Cyclotomic unit
= Cyclotomic unit
{title2=$c_n(a)$}
Cyclotomic units include roots of unity and suitable ratios $(1-\zeta^a)/(1-\zeta)$ in <cyclotomic fields>. Compatible roots $\zeta$ make these ratios norm-compatible along cyclotomic towers. Their closed norm-limit module is a concrete submodule of global units, and its local images generate the power series in the <Iwasawa main conjecture>.