Mathematicians needed to higher perceive these numbers that so intently resemble essentially the most elementary objects in quantity concept, the primes. It turned out that in 1899—a decade earlier than Carmichael’s consequence—one other mathematician, Alwin Korselt, had provide you with an equal definition. He merely hadn’t recognized if there have been any numbers that match the invoice.

In keeping with Korselt’s criterion, a quantity N is a Carmichael quantity if and provided that it satisfies three properties. First, it should have a couple of prime issue. Second, no prime issue can repeat. And third, for each prime p that divides N, p – 1 additionally divides N – 1. Contemplate once more the quantity 561. It’s equal to three × 11 × 17, so it clearly satisfies the primary two properties in Korselt’s checklist. To point out the final property, subtract 1 from every prime issue to get 2, 10 and 16. As well as, subtract 1 from 561. All three of the smaller numbers are divisors of 560. The quantity 561 is due to this fact a Carmichael quantity.

Although mathematicians suspected that there are infinitely many Carmichael numbers, there are comparatively few in comparison with the primes, which made them troublesome to pin down. Then in 1994, Pink Alford, Andrew Granville, and Carl Pomerance printed a breakthrough paper during which they lastly proved that there are certainly infinitely many of those pseudoprimes.

Sadly, the methods they developed didn’t permit them to say something about what these Carmichael numbers regarded like. Did they seem in clusters alongside the quantity line, with giant gaps in between? Or might you at all times discover a Carmichael quantity in a brief interval? “You’d suppose should you can show there’s infinitely a lot of them,” Granville mentioned, “absolutely you must be capable to show that there aren’t any massive gaps between them, that they need to be comparatively properly spaced out.”

Particularly, he and his coauthors hoped to show an announcement that mirrored this concept—that given a sufficiently giant quantity X, there’ll at all times be a Carmichael quantity between X and a couple ofX. “It’s one other means of expressing how ubiquitous they’re,” mentioned Jon Grantham, a mathematician on the Institute for Protection Analyses who has carried out associated work.

However for many years, nobody might show it. The methods developed by Alford, Granville and Pomerance “allowed us to indicate that there have been going to be many Carmichael numbers,” Pomerance mentioned, “however didn’t actually permit us to have an entire lot of management about the place they’d be.”

Then, in November 2021, Granville opened up an e mail from Larsen, then 17 years outdated and in his senior 12 months of highschool. A paper was hooked up—and to Granville’s shock, it regarded right. “It wasn’t the best learn ever,” he mentioned. “However once I learn it, it was fairly clear that he wasn’t messing round. He had good concepts.”

Pomerance, who learn a later model of the work, agreed. “His proof is basically fairly superior,” he mentioned. “It might be a paper that any mathematician could be actually proud to have written. And right here’s a highschool child writing it.”