# Left and Right Cosets of Subgroups

Definition: Let $(G, *)$ be a group and let $(H, *)$ be a subgroup. Let $g \in G$. Then the Left Coset of $H$ with Representative $g$ is the set $gH = \{ gh : h \in H \}$. The Right Coset of $H$ with Representative $g$ is the set $Hg = \{ hg : h \in H \}$. |

//When the operation symbol "$+$" is used instead of $*$ we often denote the left and right cosets of $H$ with representation $g$ with the notation $g + H$ and $H + g$ respectively.

For example, consider the group $(\mathbb{Z}, +)$ and the subgroup $(3\mathbb{Z}, +)$. Consider the element $2 \in \mathbb{Z}$. Then the left coset of $3\mathbb{Z}$ with representative $2$ is:

(1)And the right coset of $3 \mathbb{Z}$ with representative $2$ is:

(2)In this particular example we see that $2 + 3\mathbb{Z} = 3\mathbb{Z} + 2$. But in general, is $gH = Hg$ for a given subgroup $(H, *)$ of $(G, *)$ and for $g \in G$? The answer is NO. There are many examples when left cosets are not equal to corresponding right cosets.

To illustrate this, consider the symmetric group $(S_3, \circ)$. Let $G = \{ \epsilon, (12) \}$. Then $(G, \circ)$ is a subgroup of $(S_3, \circ)$ since $G \subset S_3$, $G$ is closed under $\circ$, and $\epsilon^{-1} = \epsilon$, $(12)^{-1} = (12)$ (since $(12)$ is a transposition). Now consider the element $(13) \in S_3$. Then the left coset of $G$ with representative $(13)$ is:

(3)And the right coset of $G$ with representative $(13)$ is:

(4)We note that $(123) \neq (132)$ and so $(13)G \neq G(13)$!

So, when exactly are the left and right cosets of a subgroup with representative $g$ equal? The following theorem gives us a simple criterion for a large class of groups.

Theorem 1: Let $(G, *)$ be a group and let $(H, *)$ be a subgroup. If $(G, *)$ is abelian then for all $g \in G$, $gH = Hg$. |

**Proof:**Let $g \in G$. If $G$ is abelian then for all $h \in G$ (and hence for all $h \in H$) we have that $g * h = h * g$. So:

Theorem 2: Let $(G, *)$ be a group, $(H, *)$ a subgroup, and $g_1, g_2 \in G$. Then the following statements are equivalent:a) $g_1H = g_2H$.b) $Hg_1^{-1} = Hg_2^{-1}$.c) $g_1H \subseteq g_2H$.d) $g_1 \in g_2H$.e) $g_1^{-1}g_2 \in H$. |