I have always been fascinated with “origins”: the origin of life on Earth, the origin of human beings, the origin of the Universe, and of course to fundamental particles. I reserve a sort of special place to a mathematical entity that some people regard as fundamental: prime numbers.

A

WhatIs.comprime numberis a wholenumbergreater than 1 whose only factors are 1 and itself. A factor is a wholenumbersthat can be divided evenly into anothernumber. The first fewprime numbersare 2, 3, 5, 7, 11, 13, 17, 19, 23 and 29.Numbersthat have more than two factors are called compositenumbers.

The oddity (pun intended) of these numbers attracted the curiosity of mathematicians for centuries. For example, today we still employ a simple method to tell whether a given number is prime is called the Sieve of Eratosthenes, who was a Greek mathematician known for having measured the Earth radius in 240 BC. To find all the prime numbers less than or equal to a given integer *n*:

- Create a list of consecutive integers from 2 through
*n*: (2, 3, 4, …,*n*). - Let
*p*= 2, the smallest prime number. - Enumerate the multiples of
*p*by counting in increments of*p*from 2*p*to*n*, and mark them in the list (these will be 2*p*, 3*p*, 4*p*, …; the*p*itself should not be marked). - Find the first number greater than
*p*in the list that is**not**marked. If there was no such number, stop. Otherwise, let*p*now equal this new number (which is the next prime), and repeat from step 3.

When the algorithm terminates, the numbers remaining not marked in the list are all the primes below *n*. In the following centuries, the interest in this class of numbers did not decline. In 1640 Pierre de Fermat stated the Little Theorem (not to be confused with Fermat’s Last Theorem):

If

pis a prime andais any integer not divisible byp, thena^{^(p − 1)}− 1 is divisible byp.

Leonhard Euler provided the first published proof in 1736, but Gottfired Leibniz had given basically the same proof in an unpublished manuscript fifty years before. On 7 June 1742, the German mathematician Christian Goldbach wrote a letter to Euler proposing the following conjecture:

Every even integer greater than 2 can be expressed as the sum of two prime numbers

To this date, Goldbach’s conjecture is still unproven, even though it has been demonstrated to hold true up to 3,325,581,707,333,960,528 by Tomás Oliveira e Silva.

Among the many strange properties of prime numbers is the Twin prime conjecture, i.e. the observation that many primes are in fact either 2 less or 2 more than another prime number such as 11 and 13.

### How many primes are there?

A related question is how many primes are there below a given number. The prime counting function π(*N*) is defined as the number of primes not greater than *N*. For example, π(10) = 4, since there are four primes less than or equal to ten (2, 3, 5 and 7). Intuitively, primes become less common as they become larger, but it is not obvious at what rate this occurs. This idea is formalized in the Prime Number Theorem. To start with, it was noted that for large enough *N*, the probability that a random integer not greater than *N* is prime is very close to 1 / log(*N*). So the theorem states that π(*N*) is equivalent to *N*/log(*N*) as *N* approaches infinity. However, over the years it was discovered that another function called Logarithmic Integral Function Li(*N*) was a better approximation to π(*N*).

By extrapolating the difference between π(*N*) and the other two functions, one can see that both converge towards π(*N*) at infinity, but Li(*N*) does that much faster.

### The Riemann Hypothesis

The German mathematician Bernhard Riemann worked mainly in the field of the geometry of non-Euclidean (or curved) spaces, in which he laid the basis for what will become the mathematics of Einstein’s General Relativity. Nonetheless, he’s also famous for a conjecture that to this date is still unproven (or is it?). As a matter of fact, his only paper on number theory is also probably the most important of all times in the field! In this work, Riemann discovered the **exact** formula for counting the number of primes below N, *i.e.* π(*N*) ), which can be expressed as follows:

The function ζ(s) is a complex function usually referred to as Riemann Zeta function. The Riemann hypothesis states that the zeros of ζ(s) (i.e. the numbers *s* for which ζ(s) = 0) are all either *negative even* numbers, or *complex* numbers with *real part* equal to 1/2. Riemann discovered that the exact expression of π(*N*) involves the roots of the zeta function. The Riemann conjecture is part of the 23 Hilbert’s problems and its proof is still up for grabs!

