Example:
for $n = 2$, $k = 2$, the set $\{ x | -2^2+1\leq x\leq 2^2\}$ can be split into {-1, 1, 2, 4} and {-3, -2, 0, 3}, as $(-1)^2+1^2+2^2+4^2=(-3)^2+(-2)^2+0^2+3^2$
I wonder if this problem has its own name and some related research. I would appreciate it if anyone could give some useful information. Thanks a lot.