Second cohomology group for trivial group action of elementary abelian group of prime-square order on group of prime order

Description of the group
Suppose $$p$$ is a prime number.

We consider here the second cohomology group for trivial group action

$$\! H^2(G,A)$$

where $$G$$ is the specific information about::elementary abelian group of prime-square order $$E_{p^2} = \mathbb{Z}_p \times \mathbb{Z}_p$$ and $$A$$ is the specific information about::group of prime order $$\mathbb{Z}_p$$.

The group is isomorphic to elementary abelian group of prime-cube order $$E_{p^3} = \mathbb{Z}_p \times \mathbb{Z}_p \times \mathbb{Z}_p$$. Equivalently, it is a three-dimensional vector space over the field $$\mathbb{F}_p$$ of $$p$$ elements.

The behavior is somewhat different for $$p = 2$$ and odd primes. For the anomalous case $$p = 2$$, see second cohomology group for trivial group action of V4 on Z2.

Computation in terms of group cohomology
The cohomology group can be computed as an abstract group using the group cohomology of elementary abelian group of prime-square order, which in turn can be computed using the Kunneth formula for group cohomology combined with the group cohomology of finite cyclic groups.

We explain here the part of the computation based on the group cohomology of elementary abelian group of prime-square order. As per that page, we have:

$$H^2(G;A) = (\operatorname{Ann}_A(p)) \oplus (A/pA)^2$$

Here, $$A/pA$$ is the quotient of $$A$$ by $$pA = \{ px \mid x \in A \}$$ and $$\operatorname{Ann}_A(p) = \{ x \in A \mid px = 0 \}$$.

In our case, $$A = \mathbb{Z}/p\mathbb{Z}$$, so we get that both $$A/pA$$ and $$\operatorname{Ann}_A(p)$$ are both isomorphic to $$\mathbb{Z}/p\mathbb{Z}$$. Plugging in, we get:

$$H^2(G;A) = \mathbb{Z}/p\mathbb{Z} \oplus (\mathbb{Z}/p\mathbb{Z})^2 = (\mathbb{Z}/p\mathbb{Z})^3$$

which is the elementary abelian group of order $$p^3$$.

Summary
As mentioned earlier, the information here does not apply to the case $$p = 2$$. For the case $$p = 2$$, see second cohomology group for trivial group action of V4 on Z2.

Explicit description and relation with power-commutator presentation
Consider an extension group $$E$$ with central subgroup isomorphic to $$A$$ (group of prime order) and quotient group $$G$$ isomorphic to elementary abelian group of prime-square order. Denote by $$\overline{a_1}, \overline{a_2}$$ a basis for $$G$$ (i.e., two non-identity elements of $$G$$ that do not generate the same cyclic subgroup) and by $$a_1,a_2$$ elements of $$E$$ that map to $$\overline{a_1},\overline{a_2}$$ respectively. Denote by $$a_3$$ a non-identity element of the central subgroup.

Then, $$E$$ is generated by the elements $$a_1,a_2,a_3$$. Further, we can get a power-commutator presentation for $$E$$ using these generators. Specifically, we know that $$[a_1,a_3] = e, [a_2,a_3] = e, a_3^p = e$$. We also know that the elements $$a_1^p, a_2^p, [a_1,a_2]$$ are each equal to some power of $$a_3$$.

In order to specify the cohomology class of the extension, it is necessary and sufficient to specify, for each of $$a_1^p, a_2^p, [a_1,a_2]$$, what power of $$a_3$$ it equals. In terms of the notation for the power-commutator presentation, this is equivalent to saying that $$\beta(1,2) = 0$$ and each of $$\beta(1,3), \beta(2,3), \beta(1,2,3)$$ can be represented as one of the numbers $$0,1,2,\dots,p-1$$. This can be elements of $$\mathbb{Z}/p\mathbb{Z}$$, a concrete realization of the group of prime order. Here:


 * $$\beta(1,3)$$ is the power of $$a_3$$ that $$a_1^p$$ equals. It is 0 if $$a_1^p = e$$ (i.e., is the identity element) and nonzero otherwise.
 * $$\beta(2,3)$$ is the power of $$a_3$$ that $$a_2^p$$ equals. It is 0 if $$a_2^p = e$$ (i.e., is the identity element) and nonzero otherwise.
 * $$\beta(1,2,3)$$ is the power of $$a_3$$ that $$[a_1,a_2]$$ equals. It is 0 if $$[a_1,a_2] = e$$ (i.e., is the identity element) and nonzero otherwise.

