ELI5: Explain Like I'm 5

Euler product

Imagine you have a really big number, let's call it "n." This number can be thought of as the product of all its prime factors (the numbers it can be divided by evenly without leaving a remainder). So if n = 12, it can be written as 2 x 2 x 3.

Now, the Euler product is a fancy-sounding way of expressing this idea for all numbers. It's a formula that allows us to identify all the prime numbers involved in any particular number, even if we don't know what the number is.

The formula goes like this: For any integer n, the Euler product is the infinite product of (1 - 1/p), where p are all the prime numbers that divide n evenly. This may sound a bit confusing, so let's break it down.

Let's use the example of n = 10. The prime factors of 10 are 2 and 5. So the Euler product for 10 would be:

(1 - 1/2) x (1 - 1/5) = (1/2) x (4/5) = 2/5

So the Euler product for 10 is 2/5. This means that 2/5 of all the positive integers less than or equal to 10 are relatively prime to 10 (they don't share any common factors other than 1).

The Euler product is a really powerful formula that has many applications in number theory and other branches of mathematics. But for now, just remember that it's a way of breaking down any number into its prime factors and using those factors to find out how many numbers are relatively prime to it.