Divisor 19709

To determine if any number is divisible by 19709, apply the "Guy Sperry Test":

1. If your number ("X") has 6 digits or more, separate the last (smallest) 5 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 6 digits, L = 0 and therefore R = X.
2. Multiply L by 1455 and add to R.
3. Take that result and cross off its final digit (units). Take this new number and add 1971 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 19709. That is, your original number is divisible by 19709 if (and only if) Y is. Now that it's much smaller, with basic knowledge of your 19709-times tables, it should be easy to visually see if Y is divisible by 19709. If the Y is still much larger than 19709, the above process can be repeated until it does reduce to within small multiples of 19709.

Easy!