# Second cohomology group for trivial group action of direct product of Z4 and Z4 on Z4

This article gives information about the second cohomology group for trivial group action (i.e., the second cohomology group with trivial action) of the group direct product of Z4 and Z4 on cyclic group:Z4. The elements of this classify the group extensions with cyclic group:Z4 in the center and direct product of Z4 and Z4 the corresponding quotient group. Specifically, these are precisely the central extensions with the given base group and acting group.

The value of this cohomology group is direct product of Z4 and Z4 and Z4.

Get more specific information about direct product of Z4 and Z4 |Get more specific information about cyclic group:Z4|View other constructions whose value is direct product of Z4 and Z4 and Z4

## Description of the group

This article is about the second cohomology group for trivial group action where the acting group is direct product of Z4 and Z4 and the group acted upon is cyclic group:Z4. In other words, it is about the cohomology group:

where and .

The cohomology group is isomorphic to direct product of Z4 and Z4 and Z4. In other words, it is a homocyclic group of order and exponent .

## Elements

We list here the elements, grouped by similarity under the action of the automorphism groups on both sides.

Cohomology class type | Number of cohomology classes | Corresponding group extension | Second part of GAP ID (order is 64) |
---|---|---|---|

trivial | 1 | direct product of Z4 and Z4 and Z4 | 55 |

symmetric, nontrivial, order two | 3 | direct product of Z8 and Z4 and Z2 | 83 |

symmetric, nontrivial, order four | 12 | direct product of Z16 and Z4 | 26 |

one of the non-symmetric ones, order four | 2 | unitriangular matrix group:UT(3,Z4) | 18 |

one of the non-symmetric ones, order two | 1 | SmallGroup(64,57) | 57 |

one of the non-symmetric ones | 6 | SmallGroup(64,19) | 19 |

one of the non-symmetric ones | 3 | direct product of SmallGroup(32,4) and Z2 | 84 |

one of the non-symmetric ones | 24 | semidirect product of Z16 and Z4 via fifth power map | 28 |

one of the non-symmetric ones | 12 | semidirect product of Z16 and Z4 of M-type | 27 |

## Subgroups of interest

## Generalized Baer Lie rings

For this particular choice of and , the symmetric cohomology classes (which correspond to abelian group extensions) and the cyclicity-preserving subgroup do not generate everything, but rather, they generate a subgroup of order 32 and index two, given as two cosets of the symmetric cohomology classes, giving of the 64 possible extensions. The table below shows these generalized Baer Lie rings, with each row representing a coset of the subgroup of symmetric cohomology classes and each column a coset of the subgroup of cohomology classes with a cyclicity-preserving representative. The first row and first column give the subgroup of symmetric coohmology classes and the cyclicity-preserving subgroup respectively. Note that to avoid too many columns, repeated columns are simply indicated by specifying how many columns there are of the type:

This is the smallest case where the subgroup represented by cyclicity-preserving cocycles actually has the property that there are nonzeroadditivecyclicity-preserving cocycles, i.e, they are linear. Thus, the generalized Baer Lie rings that we obtain in fact arise via the linear halving generalization of Baer correspondence.

Here now is the full table with *all* cohomology classes, including the ones that are not cyclicity-preserving: