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The Hall-Witt identity

November 20, 2011 in Groups, Lie groups, Surfaces, Visualization | Tags: , , , | by | 1 comment

The purpose of this blog post is to try to give some insight into the “meaning” of the Hall-Witt identity in group theory. This identity can look quite mysterious in its algebraic form, but there are several ways of describing it geometrically which are more natural and easier to understand.

If G is a group, and a,b are elements of G, the commutator of a and b (denoted [a,b]) is the expression aba^{-1}b^{-1} (note: algebraists tend to use the convention that [a,b]=a^{-1}b^{-1}ab instead). Commutators (as their name suggests) measure the failure of a pair of elements to commute, in the sense that ab=[a,b]ba. Since [a,b]^c = [a^c,b^c], the property of being a commutator is invariant under conjugation (here the superscript c means conjugation by c; i.e. a^c:=cac^{-1}; again, the algebraists use the opposite convention).

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