The Number of Finite Groups Whose Element Orders is Given

The spectrum $\omega(G)$ of a finite group $G$ is the set of element orders of $G$. If $\Omega$ is a non-empty subset of the set of natural numbers, $h(\Omega)$ stands for the number of isomorphism classes of finite groups $G$ with $\omega(G)=\Omega$ and put $h(G)=h(\omega(G))$. We say that $G$ is recognizable (by spectrum $\omega(G)$) if $h(G)=1$. The group $G$ is almost recognizable (resp. nonrecognizable) if $1<h(G)<\infty$ (resp. $h(G)=\infty$). In the present paper, we focus our attention on the projective general linear groups ${PGL}(2,p^n)$, where $p=2^\alpha 3^\beta+1$ is a prime, $\alpha \geq 0, \beta \geq 0$ and $n\geq 1$, and we show that these groups cannot be almost recognizable, in other words $h({PGL}(2,p^n))\in \{1, \infty\}$. It is also shown that the projective general linear groups ${PGL}(2,7)$ and ${PGL}(2,9)$ are nonrecognizable. In this paper a computer program has also been presented in order to find out the primitive prime divisors of $a^n-1$.