ELI5: Explain Like I'm 5

Abel's test

Abel's test is a fancy math trick that helps us figure out if a certain type of math problem is going to be difficult or easy to solve. Imagine you have a long list of numbers that you need to find the sum of. Sometimes this is easy to do - you just add up all the numbers and voila! You have your answer. But sometimes the list of numbers is super long and complicated, and it's hard to know if adding them all up is going to be easy or impossible.

That's where Abel's test comes in. Basically, it tells us that if the numbers on our list follow a certain pattern, we can use a different way of adding them up that will make the problem much simpler. This is kinda like breaking down a big puzzle into smaller, easier-to-solve pieces.

But what is this pattern we're looking for? Well, let's imagine that our list of numbers isn't just any old random list - instead, it follows a special rule. This rule says that as we go further down the list, each number gets smaller and smaller. This means that the first number might be 10, the second might be 5, the third might be 3, the fourth might be 2, and so on. As long as this pattern holds true, we can use Abel's test to help us solve the problem much more easily.

So what is this other way of adding up the numbers that is easier? It's called a "summation by parts" formula. This formula basically lets us break up the problem into smaller parts, and then add those parts up instead of adding up all the numbers on the list at once. It's like breaking down a huge sandwich into bite-sized pieces, so it's easier to eat.

In the case of our list of numbers that gets smaller and smaller, the summation by parts formula lets us add up the numbers in a way that cancels out a lot of the complexity. Instead of just adding up all the numbers on the list, we can subtract each number from the one before it, and then add up all those differences. This might sound confusing, but it's actually a lot simpler than adding up all the numbers on the list the normal way.

So that's Abel's test in a nutshell - a fancy math trick that helps us simplify complicated math problems by breaking them down into smaller parts. And all we have to do is look for that special pattern where the numbers get smaller and smaller as we go down the list. Cool, huh?