Action of Group on Coset Space is Group Action

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Theorem

Let $G$ be a group whose identity is $e$.

Let $H$ be a subgroup of $G$.


Let $*: G \times G / H \to G / H$ be the action on the (left) coset space:

$\forall g \in G, \forall g' H \in G / H: g * \paren {g' H} := \paren {g g'} H$

Then $G$ is a group action.


Proof

\(\displaystyle a * \paren {b * g' H}\) \(=\) \(\displaystyle a * \paren {\paren {b g'} H}\) Definition of $*$
\(\displaystyle \) \(=\) \(\displaystyle \paren {a \paren {b g'} } H\) Definition of $*$
\(\displaystyle \) \(=\) \(\displaystyle \paren {a b} g' H\) Group Axiom $G \, 1$: Associativity

demonstrating that $*$ satisfies Group Action Axiom $GA \, 2$.


Then:

\(\displaystyle e * g' H\) \(=\) \(\displaystyle \paren {e g'} H\) Definition of $*$
\(\displaystyle \) \(=\) \(\displaystyle g'H\) Group Axiom $G \, 2$: Identity

demonstrating that $*$ satisfies Group Action Axiom $GA \, 1$.

$\blacksquare$


Sources