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In mathematics, specifically in group theory, the concept of a semidirect product is a generalization of a direct product. There are two closely related concepts of semidirect product:
As with direct products, there is a natural equivalence between inner and outer semidirect products, and both are commonly referred to simply as semidirect products.
For finite groups, the Schur–Zassenhaus theorem provides a sufficient condition for the existence of a decomposition as a semidirect product (also known as splitting extension).
Given a group G with identity element e, a subgroup H, and a normal subgroup N ◁ G; then the following statements are equivalent:
If any of these statements holds (and hence all of them hold, by their equivalence), we say G is the semidirect product of N and H, written
or that G splits over N; one also says that G is a semidirect product of H acting on N, or even a semidirect product of H and N. To avoid ambiguity, it is advisable to specify which is the normal subgroup.
To introduce the notion of outer semidirect products, let us first consider the inner semidirect product. In this case, for a group , consider its normal subgroup N and the subgroup H (not necessarily normal). Let Aut(N) denote the group of all automorphisms of N, which is a group under the composition. Construct a group homomorphism φ: H → Aut(N) defined by conjugation φ(h)(n) = hnh^{−1} for all h in H and n in N. The expression φ(h) is often written as φ_{h} for brevity. In this way we can construct a group with group operation defined as for n_{1}, n_{2} in N and h_{1}, h_{2} in H. Together N, H, and φ determine G up to isomorphism, as we will show later. We can construct group G from its subgroups, this kind of construction is called inner semidirect product.
Let us now consider the outer semidirect product. Given any (even unrelated) two groups N and H and a group homomorphism φ: H → Aut(N), we can construct a new group N ⋊_{φ} H, called the (outer) semidirect product of N and H with respect to φ, defined as follows.^{[1]}
This defines 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 earlier.
Conversely, suppose that we are given a group G with a normal subgroup N and a subgroup H, 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 (written φ(h) = φ_{h}) given by
for all n ∈ N, h ∈ H.
Then G is isomorphic to the semidirect product N ⋊_{φ} H. The isomorphism λ: G → N ⋊_{φ} H is well defined by λ(a) = λ(nh) = (n, h) due to the uniqueness of the decomposition a = nh.
In G, we have
Thus, for a = n_{1}h_{1} and b = n_{2}h_{2} we obtain
which proves that λ is a homomorphism. Since λ is obviously an epimorphism and monomorphism, then it is indeed an isomorphism. This also explains the definition of the multiplication rule in N ⋊_{φ} H.
The direct product is a special case of the semidirect product. To see this, let φ be the trivial homomorphism (i.e., sending every element of H to the identity automorphism of N) then N ⋊_{φ} H is the direct product N × H.
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
and a group homomorphism γ: H → G such that α ∘ γ = id_{H}, the identity map on H. In this case, φ: H → Aut(N) is given by φ(h) = φ_{h}, where
The dihedral group D_{2n} with 2n elements is isomorphic to a semidirect product of the cyclic groups C_{n} and C_{2}.^{[2]} Here, the nonidentity 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:
More generally, a semidirect product of any two cyclic groups C_{m} with generator a and C_{n} with generator b is given by one extra relation, aba^{−1} = b^{k}, with k and n coprime; that is, the presentation:^{[2]}
If r and m are coprime, a^{r} is a generator of C_{m} and a^{r}ba^{−r} = b^{kr}, hence the presentation:
gives a group isomorphic to the previous one.
The fundamental group of the Klein bottle can be presented in the form
and is therefore a semidirect product of the group of integers, ℤ, with ℤ. The corresponding homomorphism φ: ℤ → Aut(ℤ) is given by φ(h)(n) = (−1)^{h}n.
The group of uppertriangular matrices
with nonzero determinant has a decomposition into the semidirect product
^{[3]}
where
and is the subgroup where the only nonzero entries are on the diagonal. This is called the group of diagonal matrices. The group action of on is induced by matrix multiplication. If we set
and
then their matrix multiplication is
This gives the induced group action is given by
A matrix in can be represented by a matrices in and . Hence .
The Euclidean group of all rigid motions (isometries) of the plane (maps f: ℝ^{2} → ℝ^{2} such that the Euclidean distance between x and y equals the distance between f(x) and f(y) for all x and y in ℝ^{2}) is isomorphic to a semidirect product of the abelian group ℝ^{2} (which describes translations) and the group O(2) of orthogonal 2 × 2 matrices (which describes rotations and reflections that keep the origin fixed). Applying a translation and then a rotation or reflection has the same effect as 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). This shows that the group of translations is a normal subgroup of the Euclidean group, that the Euclidean group is a semidirect product of the translation group and O(2), and that the corresponding homomorphism φ: O(2) → Aut(ℝ^{2}) is given by matrix multiplication: φ(h)(n) = hn.
The orthogonal group O(n) of all orthogonal real n × n matrices (intuitively the set of all rotations and reflections of ndimensional space that 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 ndimensional space) and C_{2}. If we represent C_{2} as the multiplicative group of matrices {I, R}, where R is a reflection of ndimensional space that 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) = HNH^{−1} for all H in C_{2} and N in SO(n). In the nontrivial 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").
The group of semilinear transformations on a vector space V over a field 𝕂, often denoted ΓL(V), is isomorphic to a semidirect product of the linear group GL(V) (a normal subgroup of ΓL(V)), and the automorphism group of 𝕂.
In crystallography, the space group of a crystal splits as the semidirect product of the point group and the translation group if and only if the space group is symmorphic. Nonsymmorphic space groups have point groups that are not even contained as subset of the space group, which is responsible for much of the complication in their analysis.^{[4]}
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. This follows from the fact that G is of the same order as the outer semidirect product of N and H, whose underlying set is the Cartesian product N × H.
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 that 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 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.
Furthermore, the result of a (proper) semidirect product by means of a nontrivial homomorphism is never an abelian group, even if the factor groups are abelian.
As opposed to the case with the direct product, a semidirect product of two groups is not, in general, unique; if G and G′ are two groups that both contain isomorphic copies of 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 because the semidirect product also depends on the choice of an action of H on N.
For example, there are four nonisomorphic groups of order 16 that are semidirect products of C_{8} and C_{2}; in this case, C_{8} is necessarily a normal subgroup because it has index 2. One of these four semidirect products is the direct product, while the other three are nonabelian groups:
If a given group is a semidirect product, then there is no guarantee that this decomposition is unique. For example, there is a group of order 24 (the only one containing six elements of order 4 and six elements of order 6) that can be expressed as semidirect product in the following ways: (D_{8} ⋉ C_{3}) ≅ (C_{2} ⋉ Q_{12}) ≅ (C_{2} ⋉ D_{12}) ≅ (D_{6} ⋉ V).^{[5]}
In general, there is no known characterization (i.e., a necessary and sufficient condition) for the existence of semidirect products in groups. However, some sufficient conditions are known, which guarantee existence in certain cases. For finite groups, the Schur–Zassenhaus theorem guarantees existence of a semidirect product when the order of the normal subgroup is coprime to the order of the quotient group.
For example, the Schur–Zassenhaus theorem implies the existence of a semidirect product among groups of order 6; there are two such products, one of which is a direct product, and the other a dihedral group. In contrast, the Schur–Zassenhaus theorem does not say anything about groups of order 4 or groups of order 8 for instance.
Within group theory, the construction of semidirect products can be pushed much further. The Zappa–Szep product of groups is a generalization that, in its internal version, does not assume that either subgroup is normal.
There is also a construction in ring theory, the crossed product of rings. This is constructed in the natural way from the group ring for a semidirect product of groups. The ringtheoretic approach can be further generalized to the semidirect sum of Lie algebras.
For geometry, there is also a crossed product for group actions on a topological space; unfortunately, it is in general noncommutative even if the group is abelian. In this context, the semidirect product is the space of orbits of the group action. The latter approach has been championed by Alain Connes as a substitute for approached by conventional topological techniques; c.f. noncommutative geometry.
There are also farreaching 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.
Another generalization is for groupoids. This occurs in topology because if a group G acts on a space X it also acts on the fundamental groupoid π_{1}(X) of the space. The semidirect product π_{1}(X) ⋊ G is then relevant to finding the fundamental groupoid of the orbit space X/G. For full details see Chapter 11 of the book referenced below, and also some details in semidirect product^{[6]} in ncatlab.
Nontrivial semidirect products do not arise in abelian categories, such as the category of modules. In this case, the splitting lemma shows that every semidirect product is a direct product. Thus the existence of semidirect products reflects a failure of the category to be abelian.
Usually the semidirect product of a group H acting on a group N (in most cases by conjugation as subgroups of a common group) is denoted by N ⋊ H or H ⋉ N. However, some sources may use this symbol with the opposite meaning. In case the action φ: H → Aut(N) should be made explicit, one also writes N ⋊_{φ} H. One way of thinking about the N ⋊ H symbol is as a combination of the symbol for normal subgroup (◁) and the symbol for the product (×). Barry Simon, in his book on group representation theory,^{[7]} employs the unusual notation for the semidirect product.
Unicode lists four variants:^{[8]}
Value  MathML  Unicode description  

⋉  U+22C9  ltimes  LEFT NORMAL FACTOR SEMIDIRECT PRODUCT 
⋊  U+22CA  rtimes  RIGHT NORMAL FACTOR SEMIDIRECT PRODUCT 
⋋  U+22CB  lthree  LEFT SEMIDIRECT PRODUCT 
⋌  U+22CC  rthree  RIGHT SEMIDIRECT PRODUCT 
Here the Unicode description of the rtimes symbol says "right normal factor", in contrast to its usual meaning in mathematical practice.
In LaTeX, the commands \rtimes and \ltimes produce the corresponding characters.