The Prüfer -group for a prime number consists of all roots of unity of order , ; that is, . The group, named after the mathematician Heinz Prüfer, is also known as the quasicyclic -group. is an example of a countable -group that is not the direct sum of groups of rank 1. The radii of the circles or disks for each element on the group in the plot decrease with .
 Wikipedia. "Prüfer Group." (Mar 5, 2015) en.wikipedia.org/wiki/Pr% C3 % BCfer_group.
 J. Baez, "Prüfer 2-Group," AMS Blogs: Visual Insight (blog). (Mar 5, 2015) blogs.ams.org/visualinsight/2014/09/15/prufer-2-group.