Divisor 353

To determine if any number is divisible by 353, 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 42 and add to R.
3. Take that result and cross off its final digit (units). Take this new number and add 106 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 353. That is, your original number is divisible by 353 if (and only if) Y is. Now that it's much smaller, with basic knowledge of your 353-times tables, it should be easy to visually see if Y is divisible by 353. If the Y is still much larger than 353, the above process can be repeated until it does reduce to within small multiples of 353.

Easy!