# Prüfer group

The Prüfer 2-group with presentation gn: gn+12 = gn, g12 = e, illustrated as a subgroup of the unit circle in the complex plane

In mathematics, specifically in group theory, the Prüfer p-group or the p-quasicyclic group or p-group, Z(p), for a prime number p is the unique p-group in which every element has p different p-th roots.

The Prüfer p-groups are countable abelian groups that are important in the classification of infinite abelian groups: they (along with the group of rational numbers) form the smallest building blocks of all divisible groups.

The groups are named after Heinz Prüfer, a German mathematician of the early 20th century.

## Constructions of Z(p∞)

The Prüfer p-group may be identified with the subgroup of the circle group, U(1), consisting of all pn-th roots of unity as n ranges over all non-negative integers:

$\displaystyle{ \mathbf{Z}(p^\infty)=\{\exp(2\pi i m/p^n) \mid 0 \leq m \lt p^n,\,n\in \mathbf{Z}^+\} = \{z\in\mathbf{C} \mid z^{(p^n)}=1 \text{ for some } n\in \mathbf{Z}^+\}.\; }$

The group operation here is the multiplication of complex numbers.

There is a presentation

$\displaystyle{ \mathbf{Z}(p^\infty) = \langle\, g_1, g_2, g_3, \ldots \mid g_1^p = 1, g_2^p = g_1, g_3^p = g_2, \dots\,\rangle. }$

Here, the group operation in Z(p) is written as multiplication.

Alternatively and equivalently, the Prüfer p-group may be defined as the Sylow p-subgroup of the quotient group Q/Z, consisting of those elements whose order is a power of p:

$\displaystyle{ \mathbf{Z}(p^\infty) = \mathbf{Z}[1/p]/\mathbf{Z} }$

(where Z[1/p] denotes the group of all rational numbers whose denominator is a power of p, using addition of rational numbers as group operation).

For each natural number n, consider the quotient group Z/pnZ and the embedding Z/pnZZ/pn+1Z induced by multiplication by p. The direct limit of this system is Z(p):

$\displaystyle{ \mathbf{Z}(p^\infty) = \varinjlim \mathbf{Z}/p^n \mathbf{Z} . }$

If we perform the direct limit in the category of topological groups, then we need to impose a topology on each of the $\displaystyle{ \mathbf{Z}/p^n \mathbf{Z} }$, and take the final topology on $\displaystyle{ \mathbf{Z}(p^\infty) }$. If we wish for $\displaystyle{ \mathbf{Z}(p^\infty) }$ to be Hausdorff, we must impose the discrete topology on each of the $\displaystyle{ \mathbf{Z}/p^n \mathbf{Z} }$, resulting in $\displaystyle{ \mathbf{Z}(p^\infty) }$ to have the discrete topology.

We can also write

$\displaystyle{ \mathbf{Z}(p^\infty)=\mathbf{Q}_p/\mathbf{Z}_p }$

where Qp denotes the additive group of p-adic numbers and Zp is the subgroup of p-adic integers.

## Properties

The complete list of subgroups of the Prüfer p-group Z(p) = Z[1/p]/Z is:

$\displaystyle{ 0 \subsetneq \left({1 \over p}\mathbf{Z}\right)/\mathbf{Z} \subsetneq \left({1 \over p^2}\mathbf{Z}\right)/\mathbf{Z} \subsetneq \left({1 \over p^3}\mathbf{Z}\right)/\mathbf{Z} \subsetneq \cdots \subsetneq \mathbf{Z}(p^\infty) }$

Here, each $\displaystyle{ \left({1 \over p^n}\mathbf{Z}\right)/\mathbf{Z} }$ is a cyclic subgroup of Z(p) with pn elements; it contains precisely those elements of Z(p) whose order divides pn and corresponds to the set of pn-th roots of unity.

The Prüfer p-groups are the only infinite groups whose subgroups are totally ordered by inclusion. This sequence of inclusions expresses the Prüfer p-group as the direct limit of its finite subgroups. As there is no maximal subgroup of a Prüfer p-group, it is its own Frattini subgroup.

Given this list of subgroups, it is clear that the Prüfer p-groups are indecomposable (cannot be written as a direct sum of proper subgroups). More is true: the Prüfer p-groups are subdirectly irreducible. An abelian group is subdirectly irreducible if and only if it is isomorphic to a finite cyclic p-group or to a Prüfer group.

The Prüfer p-group is the unique infinite p-group that is locally cyclic (every finite set of elements generates a cyclic group). As seen above, all proper subgroups of Z(p) are finite. The Prüfer p-groups are the only infinite abelian groups with this property.[1]

The Prüfer p-groups are divisible. They play an important role in the classification of divisible groups; along with the rational numbers they are the simplest divisible groups. More precisely: an abelian group is divisible if and only if it is the direct sum of a (possibly infinite) number of copies of Q and (possibly infinite) numbers of copies of Z(p) for every prime p. The (cardinal) numbers of copies of Q and Z(p) that are used in this direct sum determine the divisible group up to isomorphism.[2]

As an abelian group (that is, as a Z-module), Z(p) is Artinian but not Noetherian.[3] It can thus be used as a counterexample against the idea that every Artinian module is Noetherian (whereas every Artinian ring is Noetherian).

The endomorphism ring of Z(p) is isomorphic to the ring of p-adic integers Zp.[4]

In the theory of locally compact topological groups the Prüfer p-group (endowed with the discrete topology) is the Pontryagin dual of the compact group of p-adic integers, and the group of p-adic integers is the Pontryagin dual of the Prüfer p-group.[5]