# Wreath Products: Sum and Product Actions

During Laci Babai’s course on graph isomorphism, one of the tools we encountered is the wreath product. Here I’ll give some intuitive descriptions of the wreath product $L \wr M$.

To make sure we’re all on the same page, the wreath product $L \wr M$ for an arbitrary group $L$ and a permutation group $M \leq S_k$ is the semidirect product $(L^k) \rtimes M$, where each $M$ is an automorphism of $L^k$ by permuting the coordinates.

That is, we fix some number $k$ and some group of permutations on $[k]$, $M \leq S_k$. The group we construct takes $k$ disjoint and independent versions of $L$, $L^k$, but also allows you to make “limited rearrangements according to $M$“. There are two distinct ideas going on here; it’s the semidirect product that, if you tell it how two groups should interact, joins those two ideas into a single group.

### $L\wr S_k$ is a Graph Automorphism Group

There are two “trivial” wreath products, $L \wr 1$ (where $1$ denotes the trivial group) and $L \wr S_k$. $L \wr 1$ is $k$ disjoint and independent versions of $L$, with no interdependencies introduced by $M$; this is just $L^k$.

Let $G$ be a graph with automorphism group $L$. Claim: if you stick $k$ disjoint copies of $G$ next to each other, this new graph has automorphism group $L \wr S_k$. To see this, of course some automorphisms apply $L$ elements “in parallel” to the different copies of $G$. These automorphisms correspond to $L^k$. But any rearrangement of the copies of $G$ is also an automorphism. All copies are isomorphic, so rearranging can be done without restriction by $S_k$.

### Product Actions of Wreath Products

The previous paragraph shows that, if $L$ acts on a set $\Omega$, the wreath product $L \wr K$ acts naturally on $k\cdot \Omega$. This is called the sum action (or “imprimitive action”, because it’s nearly always an imprimitive group action)  of the wreath product. There is another natural action, called the product action (or “primitive action”). Here the domain is the set $\Omega^k$, and the action is defined by (1) applying elements of $L^k$ coordinate-wise and (2) permuting the coordinates with $M$.

I think of this like a “simultaneous tracking” version of the sum action. In the sum action, we kept track of one point/vertex across $k$ copies of our graphs, and looked at where that point went when hit with a group element. In the product action, we track one point from each of the $k$ graphs simultaneously, and look at how as a whole those elements move.

### $H \wr K$ contains all extensions of $H$ by $K$

It would be remiss to mention wreath products without mentioning the Kaluzhnin-Krasner theorem (English version). We say that the group $G$ is an extension of $H$ by $K$ if there is an exact sequence

$1 \rightarrow K \rightarrow G \rightarrow H \rightarrow 1$

Theorem (Kaluzhnin-Krasner, 1951) If $G$ is an extension of $H$ by $K$, then $G$ is isomorphic to a subgroup of $H \wr K$.

That is, $H \wr K$ is a group that’s big enough to contain all extensions of the two groups. In a sense that can probably be made more formal through order theory, $H \wr K$ is an “upper bound” on joining $H$ and $K$ together. Not bad!