Unexpected Factorial

When you use an exclamation mark with a number, you’re actually implying it’s not a normal number any more. It’s a factorial!

Ok, so how does this work?

5!=1×2×3×4×5=120

6!=720

These numbers get really large. For example:

15!≈1.3×10^12

So, next time you see a headline with 2000! in it, you’ll know what to expect.

There are also double factorials (n!!) and iterated factorials (n!)!, and they aren’t the same thing. Just add more exclamation marks and you get multifactorial. Check wikipedia to see how spicy it gets.