Definitions

# Finitely generated abelian group

In abstract algebra, an abelian group (G,+) is called finitely generated if there exist finitely many elements x1,...,xs in G such that every x in G can be written in the form
x = n1x1 + n2x2 + ... + nsxs
with integers n1,...,ns. In this case, we say that the set {x1,...,xs} is a generating set of G or that x1,...,xs generate G.

Clearly, every finite abelian group is finitely generated. The finitely generated abelian groups are of a rather simple structure and can be completely classified, as will be explained below.

## Examples

• the integers (Z,+) are a finitely generated abelian group
• the integers modulo n Zn are a finitely generated abelian group
• any direct sum of finitely many finitely generated abelian groups is again finitely generated abelian

There are no other examples. The group (Q,+) of rational numbers is not finitely generated: if x1,...,xs are rational numbers, pick a natural number w coprime to all the denominators; then 1/w cannot be generated by x1,...,xs.

## Classification

The fundamental theorem of finitely generated abelian groups (which is a special case of the structure theorem for finitely generated modules over a principal ideal domain) can be stated two ways (analogously with PIDs):

### Primary decomposition

The primary decomposition formulation states that every finitely generated abelian group G is isomorphic to a direct sum of primary cyclic groups and infinite cyclic groups. A primary cyclic group is one whose order is a power of a prime. That is, every such group is isomorphic to one of the form
$mathbb\left\{Z\right\}^n oplus mathbb\left\{Z\right\}_\left\{q_1\right\} oplus cdots oplus mathbb\left\{Z\right\}_\left\{q_t\right\}$
where the rank n ≥ 0, and the numbers q1,...,qt are powers of (not necessarily distinct) prime numbers. In particular, G is finite if and only if n = 0. The values of n, q1,...,qt are (up to rearranging the indices) uniquely determined by G.

### Invariant factor decomposition

We can also write any finitely generated abelian group G as a direct sum of the form
$mathbb\left\{Z\right\}^n oplus mathbb\left\{Z\right\}_\left\{k_1\right\} oplus cdots oplus mathbb\left\{Z\right\}_\left\{k_u\right\}$
where k1 divides k2, which divides k3 and so on up to ku. Again, the rank n and the invariant factors k1,...,ku are uniquely determined by G (here with a unique order).

### Equivalence

These statements are equivalent because of the Chinese remainder theorem, which here states that Zm is isomorphic to the direct product of Zj and Zk if and only if j and k are coprime and m = jk.

## Corollaries

Stated differently the fundamental theorem says 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 those being unique up to isomorphism. The finite abelian group is just the torsion subgroup of G. The rank of G is defined as the rank of the torsion-free part of G; this is just the number n in the above formulas.

A corollary to the fundamental theorem is that every finitely generated torsion-free abelian group is free abelian. The finitely generated condition is essential here: Q is torsion-free but not free abelian.

Every subgroup and factor group of a finitely generated abelian group is again finitely generated abelian. The finitely generated abelian groups, together with the group homomorphisms, form an abelian category which is a Serre subcategory of the category of abelian groups.

## Non-finitely generated abelian groups

Note that not every abelian group of finite rank is finitely generated; the rank-1 group Q is one example, and the rank-0 group given by a direct sum of countably many copies of Z2 is another one.