Group cohomology


In mathematics, group cohomology is a set of mathematical tools used to study groups using cohomology theory, a technique from algebraic topology. Analogous to group representations, group cohomology looks at the group actions of a group G in an associated G-module M to elucidate the properties of the group. By treating the G-module as a kind of topological space with elements of representing n-simplices, topological properties of the space may be computed, such as the set of cohomology groups. The cohomology groups in turn provide insight into the structure of the group G and G-module M themselves. Group cohomology plays a role in the investigation of fixed points of a group action in a module or space and the quotient module or space with respect to a group action. Group cohomology is used in the fields of abstract algebra, homological algebra, algebraic topology and algebraic number theory, as well as in applications to group theory proper. As in algebraic topology, there is a dual theory called group homology. The techniques of group cohomology can also be extended to the case that instead of a G-module, G acts on a nonabelian G-group; in effect, a generalization of a module to non-Abelian coefficients.
These algebraic ideas are closely related to topological ideas. The group cohomology of a discrete group G is the singular cohomology of a suitable space having G as its fundamental group, namely the corresponding Eilenberg–MacLane space. Thus, the group cohomology of can be thought of as the singular cohomology of the circle S1, and similarly for and
A great deal is known about the cohomology of groups, including interpretations of low-dimensional cohomology, functoriality, and how to change groups. The subject of group cohomology began in the 1920s, matured in the late 1940s, and continues as an area of active research today.

Motivation

A general paradigm in group theory is that a group G should be studied via its group representations. A slight generalization of those representations are the G-modules: a G-module is an abelian group M together with a group action of G on M, with every element of G acting as an automorphism of M. We will write G multiplicatively and M additively.
Given such a G-module M, it is natural to consider the submodule of G-invariant elements:
Now, if N is a G-submodule of M, it isn't in general true that the invariants in are found as the quotient of the invariants in M by those in N: being invariant 'modulo N ' is broader. The purpose of the first group cohomology is to precisely measure this difference.
The group cohomology functors in general measure the extent to which taking invariants doesn't respect exact sequences. This is expressed by a long exact sequence.

Definitions

The collection of all G-modules is a category. Sending each module M to the group of invariants yields a functor from the category of G-modules to the category Ab of abelian groups. This functor is left exact but not necessarily right exact. We may therefore form its right derived functors. Their values are abelian groups and they are denoted by, "the n-th cohomology group of G with coefficients in M". Furthermore, the group may be identified with.

Cochain complexes

The definition using derived functors is conceptually very clear, but for concrete applications, the following computations, which some authors also use as a definition, are often helpful. For, let be the group of all functions from to M. This is an abelian group; its elements are called the n-cochains. The coboundary homomorphisms
One may check that so this defines a cochain complex whose cohomology can be computed. It can be shown that the above-mentioned definition of group cohomology in terms of derived functors is isomorphic to the cohomology of this complex
Here the groups of n-cocycles, and n-coboundaries, respectively, are defined as

The functors Ext''n'' and formal definition of group cohomology

Interpreting G-modules as modules over the group ring one can note that
i.e., the subgroup of G-invariant elements in M is identified with the group of homomorphisms from, which is treated as the trivial G-module to M.
Therefore, as Ext functors are the derived functors of Hom, there is a natural isomorphism
These Ext groups can also be computed via a projective resolution of, the advantage being that such a resolution only depends on G and not on M. We recall the definition of Ext more explicitly for this context. Let F be a projective -resolution of the trivial -module :
e.g., one may always take the resolution of group rings, with morphisms
Recall that for -modules N and M, HomG is an abelian group consisting of -homomorphisms from N to M. Since is a contravariant functor and reverses the arrows, applying to F termwise and dropping produces a cochain complex :
The cohomology groups of G with coefficients in the module M are defined as the cohomology of the above cochain complex:
This construction initially leads to a coboundary operator that acts on the "homogeneous" cochains. These are the elements of, that is, functions that obey
The coboundary operator is now naturally defined by, for example,
The relation to the coboundary operator d that was defined in the previous section, and which acts on the "inhomogeneous" cochains, is given by reparameterizing so that
and so on. Thus
as in the preceding section.

