The digits of every of every whole number which a multiple of nine can be reduced to nine: 2142 > 2+1+4+2 = 9 92097 > 9+2+0+9+7 = 27 > 2+7 = 9 58841001 > 5+8+8+4+1+0+0+1 = 27 > 2+7 = 9