The total number of possibilities is $$p^3$$. Further, the mapping from $$H^2(G,A)$$ that sends a cohomology class to the tuple $$(\beta(1,3),\beta(2,3),\beta(1,2,3))$$ is an isomorphism of additive groups. This means that to add two cohomology classes, we can add the corresponding tuples.

We provide below the full list of elements. Note that $$\beta(1,2) = 0$$ in all cases:

Action of automorphism group of acting group
By pre-composition, the automorphism group of the elementary abelian group of prime-square order (which is isomorphic to the general linear group of degree two over the field of $$p$$ elements) acts on the second cohomology group. Under this action, there are a total of $$2p$$ orbits: each of the non-abelian extension types occurs in $$p - 1$$ orbits andthe abelian extension types occur in 1 orbit each. The details are given below:

Action of automorphism group of base group
By post-composition, the automorphism group of the group of prime order acts on the second cohomology group. Under this action, there are many orbits:

Action of direct product of automorphism groups
If, however, we consider the action of the direct product of these groups, with one acting by pre-composition and the other by post-composition (since the actions are at opposite ends, they commute by associativity, so this is an action of the direct product), then the orbits are precisely the four cohomology class types described here, i.e., each cohomology class type gives one orbit.

Description of group action in terms of explicit descriptions of elements
The discussion here relies on the explicit description of cohomology classes in terms of the invariants $$\beta(1,3), \beta(2,3), \beta(1,2,3)$$.

The automorphism group of the acting group, as noted above, is the general linear group $$GL(2,\mathbb{F}_p)$$. It turns out that its action on $$\beta(1,3), \beta(2,3)$$ is given precisely by the action as matrices, and it also induces an action on the $$\beta(1,2,3)$$ by a group automorphism of $$\mathbb{Z}/p\mathbb{Z}$$.

The automorphism group of the base group, which is concretely $$\mathbb{F}_p^\ast$$, acts on all coordinates via multiplication by the corresponding element mod $$p$$. Thus, this is a scalar multiplication action. The orbits of non-identity elements are thus the lines in $$\mathbb{F}_p^3$$, or equivalently, the set of orbits can be described as two-dimensional projective space over $$\mathbb{F}_p$$. There is also the orbit of the identity or zero element, so the total number of orbits is $$(p^3-1)/(p-1) + 1 = p^2 + p + 1 + 1 = p^2 + p + 2$$.

General background
We know from the general theory that there is a natural short exact sequence:

$$0 \to \operatorname{Ext}^1_{\mathbb{Z}}(G,A) \to H^2(G;A) \stackrel{\operatorname{Skew}}{\to} \operatorname{Hom}(\bigwedge^2G,A) \to 0$$

where the image of $$\operatorname{Ext}^1$$ is $$H^2_{sym}(G;A)$$, i.e., the group of cohomology classes represented by symmetric 2-cocycles and corresponding to the abelian group extensions. We also know, again from the general theory, that the short exact sequence above splits, i.e., $$H^2_{sym}(G;A)$$ has a complement inside $$H^2$$. However, there need not in general be a natural or even an automorphism-invariant choice of splitting.

Case of odd prime
In this case, we can split the short exact sequence by considering the following one-sided inverse to the skew map: the map:

$$\! \operatorname{Hom}(\bigwedge^2G,A) \to H^2(G;A)$$

that sends an alternating bihomomorphism to the cohomology class represented by the 2-cocycle that equals the "half" of the bihomomorphism. Thus, we have a natural direct sum decomposition:

$$\! H^2(G;A) = H^2_{sym}(G,A) \oplus J$$

where $$J$$ is the subgroup of $$H^2(G,A)$$ comprising those cohomology classes that have as a representative an alternating bihomomorphism from $$G$$ to $$A$$. In this case, $$H^2_{sym}(G,A)$$ is an elementary abelian group of prime-square order and $$J$$ is a group of prime order.

The $$\operatorname{Skew}$$ map on $$H^2(G,A)$$ that sends a cohomology class to the skew of any representative 2-cocycle, has $$H^2_{sym}(G,A)$$ as its kernel and its effect on $$J$$ is to output the alternating bihomomorphism that is twice of the alternating bihomomorphism representing the cohomology class.

