Mathblogging

[afgncaapvaljean]

I've got a strange hobby. You can do it in the car, you can do it while watching TV, you can do it in the rain, you can do it on a train, you can do it here or there, you can do it anywhere.

I look for numbers around me, and determine if they're prime.

This is my fun.

Let's say I've got a number, 5731

So. 2 is not a factor. It's not even.
The digits don't add up to a multiple of 3, so 3 is not a factor.
It doesn't end in 5 or 0, so 5 is not a factor.
Is 7 a factor? Well, subtract 21. 5710. Divide by 10, 571. Subtract 21. 550. 550 is not divisible by 7.
Is 11 a factor? 5731-11=5720, which is divisible by 11. 5731 is not prime. Boo hoo.

I do this because it keeps my mind busy. It's simple enough to calculate. Basically, you go through the primes up through the square root of the number, and see if it divides them. Subtraction of multiple of the desired prime and division of non-multiples preserves the quality of "has p as a factor" so you can turn it into much easier mental math.

Incidentally, 11*521=5731.

I am not sure why I am entertained by this.