because the atoms of arithmetic, prime numbers have at all times occupied a particular place on the quantity line. Now, Jared Duker Lichtman, a 26-year-old graduate scholar on the College of Oxford, has resolved a well known conjecture, establishing one other aspect of what makes the primes particular—and, in some sense, even optimum. “It provides you a bigger context to see in what methods the primes are distinctive, and in what methods they relate to the bigger universe of units of numbers,” he mentioned.

The conjecture offers with primitive units—sequences by which no quantity divides every other. Since every prime quantity can solely be divided by 1 and itself, the set of all prime numbers is one instance of a primitive set. So is the set of all numbers which have precisely two or three or 100 prime elements.

Primitive units have been launched by the mathematician Paul Erdős within the Nineteen Thirties. On the time, they have been merely a instrument that made it simpler for him to show one thing a few sure class of numbers (referred to as good numbers) with roots in historical Greece. However they shortly turned objects of curiosity in their very own proper—ones that Erdős would return to again and again all through his profession.

That’s as a result of, although their definition is simple sufficient, primitive units turned out to be unusual beasts certainly. That strangeness could possibly be captured by merely asking how large a primitive set can get. Contemplate the set of all integers as much as 1,000. All of the numbers from 501 to 1,000—half of the set—type a primitive set, as no quantity is divisible by every other. On this method, primitive units would possibly comprise a hefty chunk of the quantity line. However different primitive units, just like the sequence of all primes, are extremely sparse. “It tells you that primitive units are actually a really broad class that’s arduous to get your palms on straight,” Lichtman mentioned.

To seize attention-grabbing properties of units, mathematicians examine numerous notions of dimension. For instance, relatively than counting what number of numbers are in a set, they may do the next: For each quantity *n* within the set, plug it into the expression 1/(*n* log *n*), then add up all the outcomes. The scale of the set {2, 3, 55}, as an illustration, turns into 1/(2 log 2) + 1/(3 log 3) + 1/(55 log 55).

Erdős discovered that for any primitive set, together with infinite ones, that sum—the “Erdős sum”—is at all times finite. It doesn’t matter what a primitive set would possibly appear to be, its Erdős sum will at all times be lower than or equal to some quantity. And so whereas that sum “seems to be, not less than on the face of it, utterly alien and obscure,” Lichtman mentioned, it’s in some methods “controlling among the chaos of primitive units,” making it the proper measuring stick to make use of.

With this stick in hand, a pure subsequent query to ask is what the utmost attainable Erdős sum is likely to be. Erdős conjectured that it will be the one for the prime numbers, which comes out to about 1.64. By way of this lens, the primes represent a sort of excessive.