Commutator is Identity iff Elements Commute
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Theorem
Let $G$ be a group whose identity is $e$.
Let $x, y \in G$.
Let $\sqbrk {x, y}$ denote the commutator of $x$ and $y$.
Then $\sqbrk {x, y} = e$ if and only if $x$ and $y$ commute.
Proof
As $G$ is a group, it is a fortiori a monoid.
Hence Product of Commuting Elements with Inverses applies:
- $x y x^{-1} y^{-1} = e = x^{-1} y^{-1} x y$
$\blacksquare$
Sources
- 1978: Thomas A. Whitelaw: An Introduction to Abstract Algebra ... (previous) ... (next): Chapter $8$: Homomorphisms, Normal Subgroups and Quotient Groups: Exercise $15$