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242 Chapter 5 EXPONENTIAL FACTORING ALGORITHMS<br />

5.6.2 Factoring with quadratic form representations<br />

An old factoring strategy going back to Fermat is to try to represent n in two<br />

intrinsically different ways by the quadratic form (1, 0, 1). That is, one tries<br />

to find two different ways to write n as a sum of two squares. For example,<br />

we have 65 = 8 2 +1 2 =7 2 +4 2 .Thenthegcdof(8· 4 − 1 · 7) and 65 is the<br />

proper factor 5. In general, if<br />

n = x 2 1 + y 2 1 = x 2 2 + y 2 2, x1 ≥ y1 ≥ 0, x2 ≥ y2 ≥ 0, x1 >x2,<br />

then 1 < gcd(x1y2−y1x2,n)

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