In: Advanced Math
Let p be an odd prime and a be any integer which is not congruent to 0 modulo p.
Prove that the congruence x 2 ≡ −a 2 (mod p) has solutions if and only if p ≡ 1 (mod 4).
Hint: Naturally, you may build your proof on the fact that the statement to be proved is valid for the case a = 1.