Bases for some modules of cyclotomic units

Abstract

Let $\mathbf{Was}(\mathbb{K})$ denote the group of Washington's cyclotomic units of any abelian number field $\mathbb{K}$. If $\mathbb{K}$ coincides with its genus field in the narrow sense, we give a $\Lambda$-basis of $\lim\limits\_{\xleftarrow{}} \mathbf{Was}(\mathbb{K}\_k^{+})$ where $(\mathbb{K}\_k)\_{k \geqslant 0}$ denotes the cyclotomic $\mathbb{Z}\_p$-tower of $\mathbb{K}$ and $\Lambda$ denotes the Iwasawa's algebra. This results from a $\mathbb{Z} [1/2]$-basis of $\mathbf{Was}(\mathbb{K}) \otimes\_{\mathbb{Z}} \mathbb{Z} [1/2]$ that we give under the same hypothesis.