Holomorph (mathematics)

In mathematics, especially in the area of algebra known as group theory, the holomorph of a group $G$, denoted $\operatorname(G)$, is a group that simultaneously contains (copies of) $G$ and its automorphism group $\operatorname(G)$. It provides interesting examples of groups, and allows one to treat group elements and group automorphisms in a uniform context. The holomorph can be described as a semidirect product or as a permutation group.

Hol(''G'') as a semidirect product

If $\operatorname(G)$ is the automorphism group of $G$ then
:$\operatorname(G)=G\rtimes \operatorname(G)$
where the multiplication is given by

Typically, a semidirect product is given in the form $G\rtimes_A$ where $G$ and $A$ are groups and $\phi:A\rightarrow \operatorname(G)$ is a homomorphism and where the multiplication of elements in the semidirect product is given as
:$(g,a)(h,b)=(g\phi(a)(h),ab)$
which is well defined, since $\phi(a)\in \operatorname(G)$ and therefore $\phi(a)(h)\in G$.

For the holomorph, $A=\operatorname(G)$ and $\phi$ is the identity map, as such we suppress writing $\phi$ explicitly in the multiplication given in equation () above.

For example,
* $G=C_3=\langle x\rangle=\$ the cyclic group of order 3
* $\operatorname(G)=\langle \sigma\rangle=\$ where $\sigma(x)=x^2$
* $\operatorname(G)=\$ with the multiplication given by:
:$(x^,\sigma^)(x^,\sigma^) = (x^,\sigma^)$ where the exponents of $x$ are taken mod 3 and those of $\sigma$ mod 2.

Observe, for example
:$(x,\sigma)(x^2,\sigma)=(x^,\sigma^2)=(x^2,1)$
and this group is not abelian, as $(x^2,\sigma)(x,\sigma)=(x,1)$, so that $\operatorname(C_3)$ is a non-abelian group of order 6, which, by basic group theory, must be isomorphic to the symmetric group $S_3$.

Hol(''G'') as a permutation group

A group ''G'' acts naturally on itself by left and right multiplication, each giving rise to a homomorphism from ''G'' into the symmetric group on the underlying set of ''G''. One homomorphism is defined as ''λ'': ''G'' → Sym(''G''), ''λ_g''(''h'') = ''g''·''h''. That is, ''g'' is mapped to the permutation obtained by left-multiplying each element of ''G'' by ''g''. Similarly, a second homomorphism ''ρ'': ''G'' → Sym(''G'') is defined by ''ρ_g''(''h'') = ''h''·''g''⁻¹, where the inverse ensures that ''ρ_gh''(''k'') = ''ρ_g''(''ρ_h''(''k'')). These homomorphisms are called the left and right regular representations of ''G''. Each homomorphism is injective, a fact referred to as Cayley's theorem.

For example, if ''G'' = ''C''₃ = is a cyclic group of order three, then
* ''λ_x''(1) = ''x''·1 = ''x'',
* ''λ_x''(''x'') = ''x''·''x'' = ''x''², and
* ''λ_x''(''x''²) = ''x''·''x''² = 1,
so ''λ''(''x'') takes (1, ''x'', ''x''²) to (''x'', ''x''², 1).

The image of ''λ'' is a subgroup of Sym(''G'') isomorphic to ''G'', and its normalizer in Sym(''G'') is defined to be the holomorph ''N'' of ''G''.
For each ''n'' in ''N'' and ''g'' in ''G'', there is an ''h'' in ''G'' such that ''n''·''λ_g'' = ''λ_h''·''n''. If an element ''n'' of the holomorph fixes the identity of ''G'', then for 1 in ''G'', (''n''·''λ_g'')(1) = (''λ_h''·''n'')(1), but the left hand side is ''n''(''g''), and the right side is ''h''. In other words, if ''n'' in ''N'' fixes the identity of ''G'', then for every ''g'' in ''G'', ''n''·''λ_g'' = ''λ_n(g)''·''n''. If ''g'', ''h'' are elements of ''G'', and ''n'' is an element of ''N'' fixing the identity of ''G'', then applying this equality twice to ''n''·''λ_g''·''λ_h'' and once to the (equivalent) expression ''n''·''λ_gg'' gives that ''n''(''g'')·''n''(''h'') = ''n''(''g''·''h''). That is, every element of ''N'' that fixes the identity of ''G'' is in fact an automorphism of ''G''. Such an ''n'' normalizes ''λ_G'', and the only ''λ_g'' that fixes the identity is ''λ''(1). Setting ''A'' to be the stabilizer of the identity, the subgroup generated by ''A'' and ''λ_G'' is semidirect product with normal subgroup ''λ_G'' and complement ''A''. Since ''λ_G'' is transitive, the subgroup generated by ''λ_G'' and the point stabilizer ''A'' is all of ''N'', which shows the holomorph as a permutation group is isomorphic to the holomorph as semidirect product.

It is useful, but not directly relevant, that the centralizer of ''λ_G'' in Sym(''G'') is ''ρ_G'', their intersection is $\rho_=\lambda_$, where Z(''G'') is the center of ''G'', and that ''A'' is a common complement to both of these normal subgroups of ''N''.

Properties

* ''ρ''(''G'') ∩ Aut(''G'') = 1
* Aut(''G'') normalizes ''ρ''(''G'') so that canonically ''ρ''(''G'')Aut(''G'') ≅ ''G'' ⋊ Aut(''G'')
*$\operatorname(G)\cong \operatorname(g\mapsto \lambda(g)\rho(g))$ since ''λ''(''g'')''ρ''(''g'')(''h'') = ''ghg''⁻¹ ($\operatorname(G)$ is the group of inner automorphisms of ''G''.)
* ''K'' ≤ ''G'' is a characteristic subgroup if and only if ''λ''(''K'') ⊴ Hol(''G'')

References

*
* {{Citation , last1=Burnside , first1=William , author1-link= William Burnside , title=Theory of Groups of Finite Order, 2nd ed. , publisher=Dover , page=87 , year=2004

Group theory
Group automorphisms