ELI5: Explain Like I'm 5

Hahn–Banach theorem

The Hahn-Banach Theorem is a fancy math thing that helps us figure out how to extend functions from small spaces to bigger spaces. It's like if you have a toy car and you want to make it go farther, the Hahn-Banach theorem helps you figure out how to make the car go a longer distance.

Let's pretend you have two boxes, one big and one small. Inside the small box, there are some toys (let's say a teddy bear, a car, and a ball). We want to make a function that takes each toy and tells us how heavy it is. But we want to do this for all the toys in both the small box and the big box.

The Hahn-Banach Theorem says that we can figure out a way to make this function work for all the toys in both boxes, even though it was originally only defined for the toys in the small box. We can make it go farther!

Now, how does it work? Think of a grown-up helping you reach the top shelf. They can reach higher than you can, so they can help you get the things you need. In the same way, the Hahn-Banach Theorem uses a special trick to help us extend functions. It's like the grown-up helps us reach the toys in the big box by using their arms to make our arms longer.

In math terms, the Hahn-Banach Theorem tells us that if we have a function defined on a small space, we can always find a way to extend it to the bigger space without changing the values of the function on the small space. It's like we can make the toy weighing function work for all the toys in the big box just by using the values we already know for the toys in the small box.

So, the Hahn-Banach Theorem is like a superhero that helps us extend functions to bigger spaces. We can imagine it as a grown-up that helps us reach the top shelf or as making our toy weighing function work for all the toys in both boxes. It's a really useful tool in math!