Divisor 28109

To determine if any number is divisible by 28109, apply this test:

1. If your number ("X") has 11 digits or more, separate the last (smallest) 10 digits from the rest. This makes two smaller numbers, call them L^eft and R^ight (note: don't add in trailing zeros to L). If there are fewer than 11 digits, L = 0 and therefore R = X.
2. Multiply L by 1622 and subtract this from R.
3. Take that result and cross off its final digit (units). Take this new number and add 2811 times the digit you just crossed off. Call this final result "Y".
4. Y will be much smaller than X, but we have preserved divisibility by 28109. That is, your original number is divisible by 28109 if (and only if) Y is. Now that it's much smaller, with basic knowledge of your 28109-times tables, it should be easy to visually see if Y is divisible by 28109. If the Y is still much larger than 28109, the above process can be repeated until it does reduce to within small multiples of 28109.

Easy!