Divisor 13789

To determine if any number is divisible by 13789, apply the "ReX Test":

1. If your number ("X") has 9 digits or more, separate the last (smallest) 8 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 9 digits, L = 0 and therefore R = X.
2. Multiply L by 2172 and add to R.
3. Take that result and cross off its final digit (units). Take this new number and add 1379 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 13789. That is, your original number is divisible by 13789 if (and only if) Y is. Now that it's much smaller, with basic knowledge of your 13789-times tables, it should be easy to visually see if Y is divisible by 13789. If the Y is still much larger than 13789, the above process can be repeated until it does reduce to within small multiples of 13789.

Easy!