Group homology

Dually to the construction of group cohomology there is the following definition of group homology: given a G-module M, set DM to be the submodule generated by elements of the form g·m − m, g ∈ G, m ∈ M. Assigning to M its so-called coinvariants, the quotient
is a right exact functor. Its left derived functors are by definition the group homology
The covariant functor which assigns MG to M is isomorphic to the functor which sends M to where is endowed with the trivial G-action. Hence one also gets an expression for group homology in terms of the Tor functors,
Note that the superscript/subscript convention for cohomology/homology agrees with the convention for group invariants/coinvariants, while which is denoted "co-" switches:
Specifically, the homology groups Hn can be computed as follows. Start with a projective resolution F of the trivial -module as in the previous section. Apply the covariant functor to F termwise to get a chain complex :
Then Hn are the homology groups of this chain complex, for n ≥ 0.
Group homology and cohomology can be treated uniformly for some groups, especially finite groups, in terms of complete resolutions and the Tate cohomology groups.
The group homology of abelian groups G with values in a principal ideal domain k is closely related to the exterior algebra.

Low-dimensional cohomology groups

''H'' 1

The first cohomology group is the quotient of the so-called crossed homomorphisms, i.e. maps f : G → M satisfying f = f + af for all a, b in G, modulo the so-called principal crossed homomorphisms, i.e. maps f : G → M given by f = am−m for some fixed m ∈ M. This follows from the definition of cochains above.
If the action of G on M is trivial, then the above boils down to H1 = Hom, the group of group homomorphisms G → M.
Consider the case of where denotes the non-trivial -structure on the group of integers. Then crossed homomorphisms constitute all maps satisfying and for some integer a. Principal crossed homomorphisms satisfy additionally hence

''H'' 2

If M is a trivial G-module, the second cohomology group H2 is in one-to-one correspondence with the set of central extensions of G by M. More generally, if the action of G on M is nontrivial, H2 classifies the isomorphism classes of all extensions of G by M, in which the action of G on E, endows M with isomorphic G-module structure.
In the example as above, as the only extension of by with the given nontrivial action is the infinite dihedral group.
An example of a second group cohomology group is the Brauer group: it is the cohomology of the absolute Galois group of a field k which acts on the invertible elements in a separable closure:

Properties

In the following, let M be a G-module.

Long exact sequence of cohomology

In practice, one often computes the cohomology groups using the following fact: if
is a short exact sequence of G-modules, then a long exact sequence is induced:
The so-called connecting homomorphisms,
can be described in terms of inhomogeneous cochains as follows. If is represented by an n-cocycle then is represented by where is an n-cochain "lifting" .

Functoriality

Group cohomology depends contravariantly on the group G, in the following sense: if f : H → G is a group homomorphism, then we have a naturally induced morphism Hn → Hn. This map is called the restriction map. If the index of H in G is finite, there is also a map in the opposite direction, called transfer map,
In degree 0, it is given by the map
Given a morphism of G-modules M → N, one gets a morphism of cohomology groups in the Hn → Hn.

Products

Similarly to other cohomology theories in topology and geometry, such as singular cohomology or de Rham cohomology, group cohomology enjoys a product structure: there is a natural map called cup product:
for any two G-modules M and N. This yields a graded anti-commutative ring structure on where R is a ring such as or For a finite group G, the even part of this cohomology ring in characteristic p, carries a lot of information about the group the structure of G, for example the Krull dimension of this ring equals the maximal rank of an abelian subgroup.
For example, let G be the group with two elements, under the discrete topology. The real projective space is a classifying space for G. Let the field of two elements. Then
a polynomial k-algebra on a single generator, since this is the cellular cohomology ring of

Künneth formula

If, M = k is a field, then H* is a graded k-algebra and the cohomology of a product of groups is related to the ones of the individual groups by a Künneth formula:
For example, if G is an elementary abelian 2-group of rank r, and then the Künneth formula shows that the cohomology of G is a polynomial k-algebra generated by r classes in H1.,

Homology vs. cohomology

As for other cohomology theories, such as singular cohomology, group cohomology and homology are related to one another by means of a short exact sequence
where A is endowed with the trivial G-action and the term at the left is the first Ext group.

