Divisor 23909

To determine if any number is divisible by 23909, apply the "Jan Test":

1. If your number ("X") has 7 digits or more, separate the last (smallest) 6 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 7 digits, L = 0 and therefore R = X.
2. Multiply L by 4178 and subtract this from R.
3. Take that result and cross off its final digit (units). Take this new number and add 2391 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 23909. That is, your original number is divisible by 23909 if (and only if) Y is. Now that it's much smaller, with basic knowledge of your 23909-times tables, it should be easy to visually see if Y is divisible by 23909. If the Y is still much larger than 23909, the above process can be repeated until it does reduce to within small multiples of 23909.

Easy!