If 'p' is a prime, then the congruence x2 + 1 ≡ 0 (mod p) has a solution if and only if:

1
p = 1 or p ≡ 1 (mod 3)
2
p ≠ 1 or p ≡ 1 (mod 2)
3
p = 2 or p ≡ 1 (mod 4)
4
p = -2 or p ≡ 2 (mod 3)

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