Square root (mod p)

The Square root (mod p) of n is a number (or more properly, residue class), a, that solves the equation
 * a2=n (mod p).

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 * Main article: Square root (mod p)

If n is a quadratic residue (mod p), which just means if n is the square of some number (mod p), then the Shanks-Tonelli algorithm can find the number whose square is n.