Prüfer group

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

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)Edit

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:


The group operation here is the multiplication of complex numbers.

There is a presentation


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:


(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):


We can also write


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


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


(Here   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]

See alsoEdit


  1. ^ See Vil'yams (2001)
  2. ^ See Kaplansky (1965)
  3. ^ See also Jacobson (2009), p. 102, ex. 2.
  4. ^ See Vil'yams (2001)
  5. ^ D. L. Armacost and W. L. Armacost,"On p-thetic groups", Pacific J. Math., 41, no. 2 (1972), 295–301


  • Jacobson, Nathan (2009). Basic algebra. 2 (2nd ed.). Dover. ISBN 978-0-486-47187-7.
  • Pierre Antoine Grillet (2007). Abstract algebra. Springer. ISBN 978-0-387-71567-4.
  • Kaplansky, Irving (1965). Infinite Abelian Groups. University of Michigan Press.
  • N.N. Vil'yams (2001) [1994], "Quasi-cyclic group", Encyclopedia of Mathematics, EMS Press