Generating set of a group

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This article defines a property of subsets of groups
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This term is related to: combinatorial group theory
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Definition

Symbol-free definition

A subset of a group is termed a generating set if it satisfies the following equivalent conditions:

  • Every element of the group can be expressed in terms of the elements of this subset by means of the group operations of multiplication and inversion.
  • There is no proper subgroup of the group containing this subset
  • There is a surjective map from a free group on that many generators to the given group, that sends the generators of the free group to the elements of this generating set.

The elements of the generating set are termed generators.

Definition with symbols

A subset S of a group G is termed a generating set if it satisfies the following equivalent conditions:

  • For any element g \in G, we can write:

g = a_1a_2 \ldots a_n

where for each a_i, either a_i \in S or a_i^{-1} \in S (here, the a_is are not necessarily distinct).

  • If H is a proper subgroup of G (i.e. H is a subgroup of G that is not equal to the whole of G), then H cannot contain S.
  • Consider the natural map from the free group on as many generators as elements of S, to the group G, which maps the freely generating set to the elements of S. This gives a surjective homomorphism from the free group, to G.

Constructs

Cayley graph

Further information: Cayley graph of a group Given a generating set of a group, we can construct the Cayley graph of the group with respect to that generating set. This is a graph whose vertex set is the set of elements of the group and where there is an edge between two vertices whenever one can be taken to the other by left multiplying by a generator.

Presentation

The generators of a group may not in general be independent. That is, there may be words in the generators that simplify to the identity in the given group. Such a word is termed a relation between the generators. Relations can be viewed as elements of the free group on symbols corresponding to the generators. Thus relations form a normal subgroup of the free group. Specifying a set of relations whose normal closure is this normal subgroup is what we call a presentation of a group.

Study of this notion

Mathematical subject classification

Under the Mathematical subject classification, the study of this notion comes under the class: 20F05

External links

Definition links