher Hopf formulae exist, which can be used in different categorical contexts, based on the comonadic homology theory of Barr and Beck [7,8,10,16], on the abstract notion of Galois group [13,17,18], or on the theory of satellites [19]. These methods essentially coincide in the situation described above, namely for equation a semi-abelian category with enough projectives and equation a Birkhoff subcategory of equation.

Assume that we are in this situation. In this case, the reflector equation induces a first “centralisation functor” I1 from the category Ext (equation) of extensions (i.e., regular epimorphisms) in equation to the full subcategory CExt equation(equation) of extensions that are central with respect to equation.

equation (2)

This functor I1 is the left adjoint of the inclusion functor U1. The notion of centrality comes from categorical Galois theory and depends on the choice of the Birkhoff subcategory equation. It is defined in purely categorical terms [10,20].

The centralisation I1(f) of an extension f is given by a quotient


where [Ker(f), A]equation may be thought of as a commutator of Ker(f) and A, defined relatively to equation. For instance, in the classical case of the reflection


this relative commutator is simply the group-theoretical commutator of normal subgroups:

[Ker(f), A]Ab = [Ker(f), A].

Hence, in this case I1 is the usual centralisation functor from the category of group extensions to its full subcategory of central extensions in the classical sense.

Remark that the commutator [Ker(f), A] appears as the denominator in the Hopf formula for the second integral homology group: given a free presentation

0 → K → F → G → 0,

of a group G, Hopf’s formula tells us that the second homology group is given by the quotient


This is not a coincidence: a similar phenomenon occurs for the higher-order homology groups, where the subgroups appearing as denominator of the Brown-Ellis Hopf formulae are exactly what is required to transform higher-order extensions into a higher-order central extensions, universally.

To illustrate this idea, consider the case of the third homology group. For this, let


be a double presentation of a group G so that F, F/K1 and F/K2 are free groups and the square is a double extension: a pushout of surjective homomorphisms. As shown by Brown and Ellis, the third integral homology group equation of G is given by

equation (4)

Once more, the denominator gives precisely the normal subgroup by which one has to “quotient out” the group F in order to make this double extension a double central extension, universally:


Once again, the notion of centrality comes from categorical Galois theory, and this time depends on the induced reflection (2).

Now, the formula (4) is a special case of the general Hopf formula for the third homology corresponding to a reflection (1), with equation an arbitrary semi-abelian category with enough projectives, and equation any Birkhoff subcategory of equation [10]: starting from a double presentation f of the form (3), with F, F/K1 and F/K2 regular projective objects of the third homology object is given by a quotient

equation (5)

As in the case of groups, also in a general semi-abelian category equation the denominator L2 [f]equation of this generalised Hopf formula relative to equation is the normal subobject of F that has to be “quotiented out” in order to universally turn the double extension (3) into a double central extension. Hence, in particular, for equation = Gp and equation = Ab, we have the equality


and the formula (4) appears as a special case of (5). In general, for a given Birkhoff reflection (1) in a semi-abelian category, L2[ f]equation may be difficult to compute.

Similar formulae exist for the higher homology objects, again valid in any semi-abelian category equation with enough projectives and for any Birkhoff subcategory equation of equation. This yields, at least in principle, a description of all homology objects Hn(A,equation)(n ≥ 2). In practice, a suitable characterisation of the higher-order central extensions is required.

In some cases it has been possible to compute these formulae explicitly. For example, this has been done in literature of Everaert [10] for equation the variety of precrossed modules and equation its subvariety of crossed modules, or for equation the variety of groups and equation its subvariety of nilpotent groups of a fixed class k ≥ 1 [21], or the variety of solvable groups of a fixed class k ≥ 1. Similar results have been obtained in the categories of Leibniz and of Lie n-algebras in studies of Casas [22].

However, in general, computing the Hopf formulae explicitly is a non-trivial task. One possible strategy is to look for suitable conditions on the coefficient functor equation that facilitate such computations. In literature of Everaert [12,14] we have shown that a natural such condition is the requirement that the reflector I is a protoadditive functor. This notion extends the one of additive functor to the nonadditive context of pointed protomodular categories [23]: when equation and equation are pointed protomodular categories (for instance, equation and equation could be semi-abelian), a functor equation is protoadditive if, for any split short exact sequence


in equation (i.e., equation and k = Ker(f)), its image


under I is a split short exact sequence in equation. Whenever the coefficient functor equation is protoadditive, explicit Hopf formulae can be established in different algebraic and topological contexts. In particular, the protoadditivity condition is fundamental to explore some new Galois theories induced by torsion theories [24-26].

We refer the interested reader to the articles [12,14] for a thorough study of the theory of protoadditive functors and their use in semiabelian homological algebra. In the first article we study the homology of n-fold internal groupoids in a semi-abelian category: these results apply in particular to the so-called catn-groups in the sense of Loday [27]. The crucial point there is that the connected components functor π0: Grpd(equation) → equation is protoadditive whenever equation is semi-abelian. In studies of Everaert [14] the general theory of protoadditive functors is investigated, as well as the so-called derived torsion theories of a torsion theory having the reflector to its torsion-free subcategory protoadditive. These derived torsion theories induce a chain of Galois structures in the categories of higher order extensions. The results concerning the homology objects can be applied in particular to some new torsion theories in the category of compact groups and, more generally, in any category of compact semi-abelian algebras introduced in framework of Borceux and Clementino [28]. Further developments in this direction, including some new results in the categories of commutative rings and of topological groups, for instance, can be found in literature of Duckerts-Antoine [13,18].


This article is partly based on the text of a talk given by the second author on this joint work at the Séminaire Itinérant des Catégories that took place at the Université Paris Didérot on October 25, 2009.


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