ELI5: Explain Like I'm 5

P-adic integers

Okay, so imagine you have some regular numbers like 1, 2, 3, and so on. These numbers are called the "integers." But what if instead of just having the regular integers, we have a whole new kind of numbers called "p-adic integers"?

P-adic integers have a special property that makes them different from regular integers. This property is related to a number called "p" (which is also called the "base" of the number system).

Let's say that p is a prime number, like 2, 3, 5, or 7. Prime numbers are special numbers that can only be divided by themselves and 1 without any remainder.

In the p-adic number system, we can write any regular integer as a series of digits, just like we do in our regular number system. But there is one big difference: in the p-adic system, the digits can be any number from 0 to p-1. For example, if p is 5, then the digits can be 0, 1, 2, 3, or 4.

The p-adic integers have a special way of counting and adding that is different from our regular counting and adding. In the regular number system, when we add two numbers, we carry over to the next digit if the result is bigger than the base. But in the p-adic system, we do something different. We carry over to the next digit if the result is smaller than 0! This might seem strange, but it's actually a very useful way of doing calculations.

Now let's talk about distance. In the regular number system, the distance between two numbers is just the absolute difference between them. But in the p-adic system, the distance is calculated differently. It's based on how many digits the two numbers have in common.

For example, let's say we have two p-adic numbers, 103 and 104, where p is 5. In the regular number system, the distance between them is 1, because the absolute difference is 1. But in the p-adic system, the distance is 1/25, because the two numbers have one digit in common (the last digit). So even though the difference between the two numbers is only 1, the distance is actually very small.

This might seem strange, but it's actually very useful in some areas of mathematics, like number theory and algebraic geometry. P-adic integers help us understand numbers in a different way and solve problems that are hard to solve using regular integers.

So, in summary, p-adic integers are a special kind of numbers that have a different way of counting, adding, and measuring distance. They have a base number p, which is a prime number, and their digits can be any number from 0 to p-1. P-adic integers are very helpful in mathematics and can help us solve some difficult problems.