Commutator of Quotient Group Elements
Jump to navigation
Jump to search
Theorem
Let $G$ be a group.
Let $N$ be a normal subgroup of $G$.
Let $\sqbrk {x, y}$ denote the commutator of $x, y \in G$:
- $\sqbrk {x, y} = x^{-1} y^{-1} x y$
Then:
- $\forall x, y \in G: \sqbrk {x N, y N} = \sqbrk {x, y} N$
where $x N$ and $y N$ are left cosets of $N$, and so elements of the quotient group $G / N$ of $G$ by $N$.
Proof
\(\ds \sqbrk {x N, y N}\) | \(=\) | \(\ds \paren {x N}^{-1} \paren {y N}^{-1} \paren {x N} \paren {y N}\) | Definition of Commutator of Group Elements | |||||||||||
\(\ds \) | \(=\) | \(\ds \paren {x^{-1} N} \paren {y^{-1} N} \paren {x N} \paren {y N}\) | Quotient Group is Group: inverse of $x N$ is $x^{-1} N$ | |||||||||||
\(\ds \) | \(=\) | \(\ds \paren {x^{-1} y^{-1} N} \paren {x y N}\) | Definition of Coset Product | |||||||||||
\(\ds \) | \(=\) | \(\ds x^{-1} y^{-1} x y N\) | Definition of Coset Product | |||||||||||
\(\ds \) | \(=\) | \(\ds \sqbrk {x, y} N\) | Definition of Commutator of Group Elements |
$\blacksquare$
Sources
- 1978: Thomas A. Whitelaw: An Introduction to Abstract Algebra ... (previous) ... (next): Chapter $8$: Homomorphisms, Normal Subgroups and Quotient Groups: Exercise $16$