Roots of unity in a rational cyclotomic field
= Roots of unity in a rational cyclotomic field
The roots of unity in $\mathbb Q(\zeta_n)$ form a cyclic group of order
$$
\operatorname{lcm}(2,n)=
\begin{cases}
n,&n\text{ even},\\
2n,&n\text{ odd}.
\end{cases}
$$