In mathematics, a group extension is a general means of describing a group in terms of a particular normal subgroup and quotient group. If ${\displaystyle Q}$ and ${\displaystyle N}$ are two groups, then ${\displaystyle G}$ is an extension of ${\displaystyle Q}$ by ${\displaystyle N}$ if there is a short exact sequence

${\displaystyle 1\to N\;{\overset {\iota }{\to }}\;G\;{\overset {\pi }{\to }}\;Q\to 1.}$

If ${\displaystyle G}$ is an extension of ${\displaystyle Q}$ by ${\displaystyle N}$, then ${\displaystyle G}$ is a group, ${\displaystyle \iota (N)}$ is a normal subgroup of ${\displaystyle G}$ and the quotient group ${\displaystyle G/\iota (N)}$ is isomorphic to the group ${\displaystyle Q}$. Group extensions arise in the context of the extension problem, where the groups ${\displaystyle Q}$ and ${\displaystyle N}$ are known and the properties of ${\displaystyle G}$ are to be determined. Note that the phrasing "${\displaystyle G}$ is an extension of ${\displaystyle N}$ by ${\displaystyle Q}$" is also used by some.^[1]

Since any finite group ${\displaystyle G}$ possesses a maximal normal subgroup ${\displaystyle N}$ with simple factor group ${\displaystyle G/N}$, all finite groups may be constructed as a series of extensions with finite simple groups. This fact was a motivation for completing the classification of finite simple groups.

An extension is called a central extension if the subgroup ${\displaystyle N}$ lies in the center of ${\displaystyle G}$.

Extensions in general

One extension, the direct product, is immediately obvious. If one requires ${\displaystyle G}$ and ${\displaystyle Q}$ to be abelian groups, then the set of isomorphism classes of extensions of ${\displaystyle Q}$ by a given (abelian) group ${\displaystyle N}$ is in fact a group, which is isomorphic to

${\displaystyle \operatorname {Ext} _{\mathbb {Z} }^{1}(Q,N);}$

cf. the Ext functor. Several other general classes of extensions are known but no theory exists that treats all the possible extensions at one time. Group extension is usually described as a hard problem; it is termed the extension problem.

To consider some examples, if ${\displaystyle G=K\times H}$, then ${\displaystyle G}$ is an extension of both ${\displaystyle H}$ and ${\displaystyle K}$. More generally, if ${\displaystyle G}$ is a semidirect product of ${\displaystyle K}$ and ${\displaystyle H}$, written as ${\displaystyle G=K\rtimes H}$, then ${\displaystyle G}$ is an extension of ${\displaystyle H}$ by ${\displaystyle K}$, so such products as the wreath product provide further examples of extensions.

Classifying extensions

Solving the extension problem amounts to classifying all extensions of H by K; or more practically, by expressing all such extensions in terms of mathematical objects that are easier to understand and compute. In general, this problem is very hard, and all the most useful results classify extensions that satisfy some additional condition.

Figure 1

It is important to know when two extensions are equivalent or congruent. We say that the extensions

${\displaystyle 1\to K{\stackrel {i}{{}\to {}}}G{\stackrel {\pi }{{}\to {}}}H\to 1}$

and

${\displaystyle 1\to K{\stackrel {i'}{{}\to {}}}G'{\stackrel {\pi '}{{}\to {}}}H\to 1}$

are equivalent (or congruent) if there exists a group isomorphism ${\displaystyle T:G\to G'}$ making commutative the diagram of Figure 1. In fact it is sufficient to have a group homomorphism; due to the assumed commutativity of the diagram, the map ${\displaystyle T}$ is forced to be an isomorphism by the short five lemma.