In the 1970s, some physicists such as Freeman Dyson and Hugh Lowell Montgomery speculated that the zeros of ζ(s) may play some role in quantum mechanics. The Primon gas is an example of a quantum field theory of a set of non-interacting particles whose states depend on prime numbers. It can be shown that the Hamiltonian function describing the energy of such a system is related to the Riemann Zeta function. In some sense, there may be a chance that Nature has found a way by itself to prove the Riemann Hypothesis!

### The Prime Number Cross

Many years ago I have spent a few days in a hospital. I asked my parents to buy me a book called *God’s Secret Formula: The Deciphering of the Riddle of the Universe and the Prime Number Code* by the German Chemist Peter Plichta. I’m not sure I would recommend reading it, but it certainly contains something that kept bugging me over the years. That thing is a way to organize natural numbers in concentric rings of 24 numbers each. I’m not sure where he got the inspiration from, my best guess is that he started somehow from the distribution of twin primes. Here’s what it looks like for numbers up to 96:

The overall idea is that, apart from two exception on the first ring (integers 2 and 3), all the other prime numbers lay on 8 radii, but not all numbers that appear on those lines are primes. Also, the radius stemming from 1 contains the squares of prime numbers larger than 5, *e.g.* 5*5=25, 7*7=49, 11*11=121, 13*13=169, 17*17=289 and so on…

The authors speculates that this distribution resembles the Maltese Cross, an observation that might indicate that the Knights Hospitaller might have known that since a very long time. Furthermore, he insists that this geometrical arrangement may have some deeper implication in nuclear physics, a claim that I believe is utter nonsense.

In any event, I wrote a python code based on numpy and matplotlib to create this kind of figures. You can grab the code from my GitHub repository.

### A Deeper Pattern?

To conclude, the study of prime numbers is very fascinating, but it’s unclear to me whether cracking the code of the primes will really give us a clearer understanding of mathematics, or if it will remain confined to the realm of mere curiosities. Not being a mathematician myself, I find hard to grasp whether the Prime Number Cross can be used to further investigate (or even prove!) some of the unsolved conjectures discussed above. In any event, feel free to play around with the python code and try your chance to win the next Fields Medal.

Dear Riccardo,

I have a simple explanation for this prime cross.

The red ‘prime lines’ are simply the lines that continue on from the numbers between 1 and 24 which have no common factor with 24.

If a number Y has a common factor A with 24 such that Y=A*C and 24=A*B, then we may write all numbers in the line extending from that number as Y+24*N=A*C+A*B*N=A*(C+B*N). All the numbers in that line are therefore multiples of A and cannot be primes, except the case where N=0 and C=1, then the first number on the line may still be prime.

Therefore 2, 3, 4, 6, 8, 9, 10, 12, 14, 15, 16, 18, 20, 21, 22 are all excluded from forming a prime line, which leaves 1, 5, 7, 11, 13, 17, 19, 23, to form the prime lines, as the only places where primes may occur. Also as 2,3,4,6,8,12 all divide 24 they may be primes on a non-prime line. However only 2 and 3 here actually are primes, which explains why they are the only primes on a non-prime line.

I hope this elucidates things and that I verified and answered the crux of your question.

Kind regards,

Ethan

I was going to leave a similar comment. The “8 radii” of base 24 containing all primes is just stupidly obvious, of course they do. 12 of those “radii” are even and another 4 are always multiples of 3. Leaving you with 8 possible other streams. It’s like taking the integers, removing all the even numbers, and then removing every 3rd number after that is left (all the multiples of 3 that are not multiples or 6 or 3.2 and hence already removed)…

So yeah, there’s nothing special about 24 here, except that you have to go up to to 24 in order to establish the pattern of 3’s that you are removing (9+24x, 15+24x, 21+24x).

You can do the same thing with rings of 6, you’ll find all of the primes on 2 radii…it just won’t look as pseudo scientifically significant