Divisor 3821

To determine if any number is divisible by 3821, apply the "Gavin Stark Test":

1. If your number ("X") has 8 digits or more, separate the last (smallest) 7 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 8 digits, L = 0 and therefore R = X.
2. Multiply L by 443 and add to R.
3. Take that result and cross off its final digit (units). Take this new number and subtract 382 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 3821. That is, your original number is divisible by 3821 if (and only if) Y is. Now that it's much smaller, with basic knowledge of your 3821-times tables, it should be easy to visually see if Y is divisible by 3821. If the Y is still much larger than 3821, the above process can be repeated until it does reduce to within small multiples of 3821.

Easy!