Algorithms for $p$-rationality and for $p$-saturation of units

Abstract

Let $K$ be a number field. We give new practical algorithms that determine whether $K$ is $p$-rational. For real cyclotomic fields $K=\mathbb{Q}({\zeta }_n{)}^{+}$, our method avoids computing either the class number or the full unit group. We also use the notion of $p$-rationality to explain the practical efficiency of an algorithm for determining whether a subgroup of the unit group ${\mathcal{O}}_K^{\times }$ is $p$-saturated. This in turn yields a new algorithm for the unconditional verification of unit groups of number fields that substantially outperforms existing unconditional algorithms in practice. Finally, by combining our algorithms for determining $p$-rationality with work of Greenberg, we construct certain Galois representations $\rho :\mathrm{Gal}(\overline{\mathbb{Q}}/\mathbb{Q})\to {\mathrm{GL}}_n({\mathbb{Z}}_p)$ with open image and further prescribed properties.

38 pages; comments welcome!