Solutions for $x^2+y^2=2007$ and reference request.
$$2007\equiv3\equiv-1\pmod4$$
But $\displaystyle a\equiv0,\pm1,2\pmod4\implies a^2\equiv0,1\pmod4$
So, what are possible values of $\displaystyle x^2+y^2\pmod 4$
$$2007\equiv3\equiv-1\pmod4$$
But $\displaystyle a\equiv0,\pm1,2\pmod4\implies a^2\equiv0,1\pmod4$
So, what are possible values of $\displaystyle x^2+y^2\pmod 4$