# 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 [math]\displaystyle{ G }[/math] is

- [math]\displaystyle{ \sup\{d(H)|H\leq G\} }[/math]

where [math]\displaystyle{ d(H) }[/math] is the rank of the abelian group

- [math]\displaystyle{ H/\Phi(H) }[/math],

where [math]\displaystyle{ \Phi(H) }[/math] is the Frattini subgroup of [math]\displaystyle{ H }[/math].

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

## 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

- ↑ 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.

Original source: https://en.wikipedia.org/wiki/Prüfer rank.
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