If p is prime where p = x^2 + y^2 and p ≡ +/- 1 mod 10, then 5 | xy.
So far, I know that if p can be written as a sum of two squares, then p ≡ 1 mod 4, so x^2 + y^2 ≡ 1 mod 4, and x^2 + y^2 ≡ +/- 1 mod 10. Where do I go from here?
Okay, so if p ≡ +/- 1 mod 10, then p ≡ 1 or 4 mod 5. The quadratic residues mod 5 are 1 and 4, so any n^2 is 0, 1, or 4 mod 5. Then one of x^2 or y^2 is 0 mod 5 and the other is 1 or 4 (depending on whether p is 1 or -1 mod 10). That means either x or y is 0 mod 5, so xy is 0 mod 5. Is that it?