# Universal embedding theorem

The universal embedding theorem, or Krasner–Kaloujnine universal embedding theorem, is a theorem from the mathematical discipline of group theory first published in 1951 by Marc Krasner and Lev Kaluznin. The theorem states that any group extension of a group H by a group A is isomorphic to a subgroup of the regular wreath product A Wr H. The theorem is named for the fact that the group A Wr H is said to be universal with respect to all extensions of H by A.

## Statement

Let H and A be groups, let K = A$^H$ be the set of all functions from H to A, and consider the action of H on itself by multiplication. This action extends naturally to an action of H on K, defined as $(h\cdot \phi )(g)=\phi (h^{-1}g),$ where $\phi \in K,$ and g and h are both in H. This is an automorphism of K, so we can construct the semidirect product K ⋊ H, which is termed the regular wreath product, and denoted A Wr H or $A\wr H.$ The group K = A$^H$ (which is isomorphic to $\{(\phi ,1)\in A\wr H:\phi \in K\}$) is called the base group of the wreath product.
The Krasner–Kaloujnine universal embedding theorem states that if G has a normal subgroup A and H = G/A, then there is an injective homomorphism of groups $\theta :G\to A\wr H$ such that A maps surjectively onto ${\text{im}}(\theta )\cap K.$ This is equivalent to the wreath product A Wr H having a subgroup isomorphic to G, where G is any extension of H by A.

## Proof

This proof comes from Dixon–Mortimer.
Define a homomorphism $\psi :G\to H$ whose kernel is A. Choose a set $T=\{t_{u}:u\in H\}$ of (right) coset representatives of A in G, where $\psi (t_{u})=u.$ Then for all x in G, $t_{u}xt_{u\psi (x)}^{-1}\in \ker \psi =A.$ For each x in G, we define a function $f_{x}:H\to A$ such that $f_{x}(u)=t_{u}xt_{u\psi (x)}^{-1}.$ Then the embedding $\theta$ is given by $\theta (x)=(f_{x},\psi (x))\in A\wr H.$
We now prove that this is a homomorphism. If x and y are in G, then $\theta (x)\theta (y)=(f_{x}(f_{y}.\psi (x)^{-1}),\psi (xy)).$ Now $f_{y}(u).\psi (x)^{-1}=f_{y}(u\psi (x)),$ so for all u in H,
    $f_{x}(u)(f_{y}(u).\psi (x))=t_{u}xt_{u\psi (x)}^{-1}t_{u\psi (x)}yt_{u\psi (x)\psi (y)}^{-1}=t_{u}xyt_{u\psi (xy)}^{-1},$
so f$_x$ f$_y$ = f$_{xy}$. Hence $\theta$ is a homomorphism as required.
The homomorphism is injective. If $\theta (x)=\theta (y),$ then both f$_x$(u) = f$_y$(u) (for all u) and $\psi (x)=\psi (y).$ Then $t_{u}xt_{u\psi (x)}^{-1}=t_{u}yt_{u\psi (y)}^{-1},$ but we can cancel t$_u$ and $t_{u\psi (x)}^{-1}=t_{u\psi (y)}^{-1}$ from both sides, so x = y, hence $\theta$ is injective. Finally, $\theta (x)\in K$ precisely when $\psi (x)=1,$ in other words when $x\in A$ (as $A=\ker \psi$).

## Generalizations and related results

* The Krohn–Rhodes theorem is a statement similar to the universal embedding theorem, but for semigroups. A semigroup S is a divisor of a semigroup T if it is the image of a subsemigroup of T under a homomorphism. The theorem states that every finite semigroup S is a divisor of a finite alternating wreath product of finite simple groups (each of which is a divisor of S) and finite aperiodic semigroups.
* An alternate version of the theorem exists which requires only a group G and a subgroup A (not necessarily normal). In this case, G is isomorphic to a subgroup of the regular wreath product A Wr (G/Core(A)).