Amalgamated products

Given a group A which is the subgroup of two groups G1 and G2, the homology of the amalgamated product lies in a long exact sequence
The homology of can be computed using this:
This exact sequence can also be applied to show that the homology of the and the special linear group agree for an infinite field k.

Change of group

The Hochschild–Serre spectral sequence relates the cohomology of a normal subgroup N of G and the quotient G/N to the cohomology of the group G. From it, one gets the inflation-restriction exact sequence.

Cohomology of the classifying space

Group cohomology is closely related to topological cohomology theories such as sheaf cohomology, by means of an isomorphism
The expression BG at the left is a classifying space for G. It is an Eilenberg–MacLane space K. Classifying spaces for and are the 1-sphere S1, infinite real projective space and lens spaces, respectively. In general, BG can be constructed as the quotient EG/G, where EG is a contractible space on which G acts freely. However, BG does not usually have an easily amenable geometric description.
More generally, one can attach to any G-module M a local coefficient system on BG and the above isomorphism generalizes to an isomorphism

Cohomology of finite groups

Higher cohomology groups are torsion

The cohomology groups Hn of finite groups G are all torsion for all n≥1. Indeed, by Maschke's theorem the category of representations of a finite group is semi-simple over any field of characteristic zero, hence, viewing group cohomology as a derived functor in this abelian category, one obtains that it is zero. The other argument is that over a field of characteristic zero, the group algebra of a finite group is a direct sum of matrix algebras, while a matrix algebra is Morita equivalent to its base field and hence has trivial cohomology.
If the order of G is invertible in a G-module M, the transfer map can be used to show that for A typical application of this fact is as follows: the long exact cohomology sequence of the short exact sequence
yields an isomorphism

Tate cohomology

groups combine both homology and cohomology of a finite group G:
where is induced by the norm map:
Tate cohomology enjoys similar features, such as long exact sequences, product structures. An important application is in class field theory, see class formation.
Tate cohomology of finite cyclic groups, is 2-periodic in the sense that there are isomorphisms
A necessary and sufficient criterion for a d-periodic cohomology is that the only abelian subgroups of G are cyclic. For example, any semi-direct product has this property for coprime integers n and m.

Applications

Algebraic K-theory and homology of linear groups

is closely related to group cohomology: in Quillen's +-construction of K-theory, K-theory of a ring R is defined as the homotopy groups of a space Here is the infinite general linear group. The space has the same homology as i.e., the group homology of GL. In some cases, stability results assert that the sequence of cohomology groups
becomes stationary for large enough n, hence reducing the computation of the cohomology of the infinite general linear group to the one of some. Such results have been established when R is a field or for rings of integers in a number field.
The phenomenon that group homology of a series of groups stabilizes is referred to as homological stability. In addition to the case just mentioned, this applies to various other groups such as symmetric groups or mapping class groups.

