X = 2, series = (y, y+1);  one is div. by 2
X = 3, series = (y, y+1, y+2);  one is div. by 3
...

in nCr(17,7),
  n is of the series (17,16,15,14,13,12,11)
  i is of the series ( 1, 2, 3, 4, 5, 6, 7)
(17/1)
(17/1) (16/2)
(17/1) (16/2) (15/3)
(17/1) (16/4) (15/3)     (14/2)
(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)