Element in Right Coset iff Product with Inverse in Subgroup
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Theorem
Let $G$ be a group.
Let $H$ be a subgroup of $G$.
Let $x, y \in G$.
Let $H \circ y$ denote the right coset of $H$ by $y$.
Then:
- $x \in H y \iff x y^{-1} \in H$
Proof
Let $\struct {G, *}$ be the opposite group of $G$.
Then:
- $x \in H y \iff x \in y * H$
- $x y^{-1} \in H \iff y^{-1} * x \in H$
Since $H$ is closed under inverses:
- $x y^{-1} \in H \iff x^{-1} * y \in H$
By Element in Left Coset iff Product with Inverse in Subgroup:
- $x \in y * H \iff x^{-1} * y \in H$
Hence the result.
$\blacksquare$
Also see
Sources
- 1965: J.A. Green: Sets and Groups ... (previous) ... (next): $\S 6.1$. The quotient sets of a subgroup: Lemma $\text{(i)}$
- 1978: Thomas A. Whitelaw: An Introduction to Abstract Algebra ... (previous) ... (next): $\S 42.6 \ \text {(1R)}$ Another approach to cosets