In mathematics, more specifically group theory, the three subgroups lemma is a result concerning commutators. It is a consequence of Philip Hall and Ernst Witt's eponymous identity.

Notation

In what follows, the following notation will be employed:

  • If H and K are subgroups of a group G, the commutator of H and K, denoted by [H, K], is defined as the subgroup of G generated by commutators between elements in the two subgroups. If L is a third subgroup, the convention that [H,K,L] = [[H,K],L] will be followed.
  • If x and y are elements of a group G, the conjugate of x by y will be denoted by .
  • If H is a subgroup of a group G, then the centralizer of H in G will be denoted by CG(H).

Statement

Let X, Y and Z be subgroups of a group G, and assume

and

Then .[1]

More generally, for a normal subgroup of , if and , then .[2]

Proof and the HallWitt identity

HallWitt identity

If , then

Proof of the three subgroups lemma

Let , , and . Then , and by the HallWitt identity above, it follows that and so . Therefore, for all and . Since these elements generate , we conclude that and hence .

See also

Notes

  1. Isaacs, Lemma 8.27, p. 111
  2. Isaacs, Corollary 8.28, p. 111

References

  • I. Martin Isaacs (1993). Algebra, a graduate course (1st ed.). Brooks/Cole Publishing Company. ISBN 0-534-19002-2.
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