ELI5: Explain Like I'm 5

Ramanujan summation

Okay kiddo, have you ever done addition before? Great! Ramanujan summation is a way of adding up numbers, but unlike the usual method, it works even when the answer is infinity (which means really, really big!).

Let me give you an example. If I asked you to add up all the numbers from 1 to infinity, you might say, "That's impossible! There are too many numbers!" But Ramanujan's clever trick helps us to find an answer anyway.

First, he noticed that if we add up the numbers 1, 2, 3, 4... and so on, the answer gets bigger and bigger, without ever stopping. But if we add the same numbers, but each one squared (1 x 1, 2 x 2, 3 x 3, etc.), the answer gets bigger too, but it grows more slowly.

Now, Ramanujan's trick is to add up both of these sequences and take the average. In other words, we add up 1, 2, 3, 4... and so on, and also add up 1 x 1, 2 x 2, 3 x 3, and so on... and then we divide the sum of the two by two.

This might sound a little strange to you, but what Ramanujan realized is that when we do this, we get a very good approximation of what the sum of all the numbers from 1 to infinity would be, even though we can never actually add up all those numbers!

So, in summary, Ramanujan summation is a fancy math trick that helps us add up even really big numbers, by cleverly averaging two different ways of adding them up. Cool, huh?