### What is, in simple terms, a Beurling prime?

Simple terms of a Beurling Prime. I read the book of Leveque’s and I do not understand about the ideas of beurling primes.
What are primes?

We start with the natural numbers, naturally. We observe that they can be multiplied, and therefore some of them can be broken down into smaller pieces, as in [Math Processing Error], but some cannot: those are the primes. We distill the primes from the natural numbers of Mathematics, as those of them that are not a (non-trivial) product.

But we could also go the other way around. We can imagine having the “Lego pieces”, the primes, first: a list of numbers starting [Math Processing Error]. From these Lego pieces we can then form structures (numbers) by putting the pieces (primes) together, every which way. We pick the pieces [Math Processing Error] again and [Math Processing Error], multiply them, we get [Math Processing Error]. We pick three [Math Processing Error]’s, a dash of [Math Processing Error] and a packet of four [Math Processing Error]’s, we get [Math Processing Error]. And so on. #### Why would we want to do that?

We are interested in the relationship between the pieces and the structures they make up. If we start with a given set of structures, there’s literally only one way to cut them up into pieces: you break and separate things until you’re down to the indecomposable Lego blocks. But going the other way, we can imagine other Lego pieces, arbitrary pieces, whatever pieces, being pieced together to make structures. The process of forming structures from pieces is relatively simple: you just stick the pieces together. What is, in simple terms, a Beurling prime.

You can easily imagine different sets of Lego blocks from which to build other structures, while in the other direction, if you take the structures as a given, there isn’t a sense in which you can look for some “other” sets of basic building blocks.

And so it is with numbers. Given the natural numbers and the operation of multiplication, there’s only one way to cut them up into irreducible elements, which are the primes. You could try to introduce other numbers as ingredients, for instance writing [Math Processing Error], but then there’s too much freedom and it’s not clear what the basic, irreducible, indecomposable blocks might be.

#### When do you stop smashing?

On the other hand, the process of forming the natural numbers from the primes by multiplication is so simple that you can easily imagine doing it while starting with a completely different set of “primes”, of basic blocks. For example, imagine the “primes” are just [Math Processing Error] and that’s it. Once again, you can multiply them every which way, and you’ll get numbers like [Math Processing Error] or [Math Processing Error], but you won’t get [Math Processing Error] or [Math Processing Error]. What is, in simple terms, a Beurling prime.

Or you could even start with, I don’t know, [Math Processing Error], [Math Processing Error], [Math Processing Error], [Math Processing Error], and maybe a few more (or infinitely many more), and call these “atoms” (or “primes”), and see what you get by multiplying them together. We don’t need to stick with natural numbers: we can declare any set of real numbers “The Primes”, and see what comes out the machine when we piece them together by multiplying them in every possible combination.

#### But once again: why?

Beurling was interested in the Prime Number Theorem (PNT), which says that the number of primes up to [Math Processing Error] is very close to [Math Processing Error]. (This is the natural logarithm, but it’s traditional in analytic number theory to use [Math Processing Error] rather than [Math Processing Error], so I’ll do that.) Beurling wanted to know: how robust is that? If we take “other primes” of comparable density, do we get “other natural numbers” also of comparable density? Is the PNT a delicate, fragile beast that completely breaks down once you jigger its primes a little, or is it stable, or perhaps universal…?

Perhaps it’s always the case that if you start with any set of “primes” of whatever density, you get a set of “natural numbers” whose density bears the same relationship to the prime density as [Math Processing Error] does to [Math Processing Error]?

As we mentioned before, one of the ways to ask “why” something is true is to generalize it, or vary it, or fiddle with it. If you have a singular, one-of-a-kind phenomenon, it’s hard to know what “why” even means. Why does this happen? Well who knows, it only happened once, when conditions were just like that, and it’s like nothing else that we’ve seen elsewhere or at another time, and we don’t even have other instances when it could happen but didn’t.

So there’s no “why” here. On the other hand, if we find a million similar but somewhat different instances,. We can start seeing that the thing happens with some of them but not all of them,. And all of a sudden we have fertile ground for causal answers:

Aah, you see, when you have flarg then you do get kerflikkity, but when you don’t have flarg then kerflikkity flies out the window. Clearly, then, it is flarg which causes kerflikkity. We have a good candidate for “why”.

##### That’s exactly what Beurling sought, and that’s exactly what he found.

“Beurling primes” aren’t any special type of primes:

they are literally arbitrarily chosen increasing sequences of real numbers, which we choose to temporarily call “The Primes”,. And from them we form “The Numbers” by arbitrarily multiplying. “The Primes” together, and we compare the density of The Primes to the density of The Numbers.