Cyclotomic unit

The cyclotomic units form a subgroup of finite index in the group of units of a cyclotomic field. The index of this subgroup of real cyclotomic units (those cyclotomic units in the maximal real subfield) within the full real unit group is equal to the class number of the maximal real subfield of the cyclotomic field.[1]

If n is the power of a prime, then ζ^a
_n − 1 is not a unit; however the numbers (ζ^a
_n − 1)/(ζ
_n − 1) for (a, n) = 1, and ±ζ^a
_n generate the group of cyclotomic units in this case (n power of a prime).

If n is a composite number, the subgroup of cyclotomic units generated by (ζ^a
_n − 1)/(ζ
_n − 1)with (a, n) = 1 is not of finite index in general.[2]

The cyclotomic units satisfy distribution relations. Let a be a rational number prime to p and let g_a denote exp(2πia)−1. Then for a≠ 0 we have ${\displaystyle \prod _{pb=a}g_{b}=g_{a}}$.[3]

Using these distribution relations and the symmetry relation ζ^a
_n − 1 = -ζ^a
_n (ζ^-a
_n − 1) a basis B_n of the cyclotomic units can be constructed with the property that B_d ⊆ B_n for d | n.[4]

• Elliptic unit
• Modular unit

• Lang, Serge (1990). Cyclotomic Fields I and II. Graduate Texts in Mathematics. 121 (second combined ed.). Springer Verlag. ISBN 3-540-96671-4. Zbl 0704.11038.
• Narkiewicz, Władysław (1990). Elementary and analytic theory of numbers (Second, substantially revised and extended ed.). Springer-Verlag. ISBN 3-540-51250-0. Zbl 0717.11045.
• Washington, Lawrence C. (1997). Introduction to Cyclotomic Fields. Graduate Texts in Mathematics. 83 (2nd ed.). Springer-Verlag. ISBN 0-387-94762-0. Zbl 0966.11047.