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

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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

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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 Hall–Witt identity

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Hall–Witt identity

If  , then

 

Proof of the three subgroups lemma

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

See also

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Notes

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

References

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