# double coset

Let $H$ and $K$ be subgroups   of a group $G$. An $(H,K)$-double coset is a set of the form $HxK$ for some $x\in G$. Here $HxK$ is defined in the obvious way as

 $HxK=\{hxk\mid h\in H\hbox{ and }k\in K\}.$

Note that the $(H,\{1\})$-double cosets are just the right cosets  of $H$, and the $(\{1\},K)$-double cosets are just the left cosets of $K$. In general, every $(H,K)$-double coset is a union of right cosets of $H$, and also a union of left cosets of $K$.

The set of all $(H,K)$-double cosets is denoted $H\backslash G/K$. It is straightforward to show that $H\backslash G/K$ is a partition  (http://planetmath.org/Partition) of $G$, that is, every element of $G$ lies in exactly one $(H,K)$-double coset.

In contrast to the situation with ordinary cosets (http://planetmath.org/Coset), the $(H,K)$-double cosets need not all be of the same cardinality. For example, if $G$ is the symmetric group   (http://planetmath.org/SymmetricGroup) $S_{3}$, and $H=\langle(1,2)\rangle$ and $K=\langle(1,3)\rangle$, then the two $(H,K)$-double cosets are $\{e,(1,2),(1,3),(1,3,2)\}$ and $\{(2,3),(1,2,3)\}$.

Title double coset DoubleCoset 2013-03-22 16:17:28 2013-03-22 16:17:28 yark (2760) yark (2760) 8 yark (2760) Definition msc 20A05