Twenty-Eight

28 is a perfect number. In mathematics, a perfect number is a number that is equal to the sum of all of its factors. Think back to your arithmetic days: what are the factors of 28?

1, 2, 4, 7, 14

That’s right. And if you add them all together?

1 + 2 + 4 + 7 + 14 = 28

Very few numbers have this property. Ancient Greek mathematician Euclid knew about the first four: 6, 28, 496, and 8128. It wasn’t until the Middle Ages that the next one was discovered: 33,550,336. And from that point, they just keep getting bigger and bigger. So far, 51 perfect numbers have been discovered; the highest one has over 49 million digits in it. I can’t imagine just how much time it took a computer to crunch numbers to get to that. Or how much time it took to program said computer. (Which is a better use of computer time? Working that out, or watching cat videos on Facebook? I’m honestly not sure.)

I recently learned that 28 also has some other cool properties. Check this out. Do you remember prime numbers? They’re numbers which have only two factors: themselves and 1. Numbers like 7 and 11. There’s no whole number that can evenly divide into either 7. (Except 7 itself, and 1.) Numbers that are not prime are called composite numbers. Numbers like 6 and 12. Six is 2 x 3. Twelve is 2 x 6 or 3 x 4. Composite numbers have more factors than just themselves and 1. Okay, now here’s the cool thing. Add up the first five prime numbers.

2 + 3 + 5 + 7 + 11 = 28

Now add up the first five non-prime numbers. (We include 1, because it’s not prime, although it’s not exactly composite either.)

1 + 4 + 6 + 8 + 9 = 28

That’s cool. The first five primes and the first five non-primes add up to 28. Is there a reason for that? Like a fundamental axiom of mathematics that makes this obvious? I don’t know. Does it mean that 28 is some sort of divine number? Probably not. But I’ve always enjoyed math trivia like this. It’s like 28 is the special “adding number.” Add whatever you want together — at some point, it will equal 28.

Including all the whole numbers themselves. Check this out:

1 + 2 + 3 + 4 + 5 + 6 + 7 = 28

This is getting kind of creepy, actually. I’m not hanging out with 28 anymore. This number is like some kind of snake, you know… ahem … an adder?

I’ll get my coat.