John D. Cook 2/14/2026

Finding a square root of -1 mod p

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This technical article details the mathematical condition (p ≡ 1 mod 4) for the existence of a square root of -1 modulo an odd prime p. It explains the algorithm to find such a root using a quadratic non-residue c and provides a concrete Python implementation for the large prime p = 2²⁵⁵ - 19.

Finding a square root of -1 mod p

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