# Cochain complex for a group action

## Contents

This article gives a basic definition in the following area: group cohomology
View other basic definitions in group cohomology |View terms related to group cohomology |View facts related to group cohomology

## Definition

Given a group $G$ acting as automorphisms on an Abelian group $A$, the cochain complex for this group action is defined as follows:

• The $n^{th}$ cochain group is the group of all maps from $G^n$ to $A$ (by maps here is meant set-theoretic maps)
• The boundary map, or derivation, from the $n^{th}$ cochain group to the $(n+1)^{th}$ cochain group, is defined as follows:

$(d_nf)(g_1,g_2,\ldots,g_n,g_{n+1}) = g_1.f(g_2,g_3,\ldots,g_{n+1}) - f(g_1g_2,g_3,\ldots,g_{n+1}) + f(g_1,g_2g_3,\ldots,g_{n+1}) \ldots + (-1)^{n+1}f(g_1,g_2,\ldots,g_n)$

The lack of a left-right interchange symmetry is because the group is acting from the left side. If the group were acting on the left as well as the right, we would have a $g_{n+1}$ acting on the right in the last term.

The fact that this is a cochain complex follows from the (not-too-hard) fact that:

$d_n \circ d_{n-1} = 0$ The cohomology of this cochain complex is defined as the cohomology of the group action.

## Related notions

### Cocycle groups

The $n^{th}$ cocycle group, denoted as $Z^n(G)$, is defined as the kernel of the map $d_n$. In other words, it is the additive group of those functions $f:G^n \to A$ such that:

$g_1.f(g_2,g_3,\ldots,g_{n+1}) - f(g_1g_2,g_3,\ldots,g_{n+1}) + f(g_1,g_2g_3,\ldots,g_{n+1}) \ldots + (-1)^{n+1}f(g_1,g_2,\ldots,g_n) = 0$

for all tuples $(g_1,g_2,\ldots,g_{n+1}) \in G^{n+1}$

### Coboundary groups

The $n^{th}$ coboundary group, $B^n(G)$, is defined as the image of the map $d_{n-1}$.

Note that $B^n(G)$ is a subgroup of $Z^n(G)$, by the fact that this is a complex.

### Cohomology groups

Further information: cohomology groups for a group action

The $n^{th}$ cohomology group, $H^n(G)$ is defined as the quotient $Z^n(G)/B^n(G)$. It is in other words the homology of the cochain complex.

Particular cohomology groups ,specially the first ,second and third ones, are of direct significance even in finite group theory. These are discussed in separate articles.