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SOME EQUIVALENT DEFINITIONS


Let ''G'' be a group, ''N'' a Normal Subgroup of ''G'' (i.e., N riangleleft G) and ''H'' a Subgroup of ''G''. The following statements are equivalent:
  • ''G'' = ''NH'' and ''N'' ∩ ''H'' = {''e''} (with ''e'' being the Identity Element of ''G'')

  • ''G'' = ''HN'' and ''N'' ∩ ''H'' = {''e''}

  • Every element of ''G'' can be written in one and only one way as a product of an element of ''N'' and an element of ''H''

  • Every element of ''G'' can be written in one and only one way as a product of an element of ''H'' and an element of ''N''

  • The natural embedding ''H'' → ''G'', composed with the natural projection ''G'' → ''G / N'' , yields an Isomorphism between ''H'' and ''G / N''

  • There exists a Homomorphism ''G'' → ''H'' which is the identity on ''H'' and whose Kernel is ''N''


If one (and therefore all) of these statements hold, we say that ''G'' is a semidirect product of ''N'' and ''H'', or that ''G'' ''splits'' over ''N''.


ELEMENTARY FACTS AND CAVEATS


If ''G'' is the semidirect product of the normal subgroup ''N'' and the subgroup ''H'', and both ''N'' and ''H'' are finite, then the Order of ''G'' equals the product of the orders of ''N'' and ''H''.

Note that, as opposed to the case with the Direct Product , a semidirect product is not, in general, unique; if ''G'' and ''G' '' are two groups which both contain ''N'' as a normal subgroup and ''H'' as a subgroup, and both are a semidirect product of ''N'' and ''H'', then it does ''not'' follow that ''G'' and ''G' '' are Isomorphic . This remark leads to an Extension Problem , of describing the possibilities.


OUTER SEMIDIRECT PRODUCTS


If ''G'' is a semidirect product of ''N'' and ''H'', then the map φ : ''H'' → Aut(''N'') (where Aut(''N'') denotes the group of all Automorphisms of ''N'') defined by φ(''h'')(''n'') = ''hnh''–1 for all ''h'' in ''H'' and ''n'' in ''N''
is a group homomorphism. Together ''N'', ''H'' and φ determine ''G'' Up To isomorphism, as we now show.

Given any two groups ''N'' and ''H'' (not necessarily subgroups of a given group) and a Group Homomorphism φ : ''H'' → Aut(''N''), define a new group ''N'' ×φ ''H'', the semidirect product of ''N'' and ''H'' with respect to φ, as follows:
  • is given by

  • (''n''2, ''h''2) = (''n''1 φ(''h''1)(''n''2), ''h''1 ''h''2)

    • for all ''n''1, ''n''2 in ''N'' and ''h''1, ''h''2 in ''H''. This is a group in which the identity element is (''e''''N'', ''e''''H'') and the inverse of the element (''n'', ''h'') is (φ(''h''–1)(''n''–1), ''h''–1). Pairs ''N'' × {''e''''H''} form a normal subgroup isomorphic to ''N'', while pairs {''e''''N''} × ''H'' form a subgroup isomorphic to ''H''. The full group is a semidirect product of those two subgroups in the sense given above.


Conversely, suppose that we are given an internal semidirect product as defined above, i.e. a group ''G'' with a normal subgroup ''N'', a subgroup ''H'', and such that every element ''g'' of ''G'' may be written uniquely in the form ''g=nh'' where ''n'' lies in ''N'' and ''h'' lies in ''H''. Let φ : ''H''→Aut(''N'') be the homomorphism
:φ(''h'')(''n'')=''hnh''–1.
Then ''G'' is isomorphic to the outer semidirect product ''N'' ×φ ''H''; the isomorphism sends the product ''nh'' to the tuple (''n'',''h''). In ''G'', we have the rule
:(''n''1''h''1)(''n''2''h''2) = ''n''1(''h''1''n''2''h''1–1)(''h''1''h''2)
and this is the deeper reason for the above definition of the outer semidirect product, and an easy way to memorize it.

A version of the Splitting Lemma for groups states that a group ''G'' is isomorphic to a semidirect product of the two groups ''N'' and ''H'' if and only if there exists a Short Exact Sequence

: 0\longrightarrow N \longrightarrow^{\!\!\!\!\!\!\!\!\!\beta}\ \, G \longrightarrow^{\!\!\!\!\!\!\!\!\!\alpha}\ \, H \longrightarrow 0

and a group homomorphism γ : ''H'' → ''G'' such that α \circ γ = id''H'', the Identity Map on ''H''. In this case, φ : ''H'' → Aut(''N'') is given by
:φ(''h'')(''n'') = β−1(γ(''h'') β(''n'')γ(''h''−1)).


