In mathematics, especially group theory, the Zappa–Szép product (also known as the Zappa–Rédei–Szép product, general product, knit product, exact factorization or bicrossed 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 (1940) and Jenő Szép (1950) although it was independently studied by others including B.H. Neumann (1935), G.A. Miller (1935), and J.A. de Séguier (1904).[1]
Internal Zappa–Szép products
Let G be a group with identity elemente, and let H and K be subgroups of G. The following statements are equivalent:
G = HK and H ∩ K = {e}
For each g in G, there exists a unique h in H and a unique k in K such that g = hk.
If either (and hence both) of these statements hold, then G is said to be an internal Zappa–Szép product of H and K.
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 Hall p'-subgroup and a Sylow p-subgroup, and in fact that the group is a (multiple factor) 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(2,11) 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 subgroups H and K of the group G. For each k in K and each h in H, there exist α(k, h) in H and β(k, h) in K such that kh = α(k, h) β(k, h). This defines mappings α : K × H → H and β : K × H → K which turn out to have the following properties:
α(e, h) = h and β(k, e) = k for all h in H and k in K.
α(k1k2, h) = α(k1, α(k2, h))
β(k, h1h2) = β(β(k, h1), h2)
α(k, h1h2) = α(k, h1) α(β(k, h1), h2)
β(k1k2, h) = β(k1, α(k2, h)) β(k2, h)
for all h1, h2 in H, k1, k2 in K. From these, it follows that
For each k in K, the mapping h ↦ α(k, h) is a bijection of H.
For each h in H, the mapping k ↦ β(k, h) is a bijection of K.
(Indeed, suppose α(k, h1) = α(k, h2). Then h1 = α(k−1k, h1) = α(k−1, α(k, h1)) = α(k−1, α(k, h2)) = h2. This establishes injectivity, and for surjectivity, use h = α(k, α(k−1, h)).)
More concisely, the first three properties above assert the mapping α : K × H → H is a left action of K on (the underlying set of) H and that β : K × H → K is a right action of H on (the underlying set of) 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(h1h2) = kh1kh1h2 and (k1k2)h = k1k2hk2h.
Turning this around, suppose H and K are groups (and let e denote each group's identity element) 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,
(h1, k1) (h2, k2) = (h1 α(k1, h2), β(k1, h2) k2)
(h, k)−1 = (α(k−1, h−1), β(k−1, h−1))
Then H × K is a group called the external Zappa–Szép product of the groups H and K. The subsetsH × {e} and {e} × K are subgroups isomorphic to H and K, respectively, and H × K is, in fact, an internal Zappa–Szép product of H × {e} and {e} × 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 h k− 1 and β(k, h) = k. This is easy to see because and since by normality of , . In this case, G is an internal semidirect product of H and K.
If, in addition, K is normal in G, then α(k,h) = h. In this case, G is an internal direct product of H and K.
^Martin W. Liebeck; Cheryl E. Praeger; Jan Saxl (2010). Regular Subgroups of Primitive Permutation Groups. American Mathematical Soc. pp. 1–2. ISBN978-0-8218-4654-4.
Michor, P. W. (1989), "Knit products of graded Lie algebras and groups", Proceedings of the Winter School on Geometry and Physics, Srni, Suppl. Rendiconti Circolo Matematico di Palermo, Ser. II, 22: 171–175, arXiv:math/9204220, Bibcode:1992math......4220M.
Szép, J. (1950), "On the structure of groups which can be represented as the product of two subgroups", Acta Sci. Math. Szeged, 12: 57–61.
Takeuchi, M. (1981), "Matched pairs of groups and bismash products of Hopf algebras", Comm. Algebra, 9 (8): 841–882, doi:10.1080/00927878108822621.
Zappa, G. (1940), "Sulla costruzione dei gruppi prodotto di due dati sottogruppi permutabili traloro", Atti Secondo Congresso Un. Mat. Ital., Bologna{{citation}}: CS1 maint: location missing publisher (link); Edizioni Cremonense, Rome, (1942) 119–125.