ELI5: Explain Like I'm 5

Cartan subalgebra

Okay kiddo, so imagine you have a big toy box with lots of different toys in it. But sometimes you only want to play with one type of toy, like just the trucks or just the dolls. That's kind of like a Cartan subalgebra.

In math, we use something called an algebra to help us solve problems. An algebra is like a big toy box full of different "toys" (which are actually numbers or functions). But sometimes we only want to work with certain types of "toys" (let's call them "trucks" for example). So we take out all the other "toys" from the algebra and just work with the "trucks."

In the same way, a Cartan subalgebra is a special type of "toy box" made up of just some of the "toys" in the original algebra. These "toys" are called Cartan elements. And just like how we only want to play with one type of toy, we only want to work with these special Cartan elements.

Now, why do we care about Cartan subalgebras? Well, these special toy boxes can help us better understand the larger algebra they come from. It's kind of like how if you only play with your truck toys, you can really learn a lot about how they move and work, and maybe even make them better!

So Cartan subalgebras are a tool mathematicians can use to understand and solve problems in algebra. And they're kind of like special toy boxes made up of just the really cool truck toys!