Warning

It may happen that the extensions ${\displaystyle 1\to K\to G\to H\to 1}$ and ${\displaystyle 1\to K\to G^{\prime }\to H\to 1}$ are inequivalent but G and G' are isomorphic as groups. For instance, there are ${\displaystyle 8}$ inequivalent extensions of the Klein four-group by ${\displaystyle \mathbb {Z} /2\mathbb {Z} }$,^[2] but there are, up to group isomorphism, only four groups of order ${\displaystyle 8}$ containing a normal subgroup of order ${\displaystyle 2}$ with quotient group isomorphic to the Klein four-group.

Trivial extensions

A trivial extension is an extension

${\displaystyle 1\to K\to G\to H\to 1}$

that is equivalent to the extension

${\displaystyle 1\to K\to K\times H\to H\to 1}$

where the left and right arrows are respectively the inclusion and the projection of each factor of ${\displaystyle K\times H}$.

Classifying split extensions

A split extension is an extension

${\displaystyle 1\to K\to G\to H\to 1}$

with a homomorphism ${\displaystyle s\colon H\to G}$ such that going from H to G by s and then back to H by the quotient map of the short exact sequence induces the identity map on H i.e., ${\displaystyle \pi \circ s=\mathrm {id} _{H}}$. In this situation, it is usually said that s splits the above exact sequence.

Split extensions are very easy to classify, because an extension is split if and only if the group G is a semidirect product of K and H. Semidirect products themselves are easy to classify, because they are in one-to-one correspondence with homomorphisms from ${\displaystyle H\to \operatorname {Aut} (K)}$, where Aut(K) is the automorphism group of K. For a full discussion of why this is true, see semidirect product.

Warning on terminology

In general in mathematics, an extension of a structure K is usually regarded as a structure L of which K is a substructure. See for example field extension. However, in group theory the opposite terminology has crept in, partly because of the notation ${\displaystyle \operatorname {Ext} (Q,N)}$, which reads easily as extensions of Q by N, and the focus is on the group Q.

A paper of Ronald Brown and Timothy Porter on Otto Schreier's theory of nonabelian extensions uses the terminology that an extension of K gives a larger structure.^[3]

Central extension

A central extension of a group G is a short exact sequence of groups

${\displaystyle 1\to A\to E\to G\to 1}$

such that A is included in ${\displaystyle Z(E)}$, the center of the group E. The set of isomorphism classes of central extensions of G by A is in one-to-one correspondence with the cohomology group ${\displaystyle H^{2}(G,A)}$.

Examples of central extensions can be constructed by taking any group G and any abelian group A, and setting E to be ${\displaystyle A\times G}$. This kind of split example corresponds to the element 0 in ${\displaystyle H^{2}(G,A)}$ under the above correspondence. Another split example is given for a normal subgroup A with E set to the semidirect product ${\displaystyle A\rtimes G}$. More serious examples are found in the theory of projective representations, in cases where the projective representation cannot be lifted to an ordinary linear representation.

In the case of finite perfect groups, there is a universal perfect central extension.

Similarly, the central extension of a Lie algebra ${\displaystyle {\mathfrak {g}}}$ is an exact sequence

${\displaystyle 0\rightarrow {\mathfrak {a}}\rightarrow {\mathfrak {e}}\rightarrow {\mathfrak {g}}\rightarrow 0}$

such that ${\displaystyle {\mathfrak {a}}}$ is in the center of ${\displaystyle {\mathfrak {e}}}$.

There is a general theory of central extensions in Maltsev varieties.^[4]

See also

• Lie algebra extension
• Virasoro algebra
• HNN extension
• Group contraction
• Extension of a topological group

References

• Mac Lane, Saunders (1975), Homology, Classics in Mathematics, Springer Verlag, ISBN 3-540-58662-8
• R.L. Taylor, Covering groups of non connected topological groups, Proceedings of the American Mathematical Society, vol. 5 (1954), 753–768.
• R. Brown and O. Mucuk, Covering groups of non-connected topological groups revisited, Mathematical Proceedings of the Cambridge Philosophical Society, vol. 115 (1994), 97–110.