fermatdef.c:

/* We need standard IO */
#include <stdio.h>

/* Factor a NON-EVEN number by Fermat's method. */
/* Generating relation for this method is n = pq = (y-x)(y+x) */
/*  Dn wrt X = -(2x+1)                                        */
/*  Dn wrt Y =   2y+1                                         */
main () {
  long n, p, q, x, y, t;

  printf ("Factor a positive odd number by Fermat's method\n");
  printf ("N = \n"); scanf ("%ld", &n);
  printf ("Factoring %ld\n", n);
  if (n<1) { printf ("N must be greater than zero\n"); return 1;}
  if (n%2==0) { printf ("N must be odd\n"); return 1;}

  x = 0; y = 0; t = y*y - x*x - n;

  while (t != 0) if (t<0) { t+=2*y+1; y ++; }
                 else     { t-=2*x+1; x ++; }

  q = y + x; p = y - x; 
  printf ("%ld*%ld=%ld\n", p, q, n);
}

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© Copyright 2003 by John Halleck, All Rights Reserved.
This snapshot was last modified on January 12th, 2009
And the underlying file was last modified on December 12th, 2005