Algebraic Number Theory
《Algebraic Number Theory》 · 第 44 页 · Chapter I，Section 7，Exercise 5
Let $\zeta$ be a primitive $m$th root of unity, $m\geq3$. The numbers $(1-\zeta^k)/(1-\zeta)$ for $(k,m)=1$ are units in the ring of integers of $\mathbb Q(\zeta)$; the subgroup of the group of units they generate is called the group of cyclotomic units.