Divisor 28279

To determine if any number is divisible by 28279, apply this test:

1. If your number ("X") has 10 digits or more, separate the last (smallest) 9 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 10 digits, L = 0 and therefore R = X.
2. Multiply L by 1998 and subtract this from R.
3. Take that result and cross off its final digit (units). Take this new number and add 2828 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 28279. That is, your original number is divisible by 28279 if (and only if) Y is. Now that it's much smaller, with basic knowledge of your 28279-times tables, it should be easy to visually see if Y is divisible by 28279. If the Y is still much larger than 28279, the above process can be repeated until it does reduce to within small multiples of 28279.

Easy!