This can also be interpreted in terms of the Baer correspondence. See the section.

Case of the prime two
In this case, we do not get a direct sum decomposition of the above sort, because there is no notion of division by two or halving that would allow us to get an alternating bihomomorphism by halving the commutator map (which equals the skew of the 2-cocycle). For more, see second cohomology group for trivial group action of V4 on Z2.

Odd prime case
Recall from the discussion in the section that we have an internal direct sum decomposition:

$$\! H^2(G;A) = H^2_{\operatorname{sym}}(G;A) + J$$

where $$J$$ is the subgroup of the second cohomology group comprising those classes that can be represented by alternating bihomomorphisms.

This can be used to obtain the Baer correspondence (the class two version of the Lazard correspondence) as follows: an element of $$H^2(G,A)$$ corresponds to an extension group that is a group of nilpotency class two (which includes the abelian and non-abelian cases). It has a unique direct sum decomposition as the sum of a symmetric 2-cocycle (whose cohomology class is an element of $$H^2_{sym}(G,A)$$) and an alternating bihomomorphism (whose cohomology class is an element of $$J$$). The alternating bihomomorphism is given by halving the commutator map (which in turn is the skew of the 2-cocycle).

The Baer Lie ring for the group is defined as follows: the additive group is the extension corresponding to the symmetric 2-cocycle, and the Lie bracket is given by the alternating bihomomorphism.

Explicitly, for $$x,y$$ in the extension group, we define the alternating bihomomorphism as $$\sqrt{[x,y]}$$ and the addition as $$x + y := \frac{xy}{\sqrt{[x,y]}}$$.

We first make a picture of the cohomology group, where the top row is $$H^2_{\operatorname{sym}}$$, the left most column is $$J$$, and the remaining rows and columns are cosets of $$H^2_{\operatorname{sym}}$$ and $$J$$ respectively. The number of rows and columns depends on $$p$$, so when a row or column is repeated a certain number of times, we indicate the repetition in parentheses:

The correspondence is between each group and the abelian group in its column (i.e., the top row group of its column). We thus have two nontrivial correspondences:

Size information
We first give some quantitative size information if we use non-normalized cocycles and coboundaries:

In particular, what this means is that for every cohomology class, there are $$p^2$$ different choices of 2-cocycles that represent that cohomology class.

We give the corresponding information if we use normalized cocycles and coboundaries:

In particular, what this means is that for every cohomology class, there are $$p$$ different choices of normalized 2-cocycles that represent that cohomology class.

Finding a group of cocycle representatives
Consider the short exact sequence for cocycles and coboundaries:

$$0 \to B^2(G;A) \to Z^2(G;A) \to H^2(G;A) \to 0$$

and the corresponding one for normalized cocycles and coboundaries:

$$0 \to B^2_n(G;A) \to Z^2_n(G;A) \to H^2(G;A) \to 0$$

Since these are short exact sequences of vector spaces, they must split. Further, a splitting of the latter also gives a splitting of the former.

Construction of the cohomology group
The cohomology group can be constucted using the GAP functions ElementaryAbelianGroup, TwoCohomology, TrivialGModule, GF. Begin by setting $$p$$ equal to a specific prime number value.

gap> G := ElementaryAbelianGroup(p^2);; gap> A := TrivialGModule(G,GF(p));; gap> T := TwoCohomology(G,A);

The precise output depends on the value of $$p$$.

Construction of extensions
The extensions can be constructed using the additional command Extensions. Begin by setting $$p$$ equal to a specific prime number value.

gap> G := ElementaryAbelianGroup(p^2);; gap> A := TrivialGModule(G,GF(p));; gap> L := Extensions(G,A);; gap> List(L,IdGroup);

The precise output depends on the value of $$p$$.

Construction of automorphism group actions
This uses additionally the GAP functions AutomorphismGroup, DirectProduct, CompatiblePairs, and ExtensionRepresentatives. Begin by setting $$p$$ equal to a specific prime number value.

gap> G := ElementaryAbelianGroup(p^2);; gap> A := TrivialGModule(G,GF(p));; gap> A1 := AutomorphismGroup(G);; gap> A2 := GL(1,p);; gap> D := DirectProduct(A1,A2);; gap> P := CompatiblePairs(G,A,D);; gap> M := ExtensionRepresentatives(G,A,P);; gap> List(M,IdGroup);