# Group cohomology of Klein four-group

This article gives specific information, namely, group cohomology, about a particular group, namely: Klein four-group.

View group cohomology of particular groups | View other specific information about Klein four-group

## Contents

## Classifying space and corresponding chain complex

The classifying space of the Klein four-group is the product space , where is infinite-dimensional real projective space.

A chain complex that can be used to compute the homology of this space is given as follows:

- The chain group is a sum of copies of , indexed by ordered pairs where . In other words, the chain group is:

- The boundary map is given by adding up the following maps:

- The map is multiplication by zero if is odd and is multiplication by two if is even.
- The map is multiplication by zero if is odd and multiplication by two if is even.

## Homology groups

### Over the integers

The homology groups with coefficients in the ring of integers are given as follows:

These homology groups can be obtained from the knowledge of the homology groups of cyclic group:Z2 (see group cohomology of cyclic group:Z2) using the Kunneth formula. They can also be computed explicitly using the chain complex description above.

### Over an abelian group

The homology groups with coefficients in an abelian group (which may be equipped with additional structure as a module over a ring ) are given as follows:

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## Cohomology groups and cohomology ring

### Groups over the integers

The cohomology groups with coefficients in the integers are given as below:

## Second cohomology groups and extensions

### Schur multiplier

The Schur multiplier, defined as the second cohomoogy group for trivial group action and also as the second homology group , is isomorphic to cyclic group:Z2.

See also the projective representation theory of Klein four-group.