Orbit of Subgroup Action is Coset
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Theorem
Let $\struct {G, \circ}$ be a group.
Let $\struct {H, \circ}$ be a subgroup of $G$.
Let $*: H \times G \to G$ be the subgroup action defined for all $h \in H, g \in G$ as:
- $\forall h \in H, g \in G: h * g := h \circ g$
The orbit of $x \in G$ is the right coset by $x$ of $H$:
- $\Orb x = H x$
Proof
From Subgroup Action is Group Action we have that $*$ is a group action.
Let $x \in G$.
Then:
\(\ds \Orb x\) | \(=\) | \(\ds \set {g \in G: \exists h \in H: g = h * x}\) | Definition of Orbit | |||||||||||
\(\ds \) | \(=\) | \(\ds \set {g \in G: \exists h \in H: g = h \circ x}\) | Definition of $*$ | |||||||||||
\(\ds \) | \(=\) | \(\ds H x\) |
Hence the result, by definition of right coset.
$\blacksquare$
Sources
- 1965: J.A. Green: Sets and Groups ... (previous) ... (next): Chapter $6$: Cosets: Exercise $8$
- 1971: Allan Clark: Elements of Abstract Algebra ... (previous) ... (next): Chapter $2$: The Sylow Theorems: $\S 54 \alpha$