Divisor 587

To determine if any number is divisible by 587, apply this test:

1. If your number ("X") has 5 digits or more, separate the last (smallest) 4 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 5 digits, L = 0 and therefore R = X.
2. Multiply L by 21 and add to R.
3. Take that result and cross off its final digit (units). Take this new number and subtract 176 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 587. That is, your original number is divisible by 587 if (and only if) Y is. Now that it's much smaller, with basic knowledge of your 587-times tables, it should be easy to visually see if Y is divisible by 587. If the Y is still much larger than 587, the above process can be repeated until it does reduce to within small multiples of 587.

Easy!