Coset

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Coset

In mathematics, if G is a group, and H is a subgroup of G, and g is 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.

Only when H is normal will the right and left cosets of H coincide, which is one definition of normality of a subgroup.

A coset is a left or right coset of some subgroup in G. Since Hg = g ( g−1Hg ), the right cosets Hg (of H ) and the left cosets g ( g−1Hg ) (of the conjugate subgroup g−1Hg ) are the same. Hence it is not meaningful to speak of a coset as being left or right unless one first specifies the underlying subgroup. In other words: a right coset of one subgroup equals a left coset of a different (conjugate) subgroup. If the left cosets and right cosets are the same then H is a normal subgroup and the cosets form a group called the quotient group.

The map gH→(gH)−1=Hg−1 defines a bijection between the left cosets and the right cosets of H, so the number of left cosets is equal to the number of right cosets. The common value is called the index of H in G.

For abelian groups, left cosets and right cosets are always the same. If the group operation is written additively then the notation used changes to g+H or H+g.

Cosets are a basic tool in the study of groups, for example they play a central role in Lagrange's theorem.

Examples

Let G be the additive group of integers Z = {… , −2, −1, 0, 1, 2, …} and H the subgroup mZ = = {…, −2m, −m, 0, m, 2m, …} where m is a positive integer. Then the cosets of H in G are the m sets mZ, mZ+1, … mZ+(m−1), where mZ+a={…, −2m+a, −m+a, a, m+a, 2m+a, …}. The coset mZ+a is the congruence classes of a modulo m.[1]

Another example of a coset comes from the theory of vector spaces. The elements (vectors) of a vector space form an abelian group under vector addition. It is not hard to show that subspaces of a vector space are subgroups of this group. For a vector space V, a subspace W, and a fixed vector a in V, the sets

$\{x \in V \colon x = a + n, n \in W\}$

are called affine subspaces, and are cosets (both left and right, since the group is abelian). In terms of geometric vectors, these affine subspaces are all the "lines" or "planes" parallel to the subspace, which is a line or plane going through the origin.

Definition using equivalence classes

Some authors[2] define the left cosets of H in G to be the equivalence classes under the equivalence relation on G given by x ~ y if and only if x−1yH. The relation can also be defined by x ~ y if and only if xh=y for some h in H. It can be shown that the relation given is, in fact, an equivalence relation and that the two definitions are equivalent. It follows that any two left cosets of H in G are either identical or disjoint  — . In other words every element of G belongs to one and only one left coset and so the left cosets form a partition of G.[3] Corresponding statements are true for right cosets.

Double cosets

Given two subgroups, H and K of a group G, the double coset of H and K in G are sets of the form HgK = {hgk : h an element of H , k an element of K }. These are the left cosets of K and right cosets of H when H=1 and K=1 respectively.[4]

General properties

The identity is in precisely one left or right coset, namely H itself. Thus H is both a left and right coset of itself.

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.

Index of a subgroup

All left cosets and all right cosets have the same order (number of elements, or cardinality in the case of an infinite H), equal to the order of H (because H is itself a coset). 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, although the meaning may be less clear.

Cosets and normality

If H is not normal in G, then its left cosets are different from its right cosets. That is, there is an a in G such that no element b satisfies aH = Hb. This means that the partition of G into the left cosets of H is a different partition than the partition of G into right cosets of H. (It is important to note that some cosets may coincide. For example, if a is in the center of G, then aH = Ha.)

On the other hand, the subgroup N is normal if and only if gN = Ng for all g in G. In this case, the set of all cosets form a group called the quotient group G /N with the operation ∗ defined by (aN )∗(bN ) = abN. Since every right coset is a left coset, there is no need to differentiate "left cosets" from "right cosets".

References

1. ^ Joshi p. 323
2. ^ e.g. Zassenhaus
3. ^ Joshi Corrolary 2.3
4. ^ Scott p. 19
• Scott, W.R. (1987). "§1.7 Cosets and index". Group Theory. Courier Dover Publications. pp. 19 ff.. ISBN 0486653773.
• Joshi, K. D. (1989). "§5.2 Cosets of Subgroups". Foundations of Discrete Mathematics. New Age International. pp. 322 ff.. ISBN 8122401201.
• Zassenhaus, Hans J. (1999). "§1.4 Subgroups". The Theory of Groups. Courier Dover Publications. pp. 10 ff.. ISBN 0486409228.

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