Constructing abelian extensions with prescribed norms

Abstract

Given a number field $K$, a finite abelian group $G$ and finitely many elements $\hat{I}{\pm }_1,\dots ,\hat{I}{\pm }_t\in K$, we construct abelian extensions $L/K$ with Galois group $G$ that realise all of the elements $\hat{I}{\pm }_1,\dots ,\hat{I}{\pm }_t$ as norms of elements in $L$. In particular, this shows existence of such extensions for any given parameters. Our approach relies on class field theory and a recent formulation of Tate's characterisation of the Hasse norm principle, a local-global principle for norms. The constructions are sufficiently explicit to be implemented on a computer, and we illustrate them with concrete examples.

20 pages; minor revision