Sums of integers and sums of their squares

Suppose a positive integer $n$ is written as a sum of squares of $m$ integers. What can one say about the value $T$ of the sum of these $m$ integers itself? Which $T$ can be obtained if one considers all possible representations of $n$ as a sum of squares of $m$ integers? Denoting this set of all possible $T$ by ${\mathcal{S}}_m(n)$, Goldmakher and Pollack have given a simple characterization of ${\mathcal{S}}_4(n)$ using elementary arguments. Their result can be reinterpreted in terms of Mordell’s theory of representations of binary integral quadratic forms as sums of squares of integral linear forms. Based on this approach, we characterize ${\mathcal{S}}_m(n)$ for all $m\le 11$ and provide a few partial results for arbitrary $m$. We also show how Mordell’s results can be used to study variations of the original problem where the sum of the integers is replaced by a linear form in these integers. In this way, we recover and generalize earlier results by Z.W. Sun et. al…