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:x^2\equiv y^2 \pmod{n}\hbox{ where }x
ot\equiv \pm y\pmod{n}.

Such a relationship carries information useful in trying to factor the integer ''n'': finding a Congruence Of Squares modulo ''n'' is something sought after in Integer Factorization . There follows from it that

:
x^2\equiv y^2\pmod{n} \Rightarrow x^2-y^2\equiv 0\pmod{n} \Rightarrow
(x+y)(x-y)\equiv 0\pmod{n}.


This means that ''n'' divides (''x''+''y'')(''x''−''y'') but not (''x''+''y'') or (''x''−''y'') alone, so both (''x''+''y'') and(''x''−''y'') contain factors of ''n''. A simple s modulo ''n''.

Here is an example. Say ''n'' = 35. A Perfect Square close to 35 is 36, and, conveniently, 36 ≡ 1 (mod 35). Now 1 is also a perfect square. Thus we have our congruence:

:6^2\equiv 1^2\pmod{35}

with gcd(6 + 1, 35) = 7 and gcd(6 - 1, 35) = 5. These are the two non-trivial factors of 35.