Divisor 25981

To determine if any number is divisible by 25981, apply the "Kolbrandr 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 869 and subtract this from R.
3. Take that result and cross off its final digit (units). Take this new number and subtract 2598 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 25981. That is, your original number is divisible by 25981 if (and only if) Y is. Now that it's much smaller, with basic knowledge of your 25981-times tables, it should be easy to visually see if Y is divisible by 25981. If the Y is still much larger than 25981, the above process can be repeated until it does reduce to within small multiples of 25981.

Easy!