Representing multiples of $m$ in real quadratic fields as sums of squares

Abstract

We study real quadratic fields $\mathbb{Q}(\sqrt{D})$ such that, for a given rational integer $m$, all $m$-multiples of totally positive integers are sums of squares. We prove quite sharp necessary and sufficient conditions for this to happen. Further, we give a fast algorithm that solves this question for specific $m$, $D$ and we give complete results for $m\le 5000$.

Changed constants in Thm 2, more detailed proof. Corrected typos and updated references