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 Hall–Witt identity
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
Notes
References
- I. Martin Isaacs (1993). Algebra, a graduate course (1st ed.). Brooks/Cole Publishing Company. ISBN 0-534-19002-2.
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