# Coset

In mathematics, if *G* is a group, *H* a subgroup of *G*, and *g* an element of *G*, then

*gH*= {*gh*:*h*an element of*H*} is a**left coset**of*H*in*G*, and*Hg*= {*hg*:*h*an element of*H*} is a**right coset**of*H*in*G*.

## Some properties

We have *gH* = *H* if and only if *g* is an element of *H*.
Any two left cosets are either identical or disjoint. The left cosets form a partition of *G*: every element of *G* belongs to one and only one left coset. In particular the identity is only in one coset, and *H* itself is the only coset that is a subgroup.

The left cosets of *H* in *G* are the equivalence classes under the equivalence relation on *G* given by *x* ~ *y* if and only if *x*^{ -1}*y* ∈ *H*. Similar statements are also true for right cosets. A **coset representative** is a representative in the equivalence class sense. A set of representatives of all the cosets is called a transversal. There are other types of equivalence relations in a group, such as conjugacy, that form different classes which do not have the properties discussed here. Some books on very applied group theory erroneously identify the conjugacy class as 'the' equivalence class as opposed to a particular type of equivalence class.

All left cosets and all right cosets have the same number of elements (or cardinality in the case of an infinite *H*). Furthermore, the number of left cosets is equal to the number of right cosets and is known as the **index** of *H* in *G*, written as [*G* : *H*]. Lagrange's theorem allows us to compute the index in the case where G and H are finite, as per the formula:

- |
*G*| = [*G*:*H*] · |*H*|

This equation also holds in the case where the groups are infinite (but is somewhat less useful).

The subgroup *H* is normal if and only if *gH* = *Hg* for all *g* in *G*. In this case one can turn the set of all cosets into a group, the factor group of *G* by *H*.