0
It is ancient knowledge that every non-negative integer can be rewritten as the sum of four squared integers. For example the number 1 can be expressed as \$0^2+0^2+0^2+1^2\$. Or, in general, for any integer n, there exists integers a,b,c,d such that
$$n = a^2+b^2+c^2+d^2$$
Joseph-Louis Lagrang...