Divisor 10609

To determine if any number is divisible by 10609, apply this 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 434 and subtract this from R.
3. Take that result and cross off its final digit (units). Take this new number and add 1061 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 10609. That is, your original number is divisible by 10609 if (and only if) Y is. Now that it's much smaller, with basic knowledge of your 10609-times tables, it should be easy to visually see if Y is divisible by 10609. If the Y is still much larger than 10609, the above process can be repeated until it does reduce to within small multiples of 10609.

Easy!