TITLE: Integer roots of a symmetric polynomial
QUESTION [6 upvotes]: The question is very simple and I apologize for that, but I am not an expert of this kind of problem.
Given the polynomial
$$ P(x_1,\ldots,x_{2n})=x_1^2+\ldots+x_n^2-x_{n+1}^2-\ldots-x_{2n}^2,$$
I would like to know if there are non trivial integer roots $(y_1,\ldots, y_{2n})$ such that
$$y_1+\cdots+y_{n}=y_{n+1}+\cdots+ y_{2n}.$$
With non trivial I mean the ones like
$$y_1=y_{n+1},\ldots,y_{n}=y_{2n},$$
or their permutations.

REPLY [17 votes]: Fix large $N$ and consider all $n$-tuples $(x_1,\dots,x_n)\in \{1,\dots,N\}^n$. There are $N^n$ such $n$-tuples, at least $N^n/n!$ tuples modulo permutations, and for them the pairs $(x_1+\dots+x_n,x_1^2+\dots+x_n^2)$ take at most $n\cdot N\cdot n\cdot N^2=n^2N^3$ possible values. Thus by pigeonhole principle some value is obtained at least $N^{n-3}/(n^2\cdot n!)$ times. This is greater than 1 if $n>3$ and $N$ is chosen large enough.