Group extensions

1. Split and non-split extensions: ⋊ and .
2. Conventions and tables
3. The action of Q on N
4. Example: C2n-1⋊C2
5. Central and stem extensions

Split () and non-split (.) extensions

If N ◃ G is a normal subgroup with quotient Q = G/N, equivalently there is an exact sequence

1 → N → G → Q → 1,

then G is an extension of Q by N. (The terminology is somewhat unfortunate.) When the sequence splits, that is G has a subgroup isomorphic to Q that meets N trivially, G is called a split extension of Q by N, or a semi-direct product of N and Q, written

G = N⋊Q      or      G = N:Q

Otherwise G is a non-split extension of Q by N, written

G = N.Q

Note that the notation G = N.Q often stands for 'G is an extension which is not necessarily split' (e.g. in ATLAS), but it is only used for non-split extensions in the database.

Every finite group can be built up from simple groups using extensions, so these constructions are fundamental for describing general groups.

Conventions and tables

We exclude the trivial split extension G=N×Q from the list of extensions. For example:

S3 = C3⋊C2 valid C3◃S3 with C2 quotient, and there is a copy C2 < S3 meeting C3 trivially
C4 = C2.C2 valid C2◃C4 with C2 quotient, and there is no other C2 < C4 meeting C2 trivially
C6 = C3⋊C2 no C6≅C3×C2, and direct products are excluded from split extensions
S3 = C3.C2 no S3 ≅ C3⋊C2, and semidirect products are excluded from non-split extensions

There may be many different groups that are extensions of a given group Q by a given group N, in which case they are ordered, and the index is shown in the tables as a subscript of ⋊ or .

C2.1C22 = D4 1st non-split extension of C22 by C2
C2.2C22 = Q8 2nd non-split extension of C22 by C2

Most groups in the database are extension N⋊Q or N.Q for various choices of N and Q. For each choice, G appears in the table Name(Q)byName(N).html; for instance, C2^2byC2.html for the extensions of C22 by C2 as above.

The action of Q on N

When G=N⋊Q is a split extension, a copy of Q in G acts on N by conjugation, which gives a homomorphism Q→Aut N. Conversely, given N, Q and φ: Q→Aut N,

When G=N.Q is a non-split extension with N abelian, the action of G on N by conjugation descends to φ: Q=G/N→Aut N. When N is nonabelian, G→Aut N need not descend to Q=G/N→Aut N, and having such a homomorphism φ is neither a necessary nor a sufficient condition to having the corresponding extension. Dividing out by inner automorphisms does give a well-defined homomorphism G→Aut N/Inn N=Out N (outer automorphism group), though it is not a particularly good invariant to classify extensions.

Example: C2n-1⋊C2

n=2. The only homomorphism φ: C2→Aut(C2)=C1 is trivial, the group H2(C2,C2)=C2 has only one non-trivial class, and

C2C2 = C22 unique split extension with trivial action(direct product)
C2.C2 = C4 unique non-split extension with trivial action.

n=3. There are two homomorphisms C2→Aut(C4)=C2. The trivial one gives

C4C2 = C2×C4 unique split extension (direct product)
C4.C2 = C8 unique non-split extension
and the non-trivial one
C4C2 = D4 unique split extension with non-trivial φ
C4.C2 = Q8 unique non-split extension with non-trivial φ

n>3. There are four homomorphisms

φ: C2 → Aut(C2n-1)= (ℤ/2n-1ℤ)× (≅C2n-3×C2),
sending the non-trivial generator g of C2 to 1, -1, 2n-3-1 and 2n-3+1. The corresponding split extensions are
φ: g ↦ 1 C2n-1φC2 = C2n-1×C2 direct product
φ: g ↦ -1 C2n-1φC2 = C2n-11C2 = D2n-1 dihedral group
φ: g ↦ 2n-3-1 C2n-1φC2 = C2n-12C2 = SD2n semi​dihedral group
φ: g ↦ 2n-3+1 C2n-1φC2 = C2n-13C2 = Mn(2) modular maximal-cyclic group
In the two latter cases, H2(C2,C2n-1)=0, so there are no non-split extensions with such actions. In the first two, H2(C2,C2n-1)=C2 and there is exactly one:
φ: g ↦1 C2n-1.φC2 = C2n cyclic group
φ: g ↦-1 C2n-1.φC2 = Q2n (generalized) quaternion group

Central and stem extensions

A non-split extenson G=N.Q is a central extension if N is contained in the center of G (and is, in particular, abelian). It is a central stem extension if, in addition, N is contained in the derived subgroup G'. For perfect groups G, their central stem extensions are classified by the theory of Schur covers.