Divisor 12821
- Prime Number:
- Yes!
- Divisibility test:
- The "Chris Pleasants Test"
- Test Discovered by:
- Matt Parker
- Date:
- 11/11/2024
The "Chris Pleasants Test" for Divisibility by 12821
To determine if any number is divisible by 12821, apply the "Chris Pleasants Test":
- If your number ("X") has 7 digits or more, separate the last (smallest) 6 digits from the rest. This makes two smaller numbers, call them Left and Right (note: don't add in trailing zeros to L). If there are fewer than 7 digits, L = 0 and therefore R = X.
- Multiply L by 38 and subtract this from R.
- Take that result and cross off its final digit (units). Take this new number and subtract 1282 times the digit you just crossed off. Call this final result "Y".
- Y will be much smaller than X, but we have preserved divisibility by 12821. That is, your original number is divisible by 12821 if (and only if) Y is. Now that it's much smaller, with basic knowledge of your 12821-times tables, it should be easy to visually see if Y is divisible by 12821. If the Y is still much larger than 12821, the above process can be repeated until it does reduce to within small multiples of 12821.
Easy!