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Subgroup

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This article is about a basic definition in group theory. The article text may, however, contain more material. Rate its utility as a basic definition article on the talk page
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This article defines a property of subsets of groups

Definition

The universal algebraic definition

Let G be a group. A subset H of G is termed a subgroup if all the three conditions below are satisfied:

  • Whenever a,b belong to H, so does ab (here ab denotes the product of the two elements)
  • Whenever a belongs to H, so does a − 1 (the multiplicative inverse of a)
  • e belongs to H (where e denotes the identity element)

Definition via the subgroup condition

The equivalence of this definition with the earlier one is often called the subgroup condition. For full proof, refer: Sufficiency of subgroup condition

It has two forms (left and right):

  • A subset of a group is termed a subgroup if it is nonempty and is closed under the left quotient of elements. In other words, a subset H of a group G is termed a subgroup if and only if H is nonempty and a^{-1}b \in H whenever a,b \in H
  • A subset of a group is termed a subgroup if it is nonempty and is closed under the right quotient of elements. In other words, a subset H of a group G is termed a subgroup if and only if H is nonempty and ab^{-1} \in H whenever a,b \in H

Definition in terms of injective homomorphisms

A subgroup of a group can also be defined as another abstract group along with an injective homomorphism (or embedding) from that abstract group to the given group. Here, the other abstract group can be naturally identified via its image under the homomorphism, which is the subgroup in a more literal sense.

Often, when we want to emphasize the subgroup not just as an abstract group but in its role as a subgroup, we use the term embedding and think of it as an injective homomorphism.

Equivalence of subgroups

Given a subgroup H_1 \le G_1 and a subgroup H_2 \le G_2, we say that these two subgroups are equivalent if there is an isomorphism σ from G1 to G2 such that H1 maps to H2 under that isomorphism.

In particular, if G1 = G2 = G, then H1 and H2 are equivalent as subgroups if there is an automorphism of G under which H1 maps to H2.

This notion of equivalence of subgroups is important when dealing with and defining the notion of subgroup property.

Properties

Subgroup properties

Further information: subgroup property

Given a group and a subgroup thereof, we want answers to various questions about how the subgroup sits inside the group. These answers are encoded in various ways. One of these is by checking whether the subgroup satisfies a particular subgroup property. A subgroup property is something that takes as input a group and subgroup and outputs true/false; moreover, the answer should be the same for equivalent group-subgroup pairs.

Category:Subgroup properties is a complete list of subgroup properties; Category:Pivotal subgroup properties is a list of important ones.

Note that the property of being a subgroup is itself a subgroup property; in logical terms, it is the tautology subgroup property: the one that's always true.


Metaproperties

Intersection-closedness

This subgroup property is intersection-closed: an arbitrary (nonempty) intersection of subgroups with this property, also has this property
View a complete list of intersection-closed subgroup properties

An arbitrary intersection of subgroups is a subgroup. For full proof, refer: Intersection of subgroups is subgroup Thus, given any subset of a group, it makes sense to talk of the smallest subgroup containing that subset.

Join-closedness

This subgroup property is join-closed: an arbitrary (nonempty) join of subgroups with this property, also has this property
View a complete list of join-closed subgroup properties

Given any subset, we can talk of the subgroup generated by that subset. One way of viewing this is as the intersection of all subgroups containing that subset. Another way of viewing it is as the set of all elements in the group that can be expressed using elements of the subset, and the group operations.

Hence, in particular, given a family of subgroups, we can talk of the subgroup generated by them, as simply the subgroup generated by their union. This is the smallest subgroup containing all of them.

Transitivity

This subgroup property is transitive: a subgroup with this property in a subgroup with this property, also has this property
View a complete list of transitive subgroup properties

Any subgroup of a subgroup is again a subgroup. This follows directly from any of the equivalent definitions of subgroup.

Trimness

This subgroup property is trim -- it is both trivially true (true for the trivial subgroup) and identity-true (true for a group as a subgroup of itself)
View all trim subgroup properties OR view trivially true subgroup properties OR view identity-true subgroup properties

There are two extreme kinds of subgroups: the trivial subgroup, which comprises only the identity element, and the whole group, which comprises all elements.

Intermediate subgroup condition

This subgroup property satisfies the intermediate subgroup condition: if a subgroup has the property in the whole group, it has the property in every intermediate subgroup
View all subgroup properties satisfying intermediate subgroup condition

The property of being a subgroup satisfies the intermediate subgroup condition. That is, if H \le G is a subgroup and K is a subgroup of G containing H, then H is a subgroup of K (not merely a subset).

Image condition

This subgroup property satisfies the image condition, viz under any surjective homomorphism, the image of a subgroup satisfying the property also satisfies the property

The image of a subgroup under any homomorphism of groups is again a subgroup.

Inverse image condition

This subgroup property satisfies the inverse image condition

The inverse image of a subgroup under any homomorphism of groups is again a subgroup.

Template:ACU-closed

The union of any ascending chain of subgroups is again a subgroup. In fact, it is precisely the subgroup generated by the members of the ascending chain.

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