Conjugacy-closed abelian Hall implies retract

From Groupprops
Jump to: navigation, search

Statement

Suppose G is a finite group and H is an abelian conjugacy-closed Hall subgroup of G. Then, H is a retract of G. In other words, H possesses a normal complement N in G: a normal subgroup N of G such that N \cap H is trivial and NH = G.

Related facts

Facts used

  1. Conjugacy-closed and Hall implies commutator subgroup equals intersection with whole commutator subgroup (this is in turn an immediate corollary of the analogue of focal subgroup theorem for Hall subgroups.

Proof

Given: A finite group G and an abelian conjugacy-closed Hall subgroup H of G.

To prove: H has a normal complement in G.

Proof:

  1. H \cap [G,G] is trivial: This follows from fact (1), and the given fact that H is abelian.
  2. H has a normal complement: Consider the quotient by [G,G], and denote the image of H as \overline{H}. Since G/[G,G], the abelianization of G, is an Abelian group, and \overline{H} is a Sylow subgroup, there exists a normal complement \overline{N}. Let N be the inverse image of \overline{N} under the quotient map. Clearly, NH = G, since it contains [G,G] and its image is the whole of G/[G,G]. Further, N \cap H is trivial, because P does not intersect [G,G], and the images \overline{H} and \overline{N} intersect trivially. Thus, N is the required normal complement.