February 27, 2007


Thi s is an innagural post for a new category, and a weird hobby of mine: factoring.

Factoring is simply taking a random or pseudo-random 3,4 or even 5 digit number, and determining, in your head, all the prime factors thereof. I usually do 4 digit numbers, or 3 if I'm tired. I rarely do 5 digit numbers, but will, on occasion.

There are some tricks to it; for some numbers, there are easily rules you can apply to test if a number is divisible by it. For example, 3 (and 9): if you add up the digits and the sum is divisible by 3 (or 9), then the original number is divisible by 3 (or 9). For example, 252 gives you 2+5+2=9, so 252 is divisible by 9 (2, 2, 3, 3, 7). But 1178 yields 17 (1+1+7+8), so it's not.

There are other tricks as well, and I'll mention them in later posts (explaining why 221 is one of my favorite numbers).

What I'll do is I'll play Sudoku on my palm, then take the last digit filled in, and follow the diagonal from there towards the center, and take those 4 digits and factor them.

It's a fun thing to do while driving, too. Watch the last 3 digits of your odometer, and see how many times you can factor them on the way home (get a number, 37.9, and factor 379 (prime); read the odometer again, get another number, 41.8, and factor 418 (2, 11, and 19), etc).

Fun thing about it is, every number's different. You might find with one number that you go up to 71 or so and discover it's prime, where another number closee by is divisible by 3, 3, 17, and 31.

Give it a try! It's not as hard as it sounds.


Posted by mlv at 05:12 PM | Comments (0) | TrackBack