Projective representations and group extensions

In quantum mechanics we often have systems with a symmetry group We expect an action of on the Hilbert space by unitary matrices We might expect but the rules of quantum mechanics only require
where is a phase. This projective representation of can also be thought of as a conventional representation of a group extension of by as described by the exact sequence
Requiring associativity
leads to
which we recognise as the statement that i.e. that is a cocycle taking values in We can ask whether we can eliminate the phases by redefining
which changes
This we recognise as shifting by a coboundary The distinct projective representations are therefore classified by Note that if we allow the phases themselves to be acted on by the group, then the first term in each of the coboundary operations will have a acting on it as in the general definitions of coboundary in the previous sections. For example,

Extensions

Cohomology of topological groups

Given a topological group G, i.e., a group equipped with a topology such that product and inverse are continuous maps, it is natural to consider continuous G-modules, i.e., requiring that the action
is a continuous map. For such modules, one can again consider the derived functor of. A special case occurring in algebra and number theory is when G is profinite, for example the absolute Galois group of a field. The resulting cohomology is called Galois cohomology.

Non-abelian group cohomology

Using the G-invariants and the 1-cochains, one can construct the zeroth and first group cohomology for a group G with coefficients in a non-abelian group. Specifically, a G-group is a group A together with an action by G.
The zeroth cohomology of G with coefficients in A is defined to be the subgroup
of elements of A fixed by G.
The first cohomology of G with coefficients in A is defined as 1-cocycles modulo an equivalence relation instead of by 1-coboundaries. The condition for a map to be a 1-cocycle is that and if there is an a in A such that. In general, is not a group when A is non-abelian. It instead has the structure of a pointed set – exactly the same situation arises in the 0th homotopy group, which for a general topological space is not a group but a pointed set. Note that a group is in particular a pointed set, with the identity element as distinguished point.
Using explicit calculations, one still obtains a truncated long exact sequence in cohomology. Specifically, let
be a short exact sequence of G-groups, then there is an exact sequence of pointed sets

History and relation to other fields

The low-dimensional cohomology of a group was classically studied in other guises, well before the notion of group cohomology was formulated in 1943–45. The first theorem of the subject can be identified as Hilbert's Theorem 90 in 1897; this was recast into Emmy Noether's equations in Galois theory. The idea of factor sets for the extension problem for groups arose in the work of Otto Hölder, in Issai Schur's 1904 study of projective representations, in Otto Schreier's 1926 treatment, and in Richard Brauer's 1928 study of simple algebras and the Brauer group. A fuller discussion of this history may be found in.
In 1941, while studying , Heinz Hopf discovered what is now called Hopf's integral homology formula, which is identical to Schur's formula for the Schur multiplier of a finite, finitely presented group:
where and F is a free group.
Hopf's result led to the independent discovery of group cohomology by several groups in 1943-45: Samuel Eilenberg and Saunders Mac Lane in the United States ; Hopf and Beno Eckmann in Switzerland; and Hans Freudenthal in the Netherlands. The situation was chaotic because communication between these countries was difficult during World War II.
From a topological point of view, the homology and cohomology of G was first defined as the homology and cohomology of a model for the topological classifying space BG as discussed above. In practice, this meant using topology to produce the chain complexes used in formal algebraic definitions. From a module-theoretic point of view this was integrated into the Cartan–Eilenberg theory of homological algebra in the early 1950s.
The application in algebraic number theory to class field theory provided theorems valid for general Galois extensions. The cohomological part of class field theory was axiomatized as the theory of class formations. In turn, this led to the notion of Galois cohomology and étale cohomology . Some refinements in the theory post-1960 have been made, such as continuous cocycles and John Tate's redefinition, but the basic outlines remain the same. This is a large field, and now basic in the theories of algebraic groups.
The analogous theory for Lie algebras, called Lie algebra cohomology, was first developed in the late 1940s, by Claude Chevalley and Eilenberg, and Jean-Louis Koszul. It is formally similar, using the corresponding definition of invariant for the action of a Lie algebra. It is much applied in representation theory, and is closely connected with the BRST quantization of theoretical physics.
Group cohomology theory also has a direct application in condensed matter physics. Just like group theory being the mathematical foundation of spontaneous symmetry breaking phases, group cohomology theory is the mathematical foundation of a class of quantum states of matter—short-range entangled states with symmetry. Short-range entangled states with symmetry are also known as symmetry-protected topological states.