Question 154
Find all positive integers $k$ such that
$$
\frac{m+n}{m^2-kmn+n^2}
$$
is not a composite number for any positive integers $m$ and $n$. (Borna Vukorepa)
$$
\frac{m+n}{m^2-kmn+n^2}
$$
is not a composite number for any positive integers $m$ and $n$. (Borna Vukorepa)