EXAMPLES


The Dihedral Group ''D''''n'' with 2''n'' elements is isomorphic to a semidirect product of the Cyclic Group s ''C''''n'' and ''C''2. Here, the non-identity element of ''C''2 acts on ''C''''n'' by inverting elements; this is an automorphism since ''C''''n'' is Abelian . The Presentation for this group is:

:\langle a,\;b \mid a^2 = e,\; b^n = e,\; aba^{-1}=b^{-1} angle.

More generally, a semidirect product of any two cyclic groups C_m\; with generator a\; and C_n\; with generator b\; is given by a single relation aba^{-1}=b^k\; with k\; and n\; coprime, i.e. the presentation:

:\langle a,\;b \mid a^m=e,\;b^n = e,\;aba^{-1}=b^k\; angle.

If r\; and m\; are coprime, a^r\; is a generator of C_m\; and
a^rba^{-r}=b^{k^r}\;, hence the presentation:

:\langle a,\;b \mid a^m=e,\;b^n = e,\;aba^{-1}=b^{k^{r}}\; angle

gives an group isomorphic to the previous one.

The 2×2 matrices (which describes rotations and reflections which keep the origin fixed). ''n'' is a translation, ''h'' a rotation or reflection. Applying a translation and then a rotation or reflection corresponds to applying the rotation or reflection first and then a translation by the rotated or reflected translation vector (i.e. applying the Conjugate of the original translation). Every orthogonal matrix acts as an Automorphism on R2 by matrix multiplication.

The Orthogonal Group O(''n'') of all orthogonal real ''n''×''n'' matrices (intuitively the set of all rotations and reflections of ''n''-dimensional space which keep the origin fixed) is isomorphic to a semidirect product of the group SO(''n'') (consisting of all orthogonal matrices with Determinant 1, intuitively the rotations of ''n''-dimensional space) and ''C''2. If we represent ''C''2 as the multiplicative group of matrices {''I'', ''R''}, where ''R'' is a reflection of ''n'' dimensional space which keeps the origin fixed (i.e. an orthogonal matrix with determinant –1 representing an Involution ), then φ : ''C''2 → Aut(SO(''n'')) is given by φ(''H'')(''N'') = ''H'' ''N'' ''H''–1 for all ''H'' in ''C''2 and ''N'' in SO(''n''). In the non-trivial case ( ''H'' is not the identity) this means that φ(''H'') is conjugation of operations by the reflection (a rotation axis and the direction of rotation are replaced by their "mirror image").


RELATION TO DIRECT PRODUCTS


Suppose ''G'' is a semidirect product of the normal subgroup ''N'' and the subgroup ''H''. If ''H'' is also normal in ''G'', or equivalently, if there exists a homomorphism ''G'' → ''N'' which is the identity on ''N'', then ''G'' is the Direct Product of ''N'' and ''H''.

The direct product of two groups ''N'' and ''H'' can be thought of as the outer semidirect product of ''N'' and ''H'' with respect to φ(''h'') = id''N'' for all ''h'' in ''H''.

Note that in a direct product, the order of the factors is not important, since ''N'' × ''H'' is isomorphic to ''H'' × ''N''. This is not the case for semidirect products, as the two factors play different roles.


GENERALIZATIONS


The construction of semidirect products can be pushed much further. There is a version in Ring Theory , the Crossed Product Of Rings . This is seen naturally as soon as one constructs a Group Ring for a semidirect product of groups. There is also the Semidirect Sum of Lie Algebras . Given a Group Action on a Topological Space , there is a corresponding crossed product which will in general be non-commutative even if the group is abelian. This kind of ring (see Crossed Product for a related construction) can play the role of the ''space of orbits'' of the group action, in cases where that space cannot be approached by conventional topological techniques - for example in the work of Alain Connes (cf. Noncommutative Geometry ).

There are also far-reaching generalisations in Category Theory . They show how to construct '' Fibred Categories '' from '' Indexed Categories ''. This is an abstract form of the outer semidirect product construction.


NOTATION


Sources differ in their notation for the semidirect product. Some texts discuss it with no explicit notation. Others use the subscripted "times" symbol (×φ) as above to modify the direct product by inclusion of a homomorphism, writing the normal group on the left. Other notation reshapes the times symbol; Unicode {Link without Title} lists four variants:

Although the Unicode description of the rtimes symbol says "right normal factor", a number of authors use it with a left normal factor. Therefore the usual caution for mathematical notation applies: When reading, be careful to notice the conventions adopted by the author, and when writing, explain notation choices for the reader. The choice of symbol may vary, but putting the normal factor on the left seems fairly consistent.


SEE ALSO