Divisor 28837

To determine if any number is divisible by 28837, apply this test:

1. If your number ("X") has 8 digits or more, separate the last (smallest) 7 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 8 digits, L = 0 and therefore R = X.
2. Multiply L by 6439 and subtract this from R.
3. Take that result and cross off its final digit (units). Take this new number and subtract 8651 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 28837. That is, your original number is divisible by 28837 if (and only if) Y is. Now that it's much smaller, with basic knowledge of your 28837-times tables, it should be easy to visually see if Y is divisible by 28837. If the Y is still much larger than 28837, the above process can be repeated until it does reduce to within small multiples of 28837.

Easy!