Divisor 21683

To determine if any number is divisible by 21683, apply the "iain 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 1723 and add to R.
3. Take that result and cross off its final digit (units). Take this new number and add 6505 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 21683. That is, your original number is divisible by 21683 if (and only if) Y is. Now that it's much smaller, with basic knowledge of your 21683-times tables, it should be easy to visually see if Y is divisible by 21683. If the Y is still much larger than 21683, the above process can be repeated until it does reduce to within small multiples of 21683.

Easy!