You could say 1 plus 2 plus 3 plus, and you go all the way to plus 9 plus 10. And I clearly could have even written this whole thing out. But you can imagine it becomes a lot harder if you wanted to find the sum of the first 100 numbers. So that would be 1 plus 2 plus 3 plus, and you would go all the way to 99 plus 100. So mathematicians said, well, let's find some notation instead of having to do this dot, dot, dot thing, which you will see sometimes done, so that we can more cleanly express these types of sums. And that's where sigma notation comes from. So this sum up here right over here, this first one, it could be represented as sigma.