Element orders in extraspecial groups

Abstract

By using the structure and some properties of extraspecial and generalized/almost extraspecial $p$-groups, we explicitly determine the number of elements of specific orders in such groups. As a consequence, one may find the number of cyclic subgroups of any (generalized/almost) extraspecial group. For a finite group $G$, the ratio of the number of cyclic subgroups to the number of subgroups is called the cyclicity degree of $G$ and is denoted by $cdeg(G)$. We show that the set containing the cyclicity degrees of all finite groups is dense in $[0,1]$. This is equivalent to giving an affirmative answer to the following question posed by Tóth and Tărnăuceanu: ``For every $a\in [0,1]$, does there exist a sequence $(G_n{)}_{n\ge 1}$ of finite groups such that $\underset{n\to \infty }{\lim }cdeg(G_n)=a$?". We show that such sequences are formed of finite direct products of extraspecial groups of a specific type.

accepted for publication in Acta Math. Hung