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nCr (n,r) {
  res = 1;
...
  }
  return res;
}

X = 2, series = (y, y+1);  one is div. by 2
X = 3, series = (y, y+1, y+2);  one is div. by 3
...
(17/1) (16/4) (15/(3*5)) (14/2)     (13/1)
(17/1) (16/4) (15/(3*5)) (14/2)     (13/1) (12/6)
(17/1) (16/4) (15/(3*5)) (14/(2*7)) (13/1) (12/6) (11/1)

  r
-----
...
 | |   ---------
 | |       i
 i=1