ELI5: Explain Like I'm 5

Riesz–Markov–Kakutani representation theorem

Okay, so imagine you are trying to describe a cake recipe to your friend, but your friend doesn't know how to read or write. What do you do? Well, you could show them a picture of the cake or use other ways to help them understand the recipe, right?

Similarly, the Riesz-Markov-Kakutani Representation Theorem helps people describe certain mathematical things in a way that makes it easier to understand. In math, we often have things called linear functionals, which are basically like recipes but for numbers. Instead of telling you how to make a cake, they tell you how to work with numbers. These functionals might be something like "multiply a number by 2" or "add 5 to a number."

But just like your friend who can't read or write, sometimes it's hard for mathematicians to describe these functionals in a way that's easy to understand. That's where the Riesz-Markov-Kakutani Representation Theorem comes in! It's kind of like a set of pictures or tools we can use to make these functionals more concrete and understandable.

The theorem says that for certain kinds of functionals, we can describe them using something called a "measure." This measure is a way of assigning a number to different sets of numbers, kind of like how you assign ingredients to different steps in a recipe. And just like a recipe tells you exactly how much of each ingredient to use, the measure tells you exactly how to work with the numbers in the set.

So, to sum it up: The Riesz-Markov-Kakutani Representation Theorem helps mathematicians describe certain functionals using a "measure" that makes them easier to understand and work with. It's kind of like using pictures to describe a recipe to a friend who can't read!