Which property must a prime ppp satisfy to be written as x2+y2x^2 + y^2x2+y2?
p=2p = 2p=2 or p≡1(mod4)p \equiv 1 \pmod{4}p≡1(mod4)
p≡3(mod4)p \equiv 3 \pmod{4}p≡3(mod4)
ppp is odd
ppp is a perfect square