Answer to a Question of Hung and Tiep on Conductors of Cyclotomic Integers

Abstract

In Question 5.2 of [5], Hung and Tiep asked the following: If $\alpha$ is a sum of $k$ complex roots of unity and $\mathbb{Q}_{c(\alpha)}$ is the smallest cyclotomic field containing $\alpha$, is it true that $|\mathbb{Q}_{c(\alpha)}:\mathbb{Q}(\alpha)| \leq k$? We answer this question in the negative. Using known results on minimal vanishing sums, we also characterize all cyclotomic integers with $k \leq 4$ for which the inequality fails. In \S 6, we bound the growth of $|\mathbb{Q}_{c(\alpha)}:\mathbb{Q}(\alpha)|$ as a function of $k$.