ELI5: Explain Like I'm 5

Hilbert series and Hilbert polynomial

Hilbert series and Hilbert polynomial are math things that help us understand certain shapes, called "varieties." Think of varieties like different kinds of gumdrops – some are big and some are small, some are round and some are square, but they're all gumdrops.

First of all, what's a polynomial? A polynomial is like a bunch of terms that you add together to make a big math problem. For example, 2x^2 + 5x + 1 is a polynomial because it has terms 2x^2, 5x, and 1 that all get added together.

Now, let's talk about Hilbert polynomial. You know how when you count candy, you might count them by colors or by shapes? Hilbert polynomial helps us count how many different shapes there are in a variety by looking at how many candy pieces have the same shape. It does this by using a polynomial equation that tells us how many pieces of each shape there are.

The Hilbert polynomial is like a recipe that we can use to figure out how many candy pieces of each shape there are. Just like a recipe for making cupcakes tells you how much flour, sugar, and eggs you need, the Hilbert polynomial tells us how many pieces of gumdrops are in each shape.

Now, let's talk about Hilbert series. The Hilbert series is like a way of describing the Hilbert polynomial for really big varieties. Imagine if you had a ton of gumdrops that all looked different – it would be really hard to count how many of each shape there were! The Hilbert series helps us with that. It's like a shortcut to the Hilbert polynomial that we can use for really big varieties.

In other words, Hilbert series is a way of summing up all the different shapes of gumdrops in a variety and giving us a big equation that tells us how many there are of each shape.

So, in summary, Hilbert polynomial helps us count how many pieces of each shape there are in a variety, while Hilbert series is like a shortcut to the Hilbert polynomial for really big varieties. And that's a basic explanation of Hilbert series and polynomial for a five-year-old!