ELI5: Explain Like I'm 5

Lebesgue measurable set

Imagine you have a big container full of your favorite toys. You want to be able to count how many toys you have, but you can't just count them one by one because there are too many. So you decide to group them by size – all the small toys in one pile, medium toys in another, and large toys in a third. This makes it easier to count how many you have of each size.

In math, sometimes we have really big sets of numbers that we want to measure or count. But just like with your toys, it's not always practical to count them one by one. That's where Lebesgue measurable sets come in.

A Lebesgue measurable set is like a big container full of numbers. But instead of toys, these numbers are arranged in a certain way that makes them easier to count. For example, imagine you have a line of numbers that goes on forever in both directions. You want to find out how many of those numbers are between 0 and 1. Instead of counting each number, you can use Lebesgue measure to "measure" the set of numbers between 0 and 1.

Lebesgue measure is like a ruler that we use to measure sets of numbers. But instead of just measuring length, it can measure any kind of "volume" or "size" of a set. For example, if you have a 3D shape like a cube or a sphere, you can use Lebesgue measure to find its volume.

To make a set Lebesgue measurable, we need to follow certain rules. First, the set needs to be well-behaved – it can't have any weird or funky shapes that don't make sense. Second, we need to be able to define the Lebesgue measure of the set in a consistent way. This means that no matter how we slice or cut the set, we get the same answer for the Lebesgue measure.

So, in summary, Lebesgue measurable sets are like containers full of numbers that are arranged in a certain way to make them easier to count, and we use Lebesgue measure as a ruler to measure the size or volume of these sets.