Finitely generated subgroup
In algebra, a finitely generated group is a group G that has some finite generating set S so that every element of G can be written as the combination (under the group operation) of finitely many elements of the finite set S and of inverses of such elements.
By definition, every finite group is finitely generated, since S can be taken to be G itself. Every infinite finitely generated group must be countable but countable groups need not be finitely generated. The additive group of rational numbers Q is an example of a countable group that is not finitely generated.
Every Abelian group can be seen as a module over the ring of integers Z, and in a finitely generated Abelian group with generators x1, ..., xn, every group element x can be written as a linear combination of these generators,
with integers α1, ..., αn.
Subgroups of a finitely generated Abelian group are themselves finitely generated.
The fundamental theorem of finitely generated abelian groups states that a finitely generated Abelian group is the direct sum of a free Abelian group of finite rank and a finite Abelian group, each of which are unique up to isomorphism.
A subgroup of a finitely generated group need not be finitely generated. The commutator subgroup of the free group on two generators is an example of a subgroup of a finitely generated group that is not finitely generated.
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