12. (FRG 3) Let $z$ be an integer $>1$ and let $M$ be the set of all numbers of the form $z_{k}=1+z+\cdots+z^{k}, k=0,1, \ldots$. Determine the set $T$ of divisors of at least one of the numbers $z_{k}$ from $M$.
12. According to part (a) of the previous problem we can conclude that $T=$ $\{n \in \mathbb{N} \mid(n, z)=1\}$.