In mathematics, especially group theory, the Zappa–Szép product describes a way in which a group can be constructed from two subgroups. It is a generalization of the direct and semidirect products. It is named after Guido Zappa and Jenő Szép although it was independently studied by others including B.H. Neumann, G.A. Miller, and J.A. de Séguier.
Internal Zappa–Szép products
Let G be a group with identity elemente, and letH and K be subgroups of G. The following statements are equivalent:
G = HK and H ∩ K =
For each g in G, there exists a unique h in H and a unique k in K such that g = hk.
If either of these statements hold, then G is said to be an internal Zappa–Szép product of H and K.
Examples
Let G = GL, the general linear group of invertiblen × nmatrices over the complex numbers. For each matrix A in G, the QR decomposition asserts that there exists a unique unitary matrixQ and a unique upper triangular matrixR with positivereal entries on the main diagonal such that A = QR. Thus G is a Zappa–Szép product of the unitary groupU and the group K of upper triangular matrices with positive diagonal entries. One of the most important examples of this is Philip Hall's 1937 theorem on the existence of Sylow systems for soluble groups. This shows that every soluble group is a Zappa–Szép product of a Hallp'-subgroup and a Sylow p-subgroup, and in fact that the group is a Zappa–Szép product of a certain set of representatives of its Sylow subgroups. In 1935, George Miller showed that any non-regular transitive permutation group with a regular subgroup is a Zappa–Szép product of the regular subgroup and a point stabilizer. He gives PSL and the alternating group of degree 5 as examples, and of course every alternating group of prime degree is an example. This same paper gives a number of examples of groups which cannot be realized as Zappa–Szép products of proper subgroups, such as the quaternion group and the alternating group of degree 6.
External Zappa–Szép products
As with the direct and semidirect products, there is an external version of the Zappa–Szép product for groups which are not known a priori to be subgroups of a given group. To motivate this, let G = HK be an internal Zappa–Szép product of subgroupsH and K of the group G. For each k in K and each h in H, there exist α in H and β in K such that kh = α β. This defines mappings α : K × H → H and β : K × H → K which turn out to have the following properties:
α = h and β = k for all h in H and k in K.
α = α
β = β
α = α α
β = β β
for all h1, h2 in H, k1, k2 in K. From these, it follows that
For each k in K, the mapping h α is a bijection of H.
For each h in H, the mapping k β is a bijection of K.
=α. Then h1=α More concisely, the first three properties above assert the mapping α : K × H → H is a left action of K on H and that β : K × H → K is a right action of H on K. If we denote the left action by h → kh and the right action by k → kh, then the last two properties amount to k = kh1kh1h2 and h = k1k2hk2h. Turning this around, suppose H and K are groups and suppose there exist mappings α : K × H → H and β : K × H → K satisfying the properties above. On the cartesian productH × K, define a multiplication and an inversion mapping by, respectively,
=, β
− 1 =, β)
Then H × K is a group called the external Zappa–Szép product of the groups H and K. The subsetsH × and × K are subgroups isomorphic to H and K, respectively, and H × K is, in fact, an internal Zappa–Szép product of H × and × K.
Relation to semidirect and direct products
Let G = HK be an internal Zappa–Szép product of subgroups H and K. If H is normal in G, then the mappings α and β are given by, respectively, α = k h k− 1 and β = k. In this case, G is an internal semidirect product of H and K. If, in addition, K is normal in G, then α = h. In this case, G is an internal direct product of H and K.
