# Prüfer rank

In mathematics, especially in the area of algebra known as group theory, the Prüfer rank of a pro-p group measures the size of a group in terms of the ranks of its elementary abelian sections.[1] The rank is well behaved and helps to define analytic pro-p-groups. The term is named after Heinz Prüfer.

## Definition

The Prüfer rank of pro-p-group ${\displaystyle G}$ is

${\displaystyle \sup\{d(H)|H\leq G\}}$

where ${\displaystyle d(H)}$ is the rank of the abelian group

${\displaystyle H/\Phi (H)}$,

where ${\displaystyle \Phi (H)}$ is the Frattini subgroup of ${\displaystyle H}$.

As the Frattini subgroup of ${\displaystyle H}$ can be thought of as the group of non-generating elements of ${\displaystyle H}$, it can be seen that ${\displaystyle d(H)}$ will be equal to the size of any minimal generating set of ${\displaystyle H}$.

## Properties

Those profinite groups with finite Prüfer rank are more amenable to analysis.

Specifically in the case of finitely generated pro-p groups, having finite Prüfer rank is equivalent to having an open normal subgroup that is powerful. In turn these are precisely the class of pro-p groups that are p-adic analytic - that is groups that can be imbued with a p-adic manifold structure.

## References

1. ^ Yamagishi, Masakazu (2007), "An analogue of the Nielsen-Schreier formula for pro-p-groups", Archiv der Mathematik, 88 (4): 304–315, doi:10.1007/s00013-006-1878-4, MR 2311837, Zbl